BIOLOGY WRITE-UP - BIOLOGY ARTICLES: Human excretory system - anatomy of nephron, physiology of nephron, urine formation, regulation of urine formation, acidosis & ketosis, kidney function test, dialysis, nephrosis & nephritis, tubular function test.

Excretory system

The Nephron

Parts of a Nephron: Nephrons are the functional units of the kidneys. Each nephron consists of two parts: a renal corpuscle (KOR- pus-sul tiny body), where blood plasma is ﬁltered, and a renal tubule into which the ﬁltered ﬂuid passes. The two components of a renal corpuscle are the glomerulus (capillary network) and the glomerular (Bowman’s) capsule, a double-walled epithelial cup that surrounds the glomerular capillaries. Blood plasma is ﬁltered in the glomerular capsule, and then the ﬁltered ﬂuid passes into the renal tubule, which has three main sections. In the order that ﬂuid passes through them, the renal tubule consists of a (1) proximal convoluted tubule, (2) loop of Henle (nephron loop), and (3) distal convoluted tubule. Proximal denotes the part of the tubule attached to the glomerular capsule, and distal denotes the part that is further away. Convoluted means the tubule is tightly coiled rather than straight. The renal corpuscle and both convoluted tubules lie within the renal cortex; the loop of Henle extends into the renal medulla, makes a hairpin turn, and then returns to the renal cortex. The distal convoluted tubules of several nephrons empty into a single collecting duct. Collecting ducts then unite and con- verge into several hundred large papillary ducts, which drain into the minor calyces. The collecting ducts and papillary ducts extend from the renal cortex through the renal medulla to the renal pelvis. So one kidney has about 1 million nephrons, but a much smaller number of collecting ducts and even fewer papillary ducts. In a nephron, the loop of Henle connects the proximal and distal convoluted tubules. The ﬁrst part of the loop of Henle dips into the renal medulla, where it is called the descending limb of the loop of Henle. It then makes that hairpin turn and returns to the renal cortex as the ascending limb of the loop of Henle. About 80–85% of the nephrons are cortical nephrons. Their renal corpuscles lie in the outer portion of the renal cortex, and they have short loops of Henle that lie mainly in the cortex and penetrate only into the outer region of the renal medulla. The short loops of Henle receive their blood supply from per tubular capillaries that arise from efferent arterioles. The other 15–20% of the nephrons is juxta- medullary nephrons (juxta- near to). Their renal corpuscles lie deep in the cortex, close to the medulla, and they have a long loop of Henle that extends into the deepest region of the medulla (Figure 26.5b). Long loops of Henle receive their blood supply from peritubular capillaries and from the vasa recta that arise from efferent arterioles. In addition, the ascending limb of the loop of Henle of juxtamedullary nephrons consists of two portions: a thin ascending limb followed by a thick ascending limb. The lumen of the thin ascending limb is the same as in other areas of the renal tubule; it is only the epithelium that is thinner. Nephrons with long loops of Henle enable the kidneys to excrete very dilute or very concentrated urine.

Histology of the Nephron and Collecting Duct

A single layer of epithelial cells forms the entire wall of the glomerular capsule, renal tubule, and ducts. However, each part has distinctive histological features that reflect its particular functions. We will discuss them in the order that fluid flows through them: glomerular capsule, renal tubule, and collecting duct.

GLOMERULAR CAPSULE The glomerular (Bowman’s) capsule consists of visceral and parietal layers. The visceral layer consists of modiﬁed simple squamous epithelial cells called podocytes (PO ¯ -do¯-c¯ıts; podo- foot; -cytes cells). The many foot like projections of these cells (pedicels) wrap around the single layer of endothelial cells of the glomerular capillaries and form the inner wall of the capsule. The parietal layer of the glomerular capsule consists of simple squamous epithelium and forms the outer wall of the capsule. Fluid ﬁltered from the glomerular capillaries enters the capsular (Bowman’s) space, the space between the two layers of the glomerular capsule. Think of the glomerulus as a ﬁst punched into a limp balloon (the glomerular capsule) until the ﬁst is covered by two layers of balloon (visceral and parietal layers) with a space in between (the capsular space).

RENAL TUBULE AND COLLECTING DUCT illustrates the histology of the cells that form the renal tubule and collecting duct. In the proximal convoluted tubule, the cells are simple cuboidal epithelial cells with a prominent brush border of microvilli on their apical surface (surface facing the lumen). These microvilli, like those of the small intestine, increase the surface area for reabsorption and secretion. The descending limb of the loop of Henle and the ﬁrst part of the ascending limb of the loop of Henle (the thin ascending limb) are composed of simple squamous epithelium. (Recall that cortical or short-loop nephrons lack the thin ascending limb.) The thick ascending limb of the loop of Henle is composed of simple cuboidal to low columnar epithelium. In each nephron, the ﬁnal part of the ascending limb of the loop of Henle makes contact with the afferent arteriole serving that renal corpuscle (Figure 26.6a). Because the columnar tubule cells in this region are crowded together, they are known as the macula densa (macula spot; densa dense). Alongside the macula densa, the wall of the afferent arteriole (and sometimes the efferent arteriole) contains modiﬁed smooth muscle ﬁbers called juxtaglomerular (JG) cells. Together with the macula densa, they constitute the juxtaglomerular apparatus (JGA). As you will see later, the JGA helps regulate blood pressure within the kidneys. The distal convoluted tubule (DCT) begins a short distance past the macula densa. In the last part of the DCT and continuing into the collecting ducts, two different types of cells are present. Most are principal cells, which have receptors for both antidiuretic hormone (ADH) and aldosterone, two hormones that regulate their functions. A smaller number are intercalated cells, which play a role in the homeostasis of blood pH. The collecting ducts drain into large papillary ducts, which are lined by simple columnar epithelium. The number of nephrons is constant from birth. Any increase in kidney size is due solely to the growth of individual nephrons. If nephrons are injured or become diseased, new ones do not form. Signs of kidney dysfunction usually do not become apparent until function declines to less than 25% of normal because the remaining functional nephrons adapt to handle a larger-than- normal load. Surgical removal of one kidney, for example, stimulates hypertrophy (enlargement) of the remaining kidney, which eventually is able to ﬁlter blood at 80% of the rate of two normal kidneys.

OVERVIEW OF RENAL PHYSIOLOGY / formation of urine

To produce urine, nephrons and collecting ducts perform three basic processes—glomerular ﬁltration, tubular reabsorption, and tubular secretion:

● Glomerular ﬁltration. In the ﬁrst step of urine production, water and most solutes in blood plasma move across the wall of glomerular capillaries into the glomerular capsule and then into the renal tubule.

● Tubular reabsorption. As ﬁltered ﬂuid ﬂows along the renal tubule and through the collecting duct, tubule cells reabsorb about 99% of the ﬁltered water and many useful solutes. The water and solutes return to the blood as it ﬂows through the peritubular capillaries and vasa recta. Note that the term reabsorption refers to the return of substances to the blood- stream. The term absorption, by contrast, means entry of new substances into the body, as occurs in the gastrointestinal tract.

● Tubular secretion. As ﬂuid ﬂows along the renal tubule and through the collecting duct, the tubule and duct cells secrete other materials, such as wastes, drugs, and excess ions, into the ﬂuid. Notice that tubular secretion removes a substance from the blood. In other instances of secretion—for instance, secretion of hormones—cells release substances into interstitial ﬂuid and blood.

Solutes in the ﬂuid that drains into the renal pelvis remain in the urine and are excreted. The rate of urinary excretion of any solute is equal to its rate of glomerular ﬁltration, plus its rate of secretion, minus its rate of reabsorption. By ﬁltering, reabsorbing, and secreting, nephrons help maintain homeostasis of the blood’s volume and composition. The situation is somewhat analogous to a recycling center: Garbage trucks dump refuse into an input hopper, where the smaller refuse passes onto a conveyor belt (glomerular ﬁltration of plasma). As the conveyor belt carries the garbage along, workers remove useful items, such as aluminum cans, plastics, and glass containers (reabsorption). Other workers place additional garbage left at the center and larger items onto the conveyor belt (secretion). At the end of the belt, all remaining garbage falls into a truck for transport to the landﬁll (excretion of wastes in urine).

GLOMERULAR FILTRATION

The ﬂuid that enters the capsular space is called the glomerular ﬁltrate. The fraction of blood plasma in the afferent arterioles of the kidneys that becomes glomerular ﬁltrate is the ﬁltration fraction. Although a ﬁltration fraction of 0.16–0.20 (16–20%) is typical, the value varies considerably in both health and disease. On average, the daily volume of glomerular ﬁltrate in adults is 150 liters in females and 180 liters in males. More than 99% of the glomerular ﬁltrate returns to the bloodstream via tubular reabsorption, so only 1–2 liters (about 1–2 qt) are excreted as urine.

The Filtration Membrane

Together, the endothelial cells of glomerular capillaries and the podocytes, which completely encircle the capillaries, form a leaky barrier known as the ﬁltration membrane. This sandwich like assembly permits ﬁltration of water and small solutes but prevents ﬁltration of most plasma proteins, blood cells, and platelets. Substances ﬁltered from the blood cross three barriers—a glomerular endothelial cell, the basal lamina, and a ﬁltration slit formed by a podocytes

● Glomerular endothelial cells are quite leaky because they have large fenestrations (pores) that measure 0.07–0.1 m in diameter. This size permits all solutes in blood plasma to exit glomerular capillaries but prevents ﬁltration of blood cells and platelets. Located among the glomerular capillaries and in the cleft between afferent and efferent arterioles are mesangial cells (mes- in the middle; -angi blood vessel). These contractile cells help regulate glomerular ﬁltration.

● The basal lamina, a layer of acellular material between the endothelium and the podocytes, consists of minute collagen ﬁbers and proteoglycans in a glycoprotein matrix; it pre- vents ﬁltration of larger plasma proteins.

● Extending from each podocyte are thousands of foot like processes termed pedicels (PED-i-sels little feet) that wrap around glomerular capillaries. The spaces between pedicels are the ﬁltration slits. A thin membrane, the slit membrane, extends across each ﬁltration slit; it permits the passage of molecules having a diameter smaller than 0.006–0.007 m, including water, glucose, vitamins, amino acids, very small plasma proteins, ammonia, urea, and ions. Less than 1% of albumin, the most plentiful plasma protein, passes the slit membrane because, with a diameter of 0.007m, it is slightly too big to get through.

The principle of ﬁltration—the use of pressure to force fluids and solutes through a membrane—is the same in glomerular capillaries as in capillaries elsewhere in the body (see Starling’s law of the capillaries, page 770). However, the volume of ﬂuid ﬁltered by the renal corpuscle is much larger than in other capillaries of the body for three reasons:

1. Glomerular capillaries present a large surface area for ﬁltration because they are long and extensive. The mesangial cells regulate how much of this surface area is available for ﬁltration. When mesangial cells are relaxed, surface area is maximal, and glomerular ﬁltration is very high. Contraction of mesangial cells reduces the available surface area, and glomerular ﬁltration decreases.

2. The ﬁltration membrane is thin and porous. Despite having several layers, the thickness of the ﬁltration membrane is only 0.1 m. Glomerular capillaries also are about 50 times leakier than capillaries in most other tissues, mainly because of their large fenestrations.

3. Glomerular capillary blood pressure is high. Because the efferent arteriole is smaller in diameter than the afferent arteriole, resistance to the outﬂow of blood from the glomerulus is high. As a result, blood pressure in glomerular capillaries is considerably higher than in capillaries elsewhere in the body.

Net Filtration Pressure

Glomerular ﬁltration depends on three main pressures. One pressure promotes ﬁltration and two pressures oppose ﬁltration.

● Glomerular blood hydrostatic pressure (GBHP) is the blood pressure in glomerular capillaries. Generally, GBHP is about 55 mmHg. It promotes ﬁltration by forcing water and solutes in blood plasma through the ﬁltration membrane.

● Capsular hydrostatic pressure (CHP) is the hydrostatic pressure exerted against the ﬁltration membrane by ﬂuid already in the capsular space and renal tubule. CHP opposes ﬁltration and represents a “back pressure” of about 15 mmHg.

● Blood colloid osmotic pressure (BCOP), which is due to the presence of proteins such as albumin, globulins, and ﬁbrinogen in blood plasma, also opposes ﬁltration. The average BCOP in glomerular capillaries is 30 mmHg.

Net ﬁltration pressure (NFP), the total pressure that promotes ﬁltration, is determined as follows:

Net Filtration Pressure (NFP) + GBHP - CHP -BCOP

By substituting the values just given, normal NFP may be calculated:

NFP = 55 mmHg - 15 mmHg - 30 mmHg =10 mmHg

Thus, a pressure of only 10 mmHg causes a normal amount of blood plasma (minus plasma proteins) to ﬁlter from the glomerulus into the capsular space.

Glomerular Filtration Rate

The amount of ﬁltrate formed in all the renal corpuscles of both kidneys each minute is the glomerular ﬁltration rate (GFR). In adults, the GFR averages 125 mL/min in males and 105 mL/min in females. Homeostasis of body ﬂuids requires that the kidneys maintain a relatively constant GFR. If the GFR is too high, needed substances may pass so quickly through the renal tubules that some are not reabsorbed and are lost in the urine. If the GFR is too low, nearly all the ﬁltrate may be reabsorbed and certain waste products may not be adequately excreted. GFR is directly related to the pressures that determine net ﬁltration pressure; any change in net ﬁltration pressure will affect GFR. Severe blood loss, for example, reduces mean arterial blood pressure and decreases the glomerular blood hydrostatic pressure. Filtration ceases if glomerular blood hydrostatic pressure drops to 45 mmHg because the opposing pressures add up to 45 mmHg. Amazingly, when systemic blood pressure rises above normal, net ﬁltration pressure and GFR increase very little. GFR is nearly constant when the mean arterial blood pressure is anywhere between 80 and 180 mmHg.

The mechanisms that regulate glomerular ﬁltration rate operate in two main ways:

(1) By adjusting blood ﬂow into and out of the glomerulus and

(2) By altering the glomerular capillary surface area available for ﬁltration. GFR increases when blood ﬂow into the glomerular capillaries increases. Coordinated control of the diameter of both afferent and efferent arterioles regulates glomerular blood ﬂow. Constriction of the afferent arteriole decreases blood ﬂow into the glomerulus; dilation of the afferent arteriole increases it. Three mechanisms control GFR: renal auto regulation, neural regulation, and hormonal regulation.

Renal Auto regulation of GFR the kidneys themselves help maintain a constant renal blood ﬂow and GFR despite normal, everyday changes in blood pressure, like those that occur during exercise. This capability is called renal auto regulation and consists of two mechanisms— the myogenic mechanism and tubuloglomerular feedback. Working together, they can maintain nearly constant GFR over a wide range of systemic blood pressures. The myogenic mechanism (myo- muscle; -genic producing) occurs when stretching triggers contraction of smooth muscle cells in the walls of afferent arterioles. As blood pressure rises, GFR also rises because renal blood ﬂow increases. However, the elevated blood pressure stretches the walls of the afferent arterioles. In response, smooth muscle ﬁbers in the wall of the afferent arteriole contract, which narrows the arteriole’s lumen. As a result, renal blood ﬂow decreases, thus reducing GFR to its previous level. Conversely, when arterial blood pressure drops, the smooth muscle cells are stretched less and thus relax. The afferent arterioles dilate, renal blood ﬂow increases, and GFR increases. The myogenic mechanism normalizes renal blood ﬂow and GFR within seconds after a change in blood pressure. The second contributor to renal auto regulation, tubuloglomerular feedback, is so named because part of the renal tubules—the macula densa—provides feedback to the glomerulus. When GFR is above normal due to elevated systemic blood pressure, ﬁltered ﬂuid ﬂows more rapidly along the renal tubules. As a result, the proximal convoluted tubule and loop of Henle have less time to reabsorb Na Cl and water. Macula densa cells are thought to detect the increased delivery of Na Cl and water and to inhibit release of nitric oxide (NO) from cells in the juxtaglomerular apparatus (JGA). Because NO causes vasodilation, afferent arterioles constrict when the level of NO declines. As a result, less blood ﬂows into the glomerular capillaries and GFR decreases. When blood pressure falls, causing GFR to be lower than normal, the opposite sequence of events occurs, although to a lesser degree. Tubuloglomerular feedback operates more slowly than the myogenic mechanism.

Neural Regulation of GFR

Like most blood vessels of the body, those of the kidneys are supplied by sympathetic ANS ﬁbers that release norepinephrine. Norepinephrine causes vasoconstriction through the activation of α1 receptors, which are particularly plentiful in the smooth muscle ﬁbers of afferent arterioles. At rest, sympathetic stimulation is moderately low, the afferent and efferent arterioles are dilated, and renal auto regulation of GFR prevails. With moderate sympathetic stimulation, both afferent and efferent arterioles constrict to the same degree. Blood ﬂow into and out of the glomerulus is restricted to the same extent, which decreases GFR only slightly. With greater sympathetic stimulation, however, as occurs during exercise or hemorrhage, vasoconstriction of the afferent arterioles predominates. As a result, blood ﬂow into glomerular capillaries is greatly decreased, and GFR drops.

This lowering of renal blood ﬂow has two consequences:

(1) It reduces urine output, which helps conserve blood volume.

(2) It permits greater blood ﬂow to other body tissues.

Hormonal Regulation of GFR

Two hormones contribute to regulation of GFR. Angiotensin II reduces GFR; atrial natriuretic peptide (ANP) increases GFR. Angiotensin II is a very potent vasoconstrictor that narrows both afferent and efferent arterioles and reduces renal blood ﬂow, thereby decreasing GFR. Cells in the atria of the heart secrete atrial natriuretic peptide (ANP). Stretching of the atria, as occurs when blood volume increases, stimulates secretion of ANP. By causing relaxation of the glomerular mesangial cells, ANP increases the capillary surface area available for ﬁltration. Glomerular ﬁltration rate rises as the surface area increases.

TUBULAR REABSORPTION AND TUBULAR SECRETION

Principles of Tubular Reabsorption and Secretion

The volume of ﬂuid entering the proximal convoluted tubules in just half an hour is greater than the total blood plasma volume because the normal rate of glomerular ﬁltration is so high. Obviously some of this ﬂuid must be returned somehow to the bloodstream. Reabsorption—the return of most of the ﬁltered water and many of the ﬁltered solutes to the bloodstream—is the second basic function of the nephron and collecting duct. Normally, about 99% of the ﬁltered water is reabsorbed. Epithelial cells all along the renal tubule and duct carry out reabsorption, but proximal convoluted tubule cells make the largest contribution. Solutes that are reabsorbed by both active and passive processes include glucose, amino acids, urea, and ions such as Na (sodium), K (potassium), Ca2 (calcium), Cl (chloride), HCO3 (bicarbonate), and HPO42 (phosphate). Once ﬂuid passes through the proximal convoluted tubule, cells located more distally ﬁne-tune the reabsorption processes to maintain homeostatic balances of water and selected ions. Most small proteins and peptides that pass through the ﬁlter also are reabsorbed, usually via pinocytosis. To appreciate the magnitude of tubular reabsorption and compare the amounts of substances those are ﬁltered, reabsorbed, and excreted in urine. The third function of nephrons and collecting ducts is tubular secretion, the transfer of materials from the blood and tubule cells into tubular ﬂuid. Secreted substances include hydrogen ions (H) K, ammonium ions (NH4), creatinine, and certain drugs such as penicillin.

Tubular secretion has two important outcomes:

(1) The secretion of H helps control blood pH.

(2) The secretion of other substances helps eliminate them from the body. As a result of tubular secretion, certain substances pass from blood into urine and may be detected by a urinalysis. It is especially important to test athletes for the presence of performance-enhancing drugs such as anabolic steroids, plasma expanders, erythropoietin, hCG, hGH, and amphetamines. Urine tests can also be used to detect the presence of alcohol or illegal drugs such as marijuana, cocaine, and heroin.

Reabsorption Routes a substance being reabsorbed from the ﬂuid in the tubule lumen can take one of two routes before entering a peritubular capillary: It can move between adjacent tubule cells or through an individual tubule cell along the renal tubule, tight junctions surround and join neighboring cells to one another, much like the plastic rings that hold a six-pack of soda cans together. The apical membrane (the tops of the soda cans) contacts the tubular ﬂuid, and the basolateral membrane (the bottoms and sides of the soda cans) contacts interstitial ﬂuid at the base and sides of the cell. The tight junctions do not completely seal off the interstitial ﬂuid from the ﬂuid in the tubule lumen. Fluid can leak between the cells in a passive process known as paracellular reabsorption (para- beside). In some parts of the renal tubule, the paracellular route is thought to account for up to 50% of the reabsorption of certain ions and the water that accompanies them via osmosis. In transcellular reabsorption (trans- across), a substance passes from the ﬂuid in the tubular lumen through the apical membrane of a tubule cell, across the cytosol, and out into interstitial ﬂuid through the basolateral membrane.

Transport Mechanisms When renal cells transport solutes out of or into tubular ﬂuid, they move speciﬁc substances in one direction only. Not surprisingly, different types of transport proteins are present in the apical and basolateral membranes. The tight junctions form a barrier that prevents mixing of proteins in the apical and basolateral membrane compartments. Reabsorption of Na by the renal tubules is especially important because of the large number of sodium ions that pass through the glomerular ﬁlters. Cells lining the renal tubules, like other cells throughout the body, have a low concentration of Na in their cytosol due to the activity of sodium–potassium pumps (Na /K ATPases). These pumps are located in the basolateral membranes and eject Na from the renal tubule cells. The absence of sodium–potassium pumps in the apical membrane ensures that reabsorption of Na is a one-way process. Most sodium ions that cross the apical membrane will be pumped into interstitial ﬂuid at the base and sides of the cell. The amount of ATP used by sodium–potassium pumps in the renal tubules is about 6% of the total ATP consumption of the body at rest. This may not sound like much, but it is about the same amount of energy used by the diaphragm as it contracts during quiet breathing. Transport of materials across membranes may be either active or passive. Recall that in primary active transport the energy derived from hydrolysis of ATP is used to “pump” a substance across a membrane; the sodium– potassium pump is one such pump. In secondary active transport the energy stored in an ion’s electrochemical gradient, rather than hydrolysis of ATP, drives another substance across a membrane. Secondary active transport couples the movement of an ion down its electrochemical gradient to the “uphill” movement of a second substance against its electrochemical gradient. Symporters are membrane proteins that move two or more substances in the same direction across a membrane. Antiporters move two or more substances in opposite directions across a membrane. Each type of transporter has an upper limit on how fast it can work, just as an escalator has a limit on how many people it can carry from one level to another in a given period. This limit, called the transport maximum (Tm), is measured in mg/min. Solute reabsorption drives water reabsorption because all water reabsorption occurs via osmosis. About 90% of the reabsorption of water ﬁltered by the kidneys occurs along with the reabsorption of solutes such as Na, Cl and glucose. Water reabsorbed with solutes in tubular ﬂuid is termed obligatory water reabsorption (ob-LIG-a-tor-e¯) because the water is “obliged” to follow the solutes when they are reabsorbed. This type of water reabsorption occurs in the proximal convoluted tubule and the descending limb of the loop of Henle because these segments of the nephron are always permeable to water. Reabsorption of the ﬁnal 10% of the water, a total of 10–20 liters per day, is termed facultative water reabsorption (FAK-ul-ta¯-tiv). The word facultative means “capable of adapting to a need.” Facultative water reabsorption is regulated by antidiuretic hormone and occurs mainly in the collecting ducts.

Now that we have discussed the principles of renal transport, we will follow the ﬁltered ﬂuid from the proximal convoluted tubule, into the loop of Henle, on to the distal convoluted tubule, and through the collecting ducts. In each segment, we will ex- amine where and how speciﬁc substances are reabsorbed and secreted. The ﬁltered ﬂuid becomes tubular ﬂuid once it enters the proximal convoluted tubule. The composition of tubular ﬂuid changes as it ﬂows along the nephron tubule and through the collecting duct due to reabsorption and secretion. The ﬂuid that drains from papillary ducts into the renal pelvis is urine.

Reabsorption and Secretion in the Proximal Convoluted Tubule

The largest amount of solute and water reabsorption from ﬁltered ﬂuid occurs in the proximal convoluted tubules, which reabsorb 65% of the ﬁltered water, Na, and K; 100% of most ﬁltered organic solutes such as glucose and amino acids; 50% of the ﬁltered Cl; 80–90% of the ﬁltered HCO3; 50% of the ﬁltered urea; and a variable amount of the ﬁltered Ca2, Mg 2, and HPO42 (phosphate). In addition, proximal convoluted tubules secrete a variable amount of H ions, ammonium ions (NH4), and urea. Most solute reabsorption in the proximal convoluted tubule (PCT) involves Na. Na transport occurs via symport and anti- port mechanisms in the proximal convoluted tubule. Normally, ﬁltered glucose, amino acids, lactic acid, water-soluble vitamins, and other nutrients are not lost in the urine. Rather, they are completely reabsorbed in the ﬁrst half of the proximal convoluted tubule by several types of Na symporters located in the apical membrane. Figure 26.12 depicts the operation of one such symporter, the Na–glucose symporter in the apical membrane of a cell in the PCT. Two Na and a molecule of glucose attach to the symporter protein, which carries them from the tubular ﬂuid into the tubule cell. The glucose molecules then exit the basolateral membrane via facilitated diffusion and they diffuse into peritubular capillaries. Other Na symporters in the PCT reclaim ﬁltered HPO42 (phosphate) and SO42 (sulfate) ions, all amino acids, and lactic acid in a similar way. In another secondary active transport process, the Na/H Antiporters carry ﬁltered Na down its concentration gradient into a PCT cell as His moved from the cytosol into the lumen causing Na to be reabsorbed into blood and H to be secreted into tubular ﬂuid. PCT cells produce the H needed to keep the antiporters running in the following way. Carbon dioxide (CO2) diffuses from peritubular blood or tubular ﬂuid or is produced by metabolic reactions within the cells. As also occurs in red blood cells the enzyme carbonic anhydrase (CA) catalyzes the reaction of CO2 with water (H2O) to form carbonic acid (H2CO3), which then dissociates into H and HCO3:

CO2 + H2O ------>H 2CO3 ----------> H⁺ + HCO3^-

Most of the HCO3 in ﬁltered ﬂuid is reabsorbed in proximal convoluted tubules, thereby safeguarding the body’s supply of an important buffer. After H is secreted into the ﬂuid within the lumen of the proximal convoluted tubule, it re- acts with ﬁltered HCO3 to form H2CO3, which readily dissociates into CO2 and H2O. Carbon dioxide then diffuses into the tubule cells and joins with H2O to form H2CO3, which dissociates into H and HCO3. As the level of HCO3 rises in the cytosol, it exits via facilitated diffusion transporters in the basolateral membrane and diffuses into the blood with Na. Thus, for every H secreted into the tubular ﬂuid of the proximal convoluted tubule, one HCO3 and one Na are reabsorbed. Solute reabsorption in proximal convoluted tubules promotes osmosis of water. Each reabsorbed solute increases the osmolarity, ﬁrst inside the tubule cell, then in interstitial ﬂuid, and ﬁnally in the blood. Water thus moves rapidly from the tubular ﬂuid, via both the paracellular and transcellular routes, into the peritubular capillaries and restores osmotic balance. In other words reabsorption of the solutes creates an osmotic gradient that promotes the reabsorption of water via osmosis. Cells lining the proximal convoluted tubule and the descending limb of the loop of Henle are especially permeable to water because they have many molecules of aquaporin-1. This integral protein in the plasma membrane is a water channel that greatly increases the rate of water movement across the apical and basolateral membranes. As water leaves the tubular ﬂuid, the concentrations of the remaining ﬁltered solutes increase. In the second half of the PCT, electrochemical gradients for Cl, K , Ca 2, Mg 2, and urea promote their passive diffusion into peritubular capillaries via both paracellular and transcellular routes. Among these ions, Cl is present in the highest concentration. Diffusion of negatively charged Cl into interstitial ﬂuid via the paracellular route makes the interstitial ﬂuid electrically more negative than the tubular ﬂuid. This negativity promotes passive paracellular reabsorption of cations, such as K, Ca 2, and Mg2. Ammonia (NH3) is a poisonous waste product derived from the deamination (removal of an amino group) of various amino acids, a reaction that occurs mainly in hepatocytes (liver cells). Hepatocytes convert most of this ammonia to urea, a less-toxic compound. Although tiny amounts of urea and ammonia are present in sweat, most excretion of these nitrogen-containing waste products occurs via the urine. Urea and ammonia in blood are both ﬁltered at the glomerulus and secreted by proximal convoluted tubule cells into the tubular ﬂuid. Proximal convoluted tubule cells can produce additional NH3 by deaminating the amino acid glutamine in a reaction that also generates HCO3. The NH3 quickly binds H to become an ammonium ion (NH4), which can substitute for H aboard Na /H antiporters in the apical membrane and be secreted into the tubular ﬂuid. The HCO3 generated in this reaction moves through the basolateral membrane and then diffuses into the bloodstream, providing additional buffers in blood plasma.

Reabsorption in the Loop of Henle

Because all of the proximal convoluted tubules reabsorb about 65% of the ﬁltered water (about 80 mL/min), ﬂuid enters the next part of the nephron, the loop of Henle, at a rate of 40–45 mL/min. The chemical composition of the tubular ﬂuid now is quite different from that of glomerular ﬁltrate because glucose, amino acids, and other nutrients are no longer present. The osmolarity of the tubular ﬂuid is still close to the osmolarity of blood, however, because reabsorption of water by osmosis keeps pace with reabsorption of solutes all along the proximal convoluted tubule. The loop of Henle reabsorbs about 15% of the ﬁltered water; 20–30% of the ﬁltered Na and K; 35% of the ﬁltered Cl; 10–20% of the ﬁltered HCO3; and a variable amount of the ﬁltered Ca2 and Mg2. Here, for the first time, reabsorption of water via osmosis is not automatically coupled to reabsorption of ﬁltered solutes because part of the loop of Henle is relatively impermeable to water. The loop of Henle thus sets the stage for independent regulation of both the volume and osmolarity of body ﬂuids.

The apical membranes of cells in the thick ascending limb of the loop of Henle have Na–K–2Cl symporters that simultaneously reclaim one Na, one K, and two Cl from the ﬂuid in the tubular lumen. Na that is actively trans- ported into interstitial ﬂuid at the base and sides of the cell diffuses into the vasa recta. Cl moves through leakage channels in the basolateral membrane into interstitial ﬂuid and then into the vasa recta. Because many K leakage channels are present in the apical membrane, most K brought in by the symporters moves down its concentration gradient back into the tubular ﬂuid. Thus, the main effect of the Na–K–2Cl symporters is reabsorption of Na and Cl. The movement of positively charged K into the tubular ﬂuid through the apical membrane channels leaves the interstitial ﬂuid and blood with more negative charges relative to ﬂuid in the ascending limb of the loop of Henle. This relative negativity promotes reabsorption of cations—Na, K, Ca 2, and Mg2— via the paracellular route. Although about 15% of the ﬁltered water is reabsorbed in the descending limb of the loop of Henle, little or no water is reabsorbed in the ascending limb. In this segment of the tubule, the apical membranes are virtually impermeable to water. Because ions but not water molecules are reabsorbed, the osmolarity of the tubular ﬂuid decreases progressively as ﬂuid ﬂows toward the end of the ascending limb.

Reabsorption in the Early Distal Convoluted Tubule

Fluid enters the distal convoluted tubules at a rate of about 25 mL/min because 80% of the ﬁltered water has now been re- absorbed. The early or initial part of the distal convoluted tubule (DCT) reabsorbs about 10–15% of the ﬁltered water; 5% of the ﬁltered Na; and 5% of the ﬁltered Cl. Reabsorption of Na and Cl occurs by means of Na–Cl symporters in the apical membranes. Sodium–potassium pumps and Cl leakage channels in the basolateral membranes then permit reabsorption of Na and Cl into the peritubular capillaries. The early DCT also is a major site where parathyroid hormone (PTH) stimulates re- absorption of Ca2. The amount of Ca2reabsorption in the early DCT varies depending on the body’s needs.

Reabsorption and Secretion in the Late Distal Convoluted Tubule and Collecting Duct

By the time ﬂuid reaches the end of the distal convoluted tubule, 90–95% of the ﬁltered solutes and water have returned to the bloodstream. Recall that two different types of cells—principal cells and intercalated cells—are present at the late or terminal part of the distal convoluted tubule and throughout the collecting duct. The principal cells reabsorb Na and secrete K; the intercalated cells reabsorb K and HCO3 and secrete H. In the late distal convoluted tubules and collecting ducts, the amount of water and solute reabsorption and the amount of solute secretion vary depending on the body’s needs.

In contrast to earlier segments of the nephron, Na passes through the apical membrane of principal cells via Na leakage channels rather than by means of symporters or antiporters. The concentration of Na in the cytosol remains low, as usual, because the sodium–potassium pumps actively transport Na across the basolateral membranes. Then Na passively diffuses into the peritubular capillaries from the interstitial spaces around the tubule cells. Normally, transcellular and paracellular reabsorption in the proximal convoluted tubule and loop of Henle return most ﬁltered K to the bloodstream. To adjust for varying dietary intake of potassium and to maintain a stable level of K in body ﬂuids, principal cells secrete a variable amount of K. Because the basolateral sodium–potassium pumps continually bring K into principal cells, the intracellular concentration of K remains high. K leakage channels are present in both the apical and basolateral membranes. Thus, some K diffuses down its concentration gradient into the tubular ﬂuid, where the K concentration is very low. This secretion mechanism is the main source of K excreted in the urine.

Hormonal Regulation of Tubular Reabsorption and Tubular Secretion/kidney hormones

Five hormones affect the extent of Na, Cl, Ca 2, and water reabsorption as well as K secretion by the renal tubules. These hormones include angiotensin II, aldosterone, antidiuretic hormone, atrial natriuretic peptide, and parathyroid hormone.

Renin–Angiotensin–Aldosterone System When blood volume and blood pressure decrease, the walls of the afferent arterioles are stretched less, and the juxtaglomerular cells secrete the enzyme renin into the blood. Sympathetic stimulation also directly stimulates release of renin from juxta- glomerular cells. Renin clips off a 10-amino-acid peptide called angiotensin I from angiotensinogen, which is synthesized by hepatocytes. By clipping off two more amino acids, angiotensin converting enzyme (ACE) converts angiotensin I to angiotensin II, which is the active form of the hormone. Angiotensin II affects renal physiology in three main ways:

1. It decreases the glomerular ﬁltration rate by causing vasoconstriction of the afferent arterioles.

2. It enhances reabsorption of Na, Cl, and water in the proximal convoluted tubule by stimulating the activity of Na/H antiporters.

3. It stimulates the adrenal cortex to release aldosterone, a hormone that in turn stimulates the principal cells in the collecting ducts to reabsorb more Na and Cl and secrete more K. The osmotic consequence of reabsorbing more Na and Cl is excreting less water, which increases blood volume.

Antidiuretic Hormone Antidiuretic hormone (ADH or vasopressin) is released by the posterior pituitary. It regulates facultative water reabsorption by increasing the water permeability of principal cells in the last part of the distal convoluted tubule and throughout the collecting duct. In the absence of ADH, the apical membranes of principal cells have a very low permeability to water. Within principal cells are tiny vesicles containing many copies of a water channel protein known as aquaporin-2.* ADH stimulates insertion of the aquaporin-2–containing vesicles into the apical membranes via exocytosis. As a result, the water permeability of the principal cell’s apical membrane increases, and water molecules move more rapidly from the tubular ﬂuid into the cells. Because the basolateral membranes are always relatively permeable to water, water molecules then move rapidly into the blood. The kidneys can produce as little as 400–500 mL of very concentrated urine each day when ADH concentration is maximal, for instance during severe dehydration. When ADH level declines, the aquaporin-2 channels are removed from the apical membrane via endocytosis. The kidneys produce a large volume of dilute urine when ADH level is low. A negative feedback system involving ADH regulates facultative water reabsorption. When the osmolarity or osmotic pressure of plasma and interstitial ﬂuid increases—that is, when water concentration decreases—by as little as 1%, osmo receptors in the hypothalamus detect the change. Their nerve impulses stimulate secretion of more ADH into the blood, and the principal cells become more permeable to water. As facultative water reabsorption increases, plasma osmolarity decreases to normal. A second powerful stimulus for ADH secretion is a decrease in blood volume, as occurs in hemorrhaging or severe dehydration. In the pathological absence of ADH activity, a condition known as diabetes insipidus, a person may excrete up to 20 liters of very dilute urine daily.

Atrial Natriuretic Peptide a large increase in blood volume promotes release of atrial natriuretic peptide (ANP) from the heart. Although the importance of ANP in normal regulation of tubular function is unclear, it can inhibit reabsorption of Na and water in the proximal convoluted tubule and collecting duct. ANP also suppresses the secretion of aldosterone and ADH. These effects increase the excretion of Na in urine (natriuresis) and increase urine output (diuresis), which decreases blood volume and blood pressure.

Parathyroid Hormone A lower-than-normal level of Ca2 in the blood stimulates the parathyroid glands to release parathyroid hormone (PTH). PTH in turn stimulates cells in the early distal convoluted tubules to reabsorb more Ca2 into the blood. PTH also inhibits HPO42 (phosphate) reabsorption in proximal convoluted tubules, thereby promoting phosphate excretion. Table 26.4 summarizes hormonal regulation of tubular reabsorption and tubular secretion.

PRODUCTION OF DILUTE AND CONCENTRATED URINE

OBJECTIVE

• Describe how the renal tubule and collecting ducts pro- duce dilute and concentrated urine.

Even though your ﬂuid intake can be highly variable, the total volume of ﬂuid in your body normally remains stable. Homeostasis of body ﬂuid volume depends in large part on the ability of the kidneys to regulate the rate of water loss in urine. Normally functioning kidneys produce a large volume of dilute urine when ﬂuid intake is high, and a small volume of concentrated urine when ﬂuid intake is low or ﬂuid loss is large. ADH controls whether dilute urine or concentrated urine is formed. In the absence of ADH, urine is very dilute. However, a high level of ADH stimulates reabsorption of more water into blood, producing concentrated urine.

Formation of Dilute Urine

Glomerular ﬁltrate has the same ratio of water and solute particles as blood; its osmolarity is about 300mOsm/liter. As previously noted, ﬂuid leaving the proximal convoluted tubule is still isotonic to plasma. When dilute urine is being formed, the osmolarity of the ﬂuid in the tubular lumen increases as it ﬂows down the descending limb of the loop of Henle, decreases as it ﬂows up the ascending limb, and decreases still more as it ﬂows through the rest of the nephron and collecting duct. These changes in osmolarity result from the following conditions along the path of tubular ﬂuid:

1. Because the osmolarity of the interstitial ﬂuid of the renal medulla becomes progressively greater, more and more water is reabsorbed by osmosis as tubular ﬂuid ﬂows along the descending limb toward the tip of the loop. (The source of this medullary osmotic gradient is explained shortly.) As a result, the ﬂuid remaining in the lumen becomes progressively more concentrated.

2. Cells lining the thick ascending limb of the loop have symporters that actively reabsorb Na, K, and Cl from the tubular ﬂuid. The ions pass from the tubular ﬂuid into thick ascending limb cells, then into interstitial ﬂuid, and ﬁnally some diffuse into the blood inside the vasa recta.

3. Although solutes are being reabsorbed in the thick ascending limb, the water permeability of this portion of the nephron is always quite low, so water cannot follow by osmosis. As solutes—but not water molecules—are leaving the tubular ﬂuid, its osmolarity drops to about 150 mOsm/liter. The ﬂuid entering the distal convoluted tubule is thus more dilute than plasma.

4. While the ﬂuid continues ﬂowing along the distal convoluted tubule, additional solutes but only a few water molecule are reabsorbed. The early distal convoluted tubule cells are not very permeable to water and are not regulated by ADH.

5. Finally, the principal cells of the late distal convoluted tubules and collecting ducts are impermeable to water when the ADH level is very low. Thus, tubular ﬂuid becomes progressively more dilute as it ﬂows onward. By the time the tubular ﬂuid drains into the renal pelvis, its concentration can be as low as 65–70 mOsm/liter. This is four times more dilute than blood plasma or glomerular ﬁltrate.

Formation of Concentrated Urine

When water intake is low or water loss is high (such as during heavy sweating), the kidneys must conserve water while still eliminating wastes and excess ions. Under the inﬂuence of ADH, the kidneys produce a small volume of highly concentrated urine. Urine can be four times more concentrated (up to 1200 mOsm/liter) than blood plasma or glomerular ﬁltrate (300mOsm/liter). The ability of ADH to cause excretion of concentrated urine depends on the presence of an osmotic gradient of solutes in the interstitial ﬂuid of the renal medulla. The solute concentration of the interstitial ﬂuid in the kidney increases from about 300 mOsm/liter in the renal cortex to about 1200 mOsm/liter deep in the renal medulla. The three major solutes that contribute to this high osmolarity are Na, Cl, and urea. Two main factors contribute to building and maintaining this osmotic gradient:

(1) Differences in solute and water permeability and reabsorption in different sections of the long loops of Henle and the collecting ducts

(2) The countercurrent ﬂow of ﬂuid through tube-shaped structures in the renal medulla. Countercurrent ﬂow refers to the ﬂow of ﬂuid in opposite directions. This occurs when ﬂuid ﬂowing in one tube runs counter (opposite) to ﬂuid ﬂowing in a nearby parallel tube.

Examples of countercurrent ﬂow include the ﬂow of tubular ﬂuid through the descending and ascending limbs of the loop of Henle and the ﬂow of blood through the ascending and descending parts of the vasa recta. Two types of countercurrent mechanisms exist in the kidneys: countercurrent multiplication and countercurrent exchange.

Countercurrent Multiplication

Countercurrent multiplication is the process by which a progressively increasing osmotic gradient is formed in the interstitial ﬂuid of the renal medulla as a result of countercurrent ﬂow. Countercurrent multiplication involves the long loops of Henle of juxtamedullary nephrons. the descending limb of the loop of Henle carries tubular ﬂuid from the renal cortex deep into the medulla, and the ascending limb carries it in the opposite direction. Since countercurrent ﬂow through the descending and ascending limbs of the long loop of Henle establishes the osmotic gradient in the renal medulla, the long loop of Henle is said to function as a countercurrent multiplier. The kidneys use this osmotic gradient to excrete concentrated urine. Production of concentrated urine by the kidneys occurs in the following way.

Ø Symporters in thick ascending limb cells of the loop of Henle cause a buildup of Na and Cl in the renal medulla. In the thick ascending limb of the loop of Henle, the Na K 2Cl symporters reabsorb Na and Cl from the tubular ﬂuid. Water is not reabsorbed in this segment, however, because the cells are impermeable to water. As a result, there is a buildup of Na and Cl ions in the interstitial ﬂuid of the medulla.

Ø Countercurrent ﬂow through the descending and ascending limbs of the loop of Henle establishes an osmotic gradient in the renal medulla. Since tubular ﬂuid constantly moves from the descending limb to the thick ascending limb of the loop of Henle, the thick ascending limb is constantly reabsorbing Na and Cl Consequently, the reabsorbed Na and Cl become increasingly concentrated in the interstitial ﬂuid of the medulla, which results in the formation of an osmotic gradient that ranges from 300 mOsm/liter in the outer medulla to 1200 mOsm/liter deep in the inner medulla. The descending limb of the loop of Henle is very permeable to water but impermeable to solutes except urea. Because the osmolarity of the interstitial ﬂuid outside the descending limb is higher than the tubular ﬂuid within it, water moves out of the descending limb via osmosis. This causes the osmolarity of the tubular ﬂuid to increase. As the ﬂuid continues along the descending limb, its osmolarity increases even more: At the hairpin turn of the loop, the osmolarity can be as high as 1200 mOsm/liter in juxtamedullary nephrons. As you have already learned, the ascending limb of the loop is impermeable to water, but its symporters reabsorb Na and Cl from the tubular ﬂuid into the interstitial ﬂuid of the renal medulla, so the osmolarity of the tubular ﬂuid progressively decreases as it ﬂows through the ascending limb. At the junction of the medulla and cortex, the osmolarity of the tubular ﬂuid has fallen to about 100 mOsm/liter. Overall, tubular ﬂuid becomes progressively more concentrated as it ﬂows along the descending limb and progressively more di- lute as it moves along the ascending limb.

Ø Cells in the collecting ducts reabsorb more water and urea. When ADH increases the water permeability of the principal cells, water quickly moves via osmosis out of the collecting duct tubular ﬂuid, into the interstitial ﬂuid of the inner medulla, and then into the vasa recta. With loss of water, the urea left behind in the tubular ﬂuid of the collecting duct becomes increasingly concentrated. Because duct cells deep in the medulla are permeable to it, urea diffuses from the ﬂuid in the duct into the interstitial ﬂuid of the medulla.

Ø Urea recycling causes a buildup of urea in the renal medulla. As urea accumulates in the interstitial ﬂuid, some of it diffuses into the tubular ﬂuid in the descending and thin ascending limbs of the long loops of Henle, which also are permeable to urea. However, while the ﬂuid ﬂows through the thick ascending limb, distal convoluted tubule, and cortical portion of the collecting duct, urea remains in the lumen because cells in these segments are impermeable to it. As ﬂuid ﬂows along the collecting ducts, water reabsorption continues via osmosis because ADH is present. This water reabsorption further increases the con- centration of urea in the tubular ﬂuid, more urea diffuses into the interstitial ﬂuid of the inner renal medulla, and the cycle repeats. The constant transfer of urea between segments of the renal tubule and the interstitial ﬂuid of the medulla is termed urea recycling. In this way, reabsorption of water from the tubular ﬂuid of the ducts promotes the buildup of urea in the interstitial ﬂuid of the renal medulla, which in turn promotes water reabsorption. The solutes left behind in the lumen thus become very concentrated, and a small volume of concentrated urine is excreted.

Countercurrent Exchange

Countercurrent exchange is the process by which solutes and water are passively exchanged between the blood of the vasa recta and interstitial ﬂuid of the renal medulla as a result of countercurrent ﬂow. the vasa recta also consists of descending and ascending limbs that are parallel to each other and to the loop of Henle. Just as tubular ﬂuid ﬂows in opposite directions in the loop of Henle, blood ﬂows in opposite directions in the ascending and descending parts of the vasa recta. Since countercurrent ﬂow between the descending and ascending limbs of the vasa recta allows for exchange of solutes and water between the blood and interstitial ﬂuid of the renal medulla, the vasa recta is said to function as a countercurrent exchanger. Blood entering the vasa recta has an osmolarity of about 300 mOsm/liter. As it ﬂows along the descending part into the renal medulla, where the interstitial ﬂuid becomes increasingly concentrated, Na, Cl, and urea diffuse from interstitial ﬂuid into the blood and water diffuses from the blood into the interstitial ﬂuid. But after its osmolarity increases, the blood ﬂows into the ascending part of the vasa recta. Here blood ﬂows through a region where the interstitial ﬂuid becomes increasingly less concentrated. As a result Na, Cl, and urea diffuse from the blood back into interstitial ﬂuid, and water diffuses from interstitial ﬂuid back into the vasa recta. The osmolarity of blood leaving the vasa recta is only slightly higher than the osmolarity of blood entering the vasa recta. Thus, the vasa recta provide oxygen and nutrients to the renal medulla without washing out or diminishing the osmotic gradient. The long loop of Henle establishes the osmotic gradient in the renal medulla by countercurrent multiplication, but the vasa recta maintain the osmotic gradient in the renal medulla by countercurrent exchange.

EVALUATION OF KIDNEY FUNCTION

Routine assessment of kidney function involves evaluating both the quantity and quality of urine and the levels of wastes in the blood.

Urinalysis

An analysis of the volume and physical, chemical, and microscopic properties of urine, called a urinalysis, reveals much about the state of the body. summarizes the major characteristics of normal urine. The volume of urine eliminated per day in a normal adult is 1–2 liters (about 1–2 qt). Fluid intake, blood pressure, blood osmolarity, diet, body temperature, diuretics, mental state, and general health inﬂuence urine volume. For example, low blood pressure triggers the renin– angiotensin–aldosterone pathway. Aldosterone increases reabsorption of water and salts in the renal tubules and decreases urine volume. By contrast, when blood osmolarity decreases— for example, after drinking a large volume of water—secretion of ADH is inhibited and a larger volume of urine is excreted. Water accounts for about 95% of the total volume of urine. The remaining 5% consists of electrolytes, solutes derived from cellular metabolism, and exogenous substances such as drugs. Normal urine is virtually protein-free. Typical solutes normally present in urine include ﬁltered and secreted electrolytes that are not reabsorbed, urea (from breakdown of proteins), creatinine (from breakdown of creatine phosphate in muscle ﬁbers), uric acid (from breakdown of nucleic acids), urobilinogen (from breakdown of hemoglobin), and small quantities of other sub- stances, such as fatty acids, pigments, enzymes, and hormones. If disease alters body metabolism or kidney function, traces of substances not normally present may appear in the urine, or normal constituents may appear in abnormal amounts. Several abnormal constituents in urine that may be detected as part of a urinalysis. Normal values of urine components and the clinical implications of deviations from normal are listed in Appendix D.

Blood Tests

Renal Plasma Clearance

Even more useful than BUN and blood creatinine values in the diagnosis of kidney problems is an evaluation of how effectively the kidneys are removing a given substance from blood plasma. Renal plasma clearance is the volume of blood that is “cleaned” or cleared of a substance per unit of time, usually expressed in units of milliliters per minute. High renal plasma clearance indicates efﬁcient excretion of a substance in the urine; low clearance indicates inefﬁcient excretion. For example, the clearance of glucose normally is zero because it is completely reabsorbed; therefore, glucose is not excreted at all. Knowing a drug’s clearance is essential for deter- mining the correct dosage. If clearance is high (one example is penicillin), then the dosage must also be high, and the drug must be given several times a day to maintain an adequate therapeutic level in the blood. The following equation is used to calculate clearance:

Renal plasma clearance of substance S = ((U X V)/p)

Where U and P are the concentrations of the substance in urine and plasma, respectively (both expressed in the same units, such as mg/mL), and V is the urine ﬂow rate in mL/min. The clearance of a solute depends on the three basic processes of a nephron: glomerular ﬁltration, tubular reabsorption, and tubular secretion. Consider a substance that is ﬁltered but neither reabsorbed nor secreted. Its clearance equals the glomerular ﬁltration rate because all the molecules that pass the ﬁltration membrane appear in the urine. This is very nearly the situation for creatinine; it easily passes the ﬁlter, it is not reabsorbed, and it is secreted only to a very small extent. Measuring the creatinine clearance, which normally is 120– 140 mL/min, is the easiest way to assess glomerular ﬁltration rate. The waste product urea is ﬁltered, reabsorbed, and secreted to varying extents. Its clearance typically is less than the GFR, about 70 mL/min. The clearance of the organic anion para-aminohippuric acid (PAH) is also of clinical importance. After PAH is administered intravenously, it is ﬁltered and secreted in a single pass through the kidneys. Thus, the clearance of PAH is used to measure renal plasma ﬂow, the amount of plasma that passes through the kidneys in one minute. Typically, the renal plasma ﬂow is 650 mL per minute, which is about 55% of the renal blood ﬂow (1200 mL per minute).

Dialysis

If a person’s kidneys are so impaired by disease or injury that he or she is unable to function adequately, then blood must be cleansed artiﬁcially by dialysis (d¯ı-AL-i-sis; dialyo to separate), the separation of large solutes from smaller ones by diffusion through a selectively permeable membrane. One method of dialysis is hemodialysis (he¯-mo¯-d¯ı- AL-i-sis; hemo- blood), which directly ﬁlters the patient’s blood by re- moving wastes and excess electrolytes and ﬂuid and then returning the cleansed blood to the patient. Blood removed from the body is delivered to a hemodialyzer (artiﬁcial kidney). Inside the hemodialyzer, blood ﬂows through a dialysis membrane, which contains pores large enough to permit the diffusion of small solutes. A special solution, called the dialysate (dı¯-AL-i-sa¯t), is pumped into the hemodialyzer so that it surrounds the dialysis membrane. The dialysate is especially formulated to maintain diffusion gradients that remove wastes from the blood (for example, urea, creatinine, uric acid, excess phosphate, potassium, and sulfate ions) and add needed substances (for example, glucose and bicarbonate ions) to it. The cleansed blood is passed through an air embolus detector to remove air and then returned to the body. An anticoagulant (heparin) is added to prevent blood from clot- ting in the hemodialyzer. As a rule, most people on hemodialysis require about 6–12 hours a week, typically divided into three sessions. Another method of dialysis, called peritoneal dialysis, uses the peritoneum of the abdominal cavity as the dialysis membrane to ﬁlter the blood. The peritoneum has a large surface area and numerous blood vessels, and is a very effective ﬁlter. A catheter is inserted into the peritoneal cavity and connected to a bag of dialysate. The ﬂuid ﬂows into the peritoneal cavity by gravity and is left there for sufﬁcient time to permit wastes and excess electrolytes and ﬂuids to diffuse into the dialysate. Then the dialysate is drained out into a bag, discarded, and replaced with fresh dialysate. Each cycle is called an exchange. One variation of peritoneal dialysis, called continuous ambulatory peritoneal dialysis (CAPD), can be per- formed at home. Usually, the dialysate is drained and replenished four times a day and once at night during sleep. Between exchanges the person can move about freely with the dialysate in the peritoneal cavity.

Regulation of acid base, electrolyte and water balance

In lean adults, body ﬂuids constitute between 55% and 60% of total body mass in females and males, respectively. Body ﬂuids are present in two main “compartments”—inside cells and outside cells. About two-thirds of body ﬂuid is intracellular ﬂuid (ICF) (intra- within) or cytosol, the ﬂuid within cells. The other third, called extracellular ﬂuid (ECF) (extra- outside) is outside cells and includes all other body ﬂuids. About 80% of the ECF is interstitial ﬂuid (inter- between), which occupies the microscopic spaces between tissue cells, and 20% of the ECF is plasma, the liquid portion of the blood. Other extracellular ﬂuids that are grouped with interstitial ﬂuid include lymph in lymphatic vessels; cerebrospinal fluid in the nervous system; synovial fluid

in joints; aqueous humor and vitreous body in the eyes; endolymph and perilymph in the ears; and pleural, pericardial, and peritoneal ﬂuids between serous membranes. Two general “barriers” separate intracellular ﬂuid, interstitial ﬂuid, and blood plasma.

1. The plasma membrane of individual cells separates intra- cellular ﬂuid from the surrounding interstitial ﬂuid. the plasma membrane is a selectively permeable barrier: It allows some substances to cross but blocks the movement of other substances. In addition, active transport pumps work continuously to maintain different concentrations of certain ions in the cytosol and interstitial ﬂuid.

2. Blood vessel walls divide the interstitial ﬂuid from blood plasma. Only in capillaries, the smallest blood vessels, are the walls thin enough and leaky enough to permit the exchange of water and solutes between blood plasma and interstitial ﬂuid.

The body is in ﬂuid balance when the required amounts of water and solutes are present and are correctly proportioned among the various compartments. Water is by far the largest single component of the body, making up 45–75% of total body mass, depending on age and gender.

The processes of ﬁltration, reabsorption, diffusion, and osmosis allow continual exchange of water and solutes among body ﬂuid compartments. Yet the volume of ﬂuid in each compartment remains remarkably stable. The pressures that promote ﬁltration of ﬂuid from blood capillaries and reabsorption of ﬂuid back into capillaries can be reviewed. Because osmosis is the primary means of water movement between intracellular ﬂuid and interstitial ﬂuid, the concentration of solutes in these ﬂuids determines the direction of water movement. Because most solutes in body ﬂu- ids are electrolytes, inorganic compounds that dissociate into ions, ﬂuid balance is closely related to electrolyte balance. Because intake of water and electrolytes rarely occurs in exactly the same proportions as their presence in body ﬂuids, the ability of the kidneys to excrete excess water by producing dilute urine, or to excrete excess electrolytes by producing concentrated urine, is of utmost importance in the maintenance of homeostasis.

Sources of Body Water Gain and Loss

The body can gain water by ingestion and by metabolic synthesis. The main sources of body water are ingested liquids (about 1600 mL) and moist foods (about 700 mL) absorbed from the gastrointestinal (GI) tract, which total about 2300 mL/day. The other source of water is metabolic water that is produced in the body mainly when electrons are accepted by oxygen during aerobic cellular respiration and to a smaller extent during dehydration synthesis reactions. Metabolic water gain accounts for only 200 mL/day. Daily water gain from these two sources totals about 2500mL. Normally, body ﬂuid volume remains constant because water loss equals water gain. Water loss occurs in four ways. Each day the kidneys excrete about 1500 mL in urine, the skin evaporates about 600 mL (400 mL through insensible perspiration, sweat that evaporates before it is perceived as moisture, and 200 mL as sweat), the lungs exhale about 300 mL as water vapor, and the gastrointestinal tract eliminates about 100 mL in feces. In women of reproductive age, additional water is lost in menstrual ﬂow. On average, daily water loss totals about 2500mL. The amount of water lost by a given route can vary considerably over time. For example, water may literally pour from the skin in the form of sweat during strenuous exertion. In other cases, water may be lost in diarrhea during a GI tract infection.

Regulation of Body Water Gain

The volume of metabolic water formed in the body depends entirely on the level of aerobic cellular respiration, which reﬂects the demand for ATP in body cells. When more ATP is produced, more water is formed. Body water gain is regulated mainly by the volume of water intake, or how much ﬂuid you drink. An area in the hypothalamus known as the thirst center governs the urge to drink. When water loss is greater than water gain, dehydration— a decrease in volume and an increase in osmolarity of body fluids—stimulates thirst. When body mass decreases by 2% due to ﬂuid loss, mild dehydration exists. A decrease in blood volume causes blood pressure to fall. This change stimulates the kidneys to release renin, which promotes the formation of angiotensin II. Increased nerve impulses from osmo receptors in the hypothalamus, triggered by increased blood osmolarity, and increased angiotensin II in the blood both stimulate the thirst center in the hypothalamus. Other signals that stimulate thirst come from (1) neurons in the mouth that detect dryness due to a decreased ﬂow of saliva and (2) baroreceptors that detect lowered blood pressure in the heart and blood vessels. As a result, the sensation of thirst increases, which usually leads to increased ﬂuid intake (if ﬂuids are available) and restoration of normal ﬂuid volume. Overall, ﬂuid gain balances ﬂuid loss. Sometimes, however, the sensation of thirst does not occur quickly enough or access to ﬂuids is restricted, and signiﬁcant dehydration ensues. This happens most often in elderly people, in infants, and in those who are in a confused mental state. When heavy sweating or ﬂuid loss from diarrhea or vomiting occurs, it is wise to start replacing body ﬂuids by drinking ﬂuids even before the sensation of thirst occurs.

Regulation of Water and Solute Loss

Even though the loss of water and solutes through sweating and exhalation increases during exercise, elimination of excess body water or solutes occurs mainly by control of their loss in urine. The extent of urinary salt (NaCl) loss is the main factor that determines body ﬂuid volume. The reason for this is that “water follows solutes” in osmosis, and the two main solutes in extra cellular ﬂuid (and in urine) are sodium ions (Na) and chloride ions (Cl). In a similar way, the main factor that determines body ﬂuid osmolarity is the extent of urinary water loss. Because our daily diet contains a highly variable amount of NaCl, urinary excretion of Na and Cl must also vary to maintain homeostasis. Hormonal changes regulate the urinary loss of these ions, which in turn affects blood volume. The sequence of changes that occur after a salty meal. The increased intake of NaCl produces an increase in plasma levels of Na and Cl (the major contributors to osmolarity of extracellular ﬂuid). As a result, the osmolarity of interstitial ﬂuid increases, which causes movement of water from intracellular ﬂuid into interstitial ﬂuid and then into plasma. Such water movement increases blood volume. The three most important hormones that regulate the extent of renal Na and Cl reabsorption (and thus how much is lost in the urine) are angiotensin II, aldosterone, and atrial natriuretic peptide (ANP). When your body is dehydrated, angiotensin II and aldosterone promote urinary reabsorption of Na and Cl (and water by osmosis with the electrolytes), conserving the volume of body ﬂuids by reducing urinary loss. An increase in blood volume, as might occur after you ﬁnish one or more supersized drinks, stretches the atria of the heart and promotes release of atrial natriuretic peptide. ANP promotes natriuresis, elevated urinary excretion of Na (and Cl) followed by water excretion, which decreases blood volume. An increase in blood volume also slows release of renin from juxtaglomerular cells of the kidneys. When renin level declines, less angiotensin II is formed. Decline in angiotensin II from a moderate level to a low level increases glomerular ﬁltration rate and reduces Na, Cl, and water reabsorption in the kidney tubules. In addition, less angiotensin II leads to lower levels of aldosterone, which causes reabsorption of ﬁltered Na and Cl to slow in the renal collecting ducts. More ﬁltered Na and Cl thus remain in the tubular ﬂuid to be excreted in the urine. The osmotic consequence of excreting more Na and Cl is loss of more water in urine, which decreases blood volume and blood pressure. The major hormone that regulates water loss is antidiuretic hormone (ADH). This hormone, also known as vasopressin, is produced by neurosecretory cells that extend from the hypothalamus to the posterior pituitary. In addition to stimulating the thirst mechanism, an increase in the osmolarity of body ﬂuids stimulates release of ADH. ADH promotes the insertion of water-channel proteins (aquaporin-2) into the apical membranes of principal cells in the collecting ducts of the kidneys. As a result, the permeability of these cells to water increases. Water molecules move by osmosis from the renal tubular ﬂuid into the cells and then from the cells into the bloodstream. The result is production of a small volume of very concentrated urine. Intake of water in response to the thirst mechanism decreases the osmolarity of blood and interstitial ﬂuid. Within minutes, ADH secretion shuts down, and soon its blood level is close to zero. When the principal cells are not stimulated by ADH, aquaporin-2 molecules are removed from the apical membrane by endocytosis. As the number of water channels decreases, the water permeability of the principal cells’ apical membrane falls, and more water is lost in the urine. Under some conditions, factors other than blood osmolarity inﬂuence ADH secretion. A large decrease in blood volume, which is detected by baroreceptors (sensory neurons that respond to stretching) in the left atrium and in blood vessel walls, also stimulates ADH release. In severe dehydration, glomerular ﬁltration rate decreases because blood pressure falls, so that less water is lost in the urine. Conversely, the intake of too much water increases blood pressure, causing the rate of glomerular ﬁltration to rise, and more water to be lost in the urine. Hyperventilation (abnormally fast and deep breathing) can increase ﬂuid loss through the exhalation of more water vapor. Vomiting and diarrhea result in ﬂuid loss from the GI tract. Finally, fever, heavy sweating, and destruction of extensive areas of the skin from burns can cause excessive water loss through the skin. In all of these conditions, an increase in ADH secretion will help conserve body ﬂuids.

Movement of Water between Body Fluid Compartments

Normally, cells neither shrink nor swell because intracellular and interstitial ﬂuids have the same osmolarity. Changes in the osmolarity of interstitial ﬂuid, however, cause ﬂuid imbalances. An increase in the osmolarity of interstitial ﬂuid draws water out of cells, and they shrink slightly. A decrease in the osmolarity of interstitial ﬂuid, by contrast, causes cells to swell. Changes in osmolarity most often result from changes in the concentration of Na. A decrease in the osmolarity of interstitial ﬂuid, as may occur after drinking a large volume of water, inhibits secretion of ADH. Normally, the kidneys then excrete a large volume of di- lute urine, which restores the osmotic pressure of body ﬂuids to normal. As a result, body cells swell only slightly, and only for a brief period. But when a person steadily consumes water faster than the kidneys can excrete it (the maximum urine ﬂow rate is about 15 mL/min) or when renal function is poor, the result may be water intoxication, a state in which excessive body water causes cells to swell dangerously. If the body water and Na lost during blood loss or excessive sweating, vomiting, or diarrhea is replaced by drinking plain water, then body ﬂuids become more dilute. This dilution can cause the Na concentration of plasma and then of interstitial ﬂuid to fall below the normal range. When the Na concentration of interstitial ﬂuid decreases, its osmolarity also falls. The net result is osmosis of water from interstitial ﬂuid into the cytosol. Water entering the cells causes them to swell, producing convulsions, coma, and possibly death. To prevent this dire sequence of events in cases of severe electrolyte and water loss, solutions given for intra- venous or oral rehydration therapy (ORT) include a small amount of table salt (NaCl).

ELECTROLYTES IN BODY FLUIDS

The ions formed when electrolytes dissolve and dissociate serve four general functions in the body.

(1) Because they are largely conﬁned to particular ﬂuid compartments and are more numerous than nonelectrolytes, certain ions control the osmosis of water between ﬂuid compartments.

(2) Ions help maintain the acid–base balance required for normal cellular activities.

(3) Ions carry electrical current, which allows production of action potentials and graded potentials.

(4) Several ions serve as cofactors needed for optimal activity of enzymes.

Concentrations of Electrolytes in Body Fluids

To compare the charge carried by ions in different solutions, the concentration of ions is typically expressed in units of milli equivalents per liter (mEq/liter). These units give the concentration of cations or anions in a given volume of solution. One equivalent is the positive or negative charge equal to the amount of charge in one mole of H; a milli equivalent is one- thousandth of an equivalent. Recall that a mole of a substance is its molecular weight expressed in grams. For ions such as sodium (Na), potassium (K), and bicarbonate (HCO3), which have a single positive or negative charge, the number of mEq/liter is equal to the number of mmol/liter. For ions such as calcium (Ca2) or phosphate (HPO42), which have two positive or negative charges, the number of mEq/liter is twice the number of mmol/liter. Compares the concentrations of the main electrolytes and protein anions in blood plasma, interstitial ﬂuid, and intracellular ﬂuid. The chief difference between the two extracellular ﬂuids—blood plasma and interstitial ﬂuid—is that blood plasma contains many protein anions, in contrast to interstitial ﬂuid, which has very few. Because normal capillary membranes are virtually impermeable to proteins, only a few plasma proteins leak out of blood vessels into the interstitial fluid. This difference in protein concentration is largely responsible for the blood colloid osmotic pressure exerted by blood plasma. In other respects, the two ﬂuids are similar. The electrolyte content of intracellular ﬂuid differs consider- ably from that of extracellular ﬂuid. In extracellular ﬂuid, the most abundant cation is Na, and the most abundant anion is Cl. In intracellular ﬂuid, the most abundant cation is K, and the most abundant anions are proteins and phosphates (HPO42). By actively transporting Na out of cells and K into cells, sodium–potassium pumps (Na/K ATPase) play a major role in maintaining the high intracellular concentration of K and high extracellular concentration of Na.

Sodium ions (Na) are the most abundant ions in extracellular ﬂuid, accounting for 90% of the extracellular cations. The normal blood plasma Na concentration is 136–148 mEq/liter. As we have already seen, Na plays a pivotal role in ﬂuid and electrolyte balance because it accounts for almost half of the osmolarity of extracellular ﬂuid (142 of about 300 mOsm/liter). The ﬂow of Na through voltage-gated channels in the plasma membrane also is necessary for the generation and conduction of action potentials in neurons and muscle ﬁbers. The typical daily intake of Na in North America often far exceeds the body’s normal daily requirements, due largely to excess dietary salt the kidneys excrete excess Na, but they also can conserve it during periods of shortage. The Na level in the blood is controlled by aldosterone, antidiuretic hormone (ADH), and atrial natriuretic peptide (ANP). Aldosterone increases renal reabsorption of Na. When the blood plasma concentration of Na drops below 135 mEq/liter, a condition called hyponatremia, ADH release ceases. The lack of ADH in turn permits greater excretion of water in urine and restoration of the normal Na level in ECF. Atrial natriuretic peptide (ANP) increases Na excretion by the kidneys when Na level is above normal, a condition called hypernatremia.

Chloride ions (Cl) are the most prevalent anions in extracellular ﬂuid. The normal blood plasma Cl concentration is 95–105 mEq/liter. Cl moves relatively easily between the extracellular and intracellular compartments because most plasma membranes contain many Cl leakage channels and antiporters. For this reason, Cl can help balance the level of anions in different ﬂuid compartments. One example is the chloride shift that occurs between red blood cells and blood plasma as the blood level of carbon dioxide either increases or decreases. In this case, the antiporter exchange of Cl for HCO3 maintains the correct balance of anions between ECF and ICF. Chloride ions also are part of the hydrochloric acid secreted into gastric juice. ADH helps regulate Cl balance in body ﬂuids because it governs the extent of water loss in urine. Processes that increase or decrease renal reabsorption of sodium ions also affect reabsorption of chloride ions. (Recall that reabsorption of Na and Cl occurs by means of Na Cl symporters.)

Potassium ions (K) are the most abundant cations in intracellular ﬂuid (140 mEq/liter). K plays a key role in establishing the resting membrane potential and in the repolarization phase of action potentials in neurons and muscle ﬁbers; K also helps maintain normal intracellular ﬂuid volume. When K moves into or out of cells, it often is exchanged for H and thereby helps regulate the pH of body ﬂuids. The normal blood plasma K concentration is 3.5– 5.0 mEq/liter and is controlled mainly by aldosterone. When blood plasma K concentration is high, more aldosterone is secreted into the blood. Aldosterone then stimulates principal cells of the renal collecting ducts to secrete more K so excess K is lost in the urine. Conversely, when blood plasma K concentration is low, aldosterone secretion decreases and less K is excreted in urine. Because K is needed during the repolarization phase of action potentials, abnormal K levels can be lethal. For instance, hyperkalemia (above-normal concentration of K in blood) can cause death due to ventricular ﬁbrillation.

Bicarbonate ions (HCO3) are the second most prevalent extracellular anions. Normal blood plasma HCO3 concentration is 22–26 mEq/liter in systemic arterial blood and 23–27 mEq/ liter in systemic venous blood. HCO3 concentration increases as blood ﬂows through systemic capillaries because the carbon dioxide released by metabolically active cells combines with water to form carbonic acid; the carbonic acid then dissociates into H and HCO3. As blood ﬂows through pulmonary capillaries, however, the concentration of HCO3 decreases again as carbon dioxide is exhaled. Intracellular ﬂuid also contains a small amount of HCO3. As previously noted, the exchange of Cl for HCO3 helps maintain the correct balance of anions in extracellular ﬂuid and intracellular ﬂuid. The kidneys are the main regulators of blood HCO3 concentration. The intercalated cells of the renal tubule can either form HCO3 and release it into the blood when the blood level is low or excrete excess HCO3 in the urine when the level in blood is too high. Changes in the blood level of HCO3 are considered later in this chapter in the section on acid–base balance.

Calcium

Because such a large amount of calcium is stored in bone, it is the most abundant mineral in the body. About 98% of the calcium in adults is located in the skeleton and teeth, where it is combined with phosphates to form a crystal lattice of mineral salts. In body ﬂuids, calcium is mainly an extracellular cation (Ca2). The normal concentration of free or unattached Ca2 in blood plasma is 4.5–5.5 mEq/liter. About the same amount of Ca2 is attached to various plasma proteins. Besides contributing to the hardness of bones and teeth, Ca2 plays important roles in blood clotting, neurotransmitter release, maintenance of muscle tone, and excitability of nervous and muscle tissue. The most important regulator of Ca2 concentration in blood plasma is parathyroid hormone (PTH). A low level of Ca2 in blood plasma promotes release of more PTH, which stimulates osteoclasts in bone tissue to release calcium (and phosphate) from bone extracellular matrix. Thus, PTH increases bone resorption. Parathyroid hormone also enhances reabsorption of Ca2 from glomerular ﬁltrate through renal tubule cells and back into blood, and increases production of calcitriol (the form of vitamin D that acts as a hormone), which in turn increases Ca2 absorption from food in the gastrointestinal tract. Recall that calcitonin (CT) produced by the thyroid gland inhibits the activity of osteoclasts, accelerates Ca2 deposition into bones, and thus lowers blood Ca2 levels.

Phosphate

About 85% of the phosphate in adults is present as calcium phosphate salts, which are structural components of bone and teeth. The remaining 15% is ionized. Three phosphate ions (H2PO4, HPO42, and PO43) are important intracellular anions. At the normal pH of body ﬂuids, HPO42 is the most prevalent form. Phosphates contribute about 100 mEq/liter of anions to intracellular ﬂuid. HPO42 is an important buffer of H, both in body ﬂuids and in the urine. Although some are “free,” most phosphate ions are covalently bound to organic molecules such as lipids (phospholipids), proteins, carbohydrates, nucleic acids (DNA and RNA), and adenosine triphosphate (ATP). The normal blood plasma concentration of ionized phosphate is only 1.7–2.6 mEq/liter. The same two hormones that govern calcium homeostasis—parathyroid hormone (PTH) and calcitriol—also regulate the level of HPO42 in blood plasma. PTH stimulates resorption of bone extracellular matrix by osteoclasts, which releases both phosphate and calcium ions into the blood- stream. In the kidneys, however, PTH inhibits reabsorption of phosphate ions while stimulating reabsorption of calcium ions by renal tubular cells. Thus, PTH increases urinary excretion of phosphate and lowers blood phosphate level. Calcitriol promotes absorption of both phosphates and calcium from the gastrointestinal tract. Fibroblast growth factor 23 (FGF 23) is a polypeptide paracrine (local hormone) that also helps regulate blood plasma levels of HPO42. This hormone decreases HPO42 blood levels by increasing HPO42 excretion by the kidneys and decreasing absorption of HPO42 by the gastrointestinal tract.

Magnesium

In adults, about 54% of the total body magnesium is part of bone matrix as magnesium salts. The remaining 46% occurs as magnesium ions (Mg2) in intracellular ﬂuid (45%) and extra- cellular ﬂuid (1%). Mg2 is the second most common intracellular cation (35 mEq/liter). Functionally, Mg2 is a cofactor for certain enzymes needed for the metabolism of carbohydrates and proteins and for the sodium–potassium pump. Mg2 is essential for normal neuromuscular activity, synaptic transmission, and myocardial functioning. In addition, secretion of parathyroid hormone (PTH) depends on Mg2. Normal blood plasma Mg2 concentration is low, only 1.3–2.1 mEq/liter. Several factors regulate the blood plasma level of Mg2 by varying the rate at which it is excreted in the urine. The kidneys increase urinary excretion of Mg2 in response to hypercalcemia, hypomagnesaemia, increases in extracellular ﬂuid volume, decreases in parathyroid hormone, and acidosis. The opposite conditions decrease renal excretion of Mg2. People at risk for ﬂuid and electrolyte imbalances include those who depend on others for ﬂuid and food, such as infants, the elderly, and the hospitalized; individuals undergoing medical treatment that involves intravenous infusions, drainages or suctions, and urinary catheters; and people who receive diuretics, experience excessive ﬂuid losses and require increased ﬂuid intake, or experience ﬂuid retention and have ﬂuid restrictions. Finally, athletes and military personnel in extremely hot environments, postoperative individuals, severe burn or trauma cases, individuals with chronic diseases (congestive heart failure, diabetes, chronic obstructive lung disease, and cancer), people in conﬁnement, and individuals with altered levels of conscious- ness who may be unable to communicate needs or respond to thirst are also subject to ﬂuid and electrolyte imbalances.

ACID–BASE BALANCE

From our discussion thus far, it should be clear that various ions play different roles that help maintain homeostasis. A major homeostatic challenge is keeping the H concentration (pH) of body ﬂuids at an appropriate level. This task—the maintenance of acid–base balance—is of critical importance to normal cellular function. For example, the three-dimensional shape of all body proteins, which enable them to perform speciﬁc functions, is very sensitive to pH changes. When the diet contains a large amount of protein, as is typical in North America, cellular metabolism produces more acids than bases, which tends to acidify the blood. Before proceeding with this section of the chapter, you may wish to review the discussion of acids, bases, and pH on pages 41–43. In a healthy person, several mechanisms help maintain the pH of systemic arterial blood between 7.35 and 7.45. (A pH of 7.4 corresponds to an H concentration of 0.00004 mEq/liter 40 nEq /liter.) Because metabolic reactions often produce a huge excess of H, the lack of any mechanism for the disposal of H would cause H level in body ﬂuids to rise quickly to a lethal level. Homeostasis of H concentration within a narrow range is thus essential to survival. The removal of H from body ﬂuids and its subsequent elimination from the body depend on the following three major mechanisms:

1. Buffer systems. Buffers act quickly to temporarily bind H, removing the highly reactive, excess H from solution. Buffers thus raise pH of body ﬂuids but do not remove H from the body.

2. Exhalation of carbon dioxide. By increasing the rate and depth of breathing, more carbon dioxide can be exhaled. Within minutes this reduces the level of carbonic acid in blood, which raises the blood pH (reduces blood H level).

3. Kidney excretion of H. The slowest mechanism, but the only way to eliminate acids other than carbonic acid, is through their excretion in urine.

The Actions of Buffer Systems

Most buffer systems in the body consist of a weak acid and the salt of that acid, which functions as a weak base. Buffers prevent rapid, drastic changes in the pH of body ﬂuids by converting strong acids and bases into weak acids and weak bases within fractions of a second. Strong acids lower pH more than weak acids because strong acids release H more readily and thus contribute more free hydrogen ions. Similarly, strong bases raise pH more than weak ones. The principal buffer systems of the body ﬂuids are the protein buffer system, the carbonic acid–bicarbonate buffer system, and the phosphate buffer system.

Protein Buffer System

The protein buffer system is the most abundant buffer in intra- cellular ﬂuid and blood plasma. For example, the protein hemoglobin is an especially good buffer within red blood cells, and albumin is the main protein buffer in blood plasma. Proteins are composed of amino acids, organic molecules that contain at least one carboxyl group (HCOOH) and at least one amino group (NH2); these groups are the functional components of the protein buffer system. The free carboxyl group at one end of a protein acts like an acid by releasing H when pH rises; it dissociates as follows: a.a COOH group into coo- and h+…..

The H is then able to react with any excess OH in the solution to form water. The free amino group at the other end of a protein can act as a base by combining with H when pH falls, as follows:

Accept of h+ by nh2 to become nh3+…….

So proteins can buffer both acids and bases. In addition to the terminal carboxyl and amino groups, side chains that can buffer H are present on seven of the 20 amino acids.

As we have already noted, the protein hemoglobin is an important buffer of H in red blood cells (see Figure 23.23 on page 904). As blood ﬂows through the systemic capillaries, carbon dioxide (CO2) passes from tissue cells into red blood cells, where it combines with water (H2O) to form carbonic acid (H2CO3). Once formed, H2CO3 dissociates into H and HCO3. At the same time that CO2 is entering red blood cells, oxyhemoglobin (Hb.O2) is giving up its oxygen to tissue cells. Reduced hemoglobin (deoxyhemoglobin) picks up most of the H. For this reason, reduced hemoglobin usually is written as Hb.H. The following reactions summarize these relations:

H2O + CO2 H2CO3 Water+ Carbon dioxide Carbonic acid

(Entering RBCs) H2CO3 H + HCO3 Carbonic acid Hydrogen ion Bicarbonate ion

Hb–O2 + H Hb–H + O2 Oxyhemoglobin + Hydrogen ion Reduced hemoglobin + Oxygen (released to tissue cells)

Carbonic Acid–Bicarbonate Buffer System

The carbonic acid–bicarbonate buffer system is based on the bicarbonate ion (HCO3), which can act as a weak base, and carbonic acid (H2CO3), which can act as a weak acid. As you have already learned, HCO3 is a signiﬁcant anion in both intracellular and extracellular ﬂuids (see Figure 27.6). Because the kidneys also synthesize new HCO3 and reabsorb ﬁltered HCO3, this important buffer is not lost in the urine. If there is an excess of H, the HCO3, can function as a weak base and remove the excess H as follows:

H + HCO3 H2CO3 Hydrogen ion + Bicarbonate ion (weak base) Carbonic acid

Then, H2CO3 dissociates into water and carbon dioxide, and the CO2 is exhaled from the lungs. Conversely, if there is a shortage of H, the H2CO3 can function as a weak acid and provide H as follows:

H2CO3 H + HCO3 Carbonic acid (weak acid) Hydrogen ion + Bicarbonate ion

At a pH of 7.4, HCO3 concentration is about 24 mEq/liter and H2CO3 concentration is about 1.2 mmol/liter, so bicarbonate ions outnumber carbonic acid molecules by 20 to 1. Because CO2 and H2O combine to form H2CO3, this buffer system cannot protect against pH changes due to respiratory problems in which there is an excess or shortage of CO2.

Phosphate Buffer System

The phosphate buffer system acts via a mechanism similar to the one for the carbonic acid–bicarbonate buffer system. The components of the phosphate buffer system are the ions dihydrogen phosphate (H2PO4) and monohydrogen phosphate (HPO42). Recall that phosphates are major anions in intracellular ﬂuid and minor ones in extracellular ﬂuids (Figure 27.6). The dihydrogen phosphate ion acts as a weak acid and is capable of buffering strong bases such as OH, as follows:

OH + H2PO4 H2O + HPO42

Hydroxide ion (strong base) + Dihydrogen phosphate (weak acid) Water + Monohydrogen phosphate (weak base)

The monohydrogen phosphate ion is capable of buffering the H released by a strong acid such as hydrochloric acid (HCl) by acting as a weak base:

H + HPO42 H2PO4 Hydrogen ion(strong acid) + Monohydrogen phosphate (weak base) gives Dihydrogen phosphate (weak acid)

Because the concentration of phosphates is highest in intra- cellular ﬂuid, the phosphate buffer system is an important regulator of pH in the cytosol. It also acts to a smaller degree in extracellular ﬂuids and buffers acids in urine. H2PO42 is formed when excess H in the kidney tubule ﬂuid combines with HPO42. The H that becomes part of the H2PO4 passes into the urine. This reaction is one way the kidneys help maintain blood pH by excreting H in the urine.

Exhalation of Carbon Dioxide

The simple act of breathing also plays an important role in maintaining the pH of body ﬂuids. An increase in the carbon dioxide (CO2) concentration in body ﬂuids increases H concentration and thus lowers the pH (makes body ﬂuids more acidic). Because H2CO3 can be eliminated by exhaling CO2, it is called a volatile acid. Conversely, a decrease in the CO2 concentration of body ﬂuids raises the pH (makes body ﬂuids more alkaline). This chemical interaction is illustrated by the following reversible reactions:

CO2 + H2O H 2CO3 H + HCO3 Carbon dioxide Water Carbonic acid Hydrogen & Bicarbonate ion

Changes in the rate and depth of breathing can alter the pH of body ﬂuids within a couple of minutes. With increased ventilation, more CO2 is exhaled. When CO2 levels decrease, the reaction is driven to the left (blue arrows), H concentration falls, and blood pH increases. Doubling the ventilation increases pH by about 0.23 units, from 7.4 to 7.63. If ventilation is slower than normal, less carbon dioxide is exhaled. When CO2 levels increase, the reaction is driven to the right (red arrows), the H concentration increases, and blood pH decreases. Reducing ventilation to one-quarter of normal lowers the pH by 0.4 units, from 7.4 to 7.0. These examples show the powerful effect of alterations in breathing on the pH of body ﬂuids. The pH of body ﬂuids and the rate and depth of breathing interact via a negative feedback loop (Figure 27.7). When the blood acidity increases, the decrease in pH (increase in concentration of H is detected by central chemoreceptors in the medulla oblongata and peripheral chemoreceptors in the aortic and carotid bodies, both of which stimulate the inspiratory area in the medulla oblongata. As a result, the diaphragm and other respiratory muscles contract more forcefully and frequently, so more CO2 is exhaled. As less H2CO3 forms and fewer H are present, blood pH increases. When the response brings blood pH (H concentration) back to normal, there is a return to acid–base homeostasis. The same negative feedback loop operates if the blood level of CO2 increases. Ventilation increases, which remove more CO2, reducing the H concentration and increasing the blood’s pH. By contrast, if the pH of the blood increases, the respiratory center is inhibited and the rate and depth of breathing decreases. A decrease in the CO2 concentration of the blood has the same effect. When breathing decreases, CO2 accumulates in the blood so its H concentration increases.

Kidney Excretion of H

Metabolic reactions produce nonvolatile acids such as sulfuric acid at a rate of about 1 mEq of H per day for every kilogram of body mass. The only way to eliminate this huge acid load is to excrete H in the urine. Given the magnitude of these contributions to acid–base balance, it’s not surprising that renal failure can quickly cause death. As you learned in Chapter 26, cells in both the proximal convoluted tubules (PCT) and the collecting ducts of the kidneys secrete hydrogen ions into the tubular ﬂuid. In the PCT, Na/H antiporters secrete H as they reabsorb Na. Even more important for regulation of pH of body ﬂuids, however, are the intercalated cells of the collecting duct. The apical membranes of some intercalated cells include proton pumps (HATPases) that secrete H into the tubular ﬂuid. Intercalated cells can secrete H against a concentration gradient so effectively that urine can be up to 1000 times (3 pH units) more acidic than blood. HCO3 produced by dissociation of H2CO3 inside intercalated cells crosses the basolateral membrane by means of Cl/HCO3 antiporters and then diffuses into peritubular capillaries. The HCO3 that enters the blood in this way is new (not ﬁltered). For this reason, blood leaving the kidney in the renal vein may have a higher HCO3 concentration than blood entering the kidney in the renal artery. Interestingly, a second type of intercalated cell has proton pumps in its basolateral membrane and Cl/HCO3 antiporters in its apical membrane. These intercalated cells secrete HCO3 and reabsorb H. Thus, the two types of intercalated cells help maintain the pH of body ﬂuids in two ways—by excreting excess H when pH of body ﬂuids is too low and by excreting excess HCO3 when pH is too high. Some H secreted into the tubular ﬂuid of the collecting duct is buffered, but not by HCO3, most of which has been ﬁltered and reabsorbed. Two other buffers combine with H in the collecting duct. The most plentiful buffer in the tubular ﬂuid of the collecting duct is HPO42 (monohydrogen phosphate ion). In addition, a small amount of NH3 (ammonia) also is present. H combines with HPO42 to form H2PO4 (di- hydrogen phosphate ion) and with NH3 to form NH4 (ammonium ion). Because these ions cannot diffuse back into tubule cells, they are excreted in the urine.

Acid–Base Imbalances: The normal pH range of systemic arterial blood is between 7.35 (45 nEq of H /liter) and 7.45 (35 nEq of H/liter). Acidosis (or acidemia) is a condition in which blood pH is below 7.35; alkalosis (or alkalemia) is a condition in which blood pH is higher than 7.45. The major physiological effect of acidosis is depression of the central nervous system through depression of synaptic transmission. If the systemic arterial blood pH falls below 7, depression of the nervous system is so severe that the individual becomes disoriented, then comatose, and may die. Patients with severe acidosis usually die while in a coma. A major physiological effect of alkalosis, by contrast, is over excitability in both the central nervous system and peripheral nerves. Neurons conduct impulses repetitively, even when not stimulated by normal stimuli; the results are nervousness, muscle spasms, and even convulsions and death. A change in blood pH that leads to acidosis or alkalosis may be countered by compensation, the physiological response to an acid–base imbalance that acts to normalize arterial blood pH. Compensation may be either complete, if pH indeed is brought within the normal range, or partial, if systemic arterial blood pH is still lower than 7.35 or higher than 7.45. If a person has altered blood pH due to metabolic causes, hyperventilation or hypoventilation can help bring blood pH back toward the normal range; this form of compensation, termed respiratory compensation, occurs within minutes and reaches its maximum within hours. If, however, a person has altered blood pH due to respiratory causes, then renal compensation—changes in secretion of H and reabsorption of HCO3 by the kidney tubules—can help reverse the change. Renal compensation may begin in minutes, but it takes days to reach maximum effectiveness. In the discussion that follows, note that both respiratory acidosis and respiratory alkalosis are disorders resulting from changes in the partial pressure of CO2 (PCO2) in systemic arterial blood (normal range is 35–45 mmHg). By contrast, both metabolic acidosis and metabolic alkalosis are disorders resulting from changes in HCO3 concentration (normal range is 22–26 mEq/liter in systemic arterial blood).

Respiratory Acidosis

The hallmark of respiratory acidosis is an abnormally high PCO2 in systemic arterial blood—above 45 mmHg. Inadequate exhalation of CO2 causes the blood pH to drop. Any condition that decreases the movement of CO2 from the blood to the alveoli of the lungs to the atmosphere causes a buildup of CO2, H2CO3, and H. Such conditions include emphysema, pulmonary edema, injury to the respiratory center of the medulla oblongata, airway obstruction, or disorders of the muscles involved in breathing. If the respiratory problem is not too severe, the kidneys can help raise the blood pH into the normal range by increasing excretion of H and reabsorption of HCO3 (renal compensation). The goal in treatment of respiratory acidosis is to increase the exhalation of CO2, as, for instance, by providing ventilation therapy. In addition, intravenous administration of HCO3 may be helpful.

Respiratory Alkalosis

In respiratory alkalosis, systemic arterial blood PCO2 falls below 35 mmHg. The cause of the drop in PCO2 and the resulting increase in pH is hyperventilation, which occurs in conditions that stimulate the inspiratory area in the brain stem. Such conditions include oxygen deﬁciency due to high altitude or pulmonary disease, cerebrovascular accident (stroke), or severe anxiety. Again, renal compensation may bring blood pH into the normal range if the kidneys are able to decrease excretion of H and reabsorption of HCO3. Treatment of respiratory alkalosis is aimed at increasing the level of CO2 in the body. One simple treatment is to have the person inhale and exhale into a paper bag for a short period; as a result, the person inhales air containing a higher-than-normal concentration of CO2.

Metabolic Acidosis

In metabolic acidosis, the systemic arterial blood HCO3 level drops below 22 mEq/liter. Such a decline in this important buffer causes the blood pH to decrease. Three situations may lower the blood level of HCO3 (1) actual loss of HCO3, such as may occur with severe diarrhea or renal dysfunction; (2) accumulation of an acid other than carbonic acid, as may occur in ketosis; or (3) failure of the kidneys to excrete H, from metabolism of dietary proteins. If the problem is not too severe, hyperventilation can help bring blood pH into the normal range (respiratory compensation). Treatment of metabolic acidosis consists of administering intra- venous solutions of sodium bicarbonate and correcting the cause of the acidosis.

Metabolic Alkalosis

In metabolic alkalosis, the systemic arterial blood HCO3 concentration is above 26 mEq/liter. A non-respiratory loss of acid or excessive intake of alkaline drugs causes the blood pH to increase above 7.45. Excessive vomiting of gastric contents, which results in a substantial loss of hydrochloric acid, is probably the most frequent cause of metabolic alkalosis. Other causes include gastric suctioning, use of certain diuretics, endocrine disorders, excessive intake of alkaline drugs (antacids), and severe dehydration. Respiratory compensation through hypoventilation may bring blood pH into the normal range. Treatment of metabolic alkalosis consists of giving ﬂuid solutions to correct Cl, K, and other electrolyte deﬁciencies plus correcting the cause of alkalosis.

Composition of urine

Urine is approximately 95% water. The other components of normal urine are the solutes that are dissolved in the water component of the urine. These solutes can be divided into two categories according to their chemical structure (e.g. size and electrical charge).

Organic molecules are electrically neutral and can be relatively large (compared with the 'simpler' ions - below).
These include:

Urea - Urea is an organic (i.e. carbon-based) compound whose chemical formula is: CON₂H₄ or (NH₂)₂CO. It is also known as carbamide. Urea is derived from ammonia and produced by the deamination of amino acids. The amount of urea in urine is related to quantity of dietary protein.
Creatinine - Creatinine is a normal (healthy) constituent of blood. It is produced mainly as a result of the breakdown of creatine phosphate in muscle tissue. It is usually produced by the body at a fairly constant rate (which depends on the muscle mass of the body).
Uric acid - Uric acid is an organic (i.e. carbon-based) compound whose chemical formula is: C₅H₄N₄O₃.
Due to its insolubility, uric acid has a tendency to crystallize, and is a common part of kidney stones.

Other substances/molecules - Example of other substances that may be found in small amounts in normal urine include carbohydrates, enzymes, fatty acids, hormones, pigments, and mucins (a group of large, heavily glycosylated proteins found in the body).

Ions are atoms or groups of atoms that have either, lost some outer electrons, hence have a positive electric charge, or have gained some outer electrons (to the atom or group of atoms), and hence have a negative electric charge. Even in the cases of ions formed by groups of atoms (they are ions due to the few lost or gained electrons), the groups are formed from only a small number of particles and therefore tend to be relatively small.
These include:

Individual elements:

Sodium (Na⁺): Amount in urine varies with diet and the amount of aldosterone (a steroid hormone) in the body.
Potassium (K⁺): Amount in urine varies with diet and the amount of aldosterone (a steroid hormone) in the body.
Chloride (Cl^-): Amount in urine varies with dietary intake (chloride is a part of common salt, NaCl).
Magnesium (Mg²⁺): Amount in urine varies with diet and the amount in the body. (Parathyroid hormone increases the reabsorption of magnesium by the body, which therefore decreases the quantity of magnesium in urine.)
Calcium (Ca²⁺): Amount in urine varies with diet and the amount of parathyroid hormone in the body. (Parathyroid hormone increases the reabsorption of calcium by the body, which therefore decreases the quantity of calcium in urine.)

Small groups formed from a few different elements:

Ammonium (NH₄⁺): The amount of ammonia produced by the kidneys, may vary according to the pH of the blood and tissues in the body.
Sulphates (SO₄^2-): Sulphates are derived from aa. The quantity of sulphates excreted in urine varies according to the quantity and type of protein in the person's diet.
Phosphates (H₂PO₄^-, HPO₄^2-, PO₄^3-) : Amount in urine varies with the amount of parathyroid hormone in the body - parathyroid hormone increases the quantity of phosphates in urine.

Nephrosis and nephritis

An inflammation of one or both kidneys is called nephritis. It is more common in childhood and adolescence. Nephritis most often affects the glomeruli, tufts of microscopic vessels that filter blood. Less commonly, nephritis affects the tubules that surround the glomeruli or the blood vessels inside the kidneys. Nephritis may be acute or chronic. People with the acute form usually recover. Nephrosis is a non-inflammatory disease of the kidney also affecting the glomeruli.

Chronic nephritis can lead to severe hypertension which can, in extreme cases, result in death from kidney or heart failure.

Nephrosis occurs when the glomeruli are damaged; instead of filtering only wastes and water from the blood, the glomeruli also filter out protein. Without sufficient protein, a person may develop edema (swelling) in the feet, ankles, and abdomen and around the eyes. Nephrosis can occur in people of all ages, but is more common in children.

Symptoms

· Loss of appetite

· Fatigue

· Stomached ache/ abdominal pain

· Dark urine

· Facial swelling

· Edema.

Causes

Acute nephritis: can be caused by drug allergies, especially analgesic such as aspirin and paracetomol, immunosuppressant drugs such as cyclosporine, anti-cancer drugs such as cisplatin and carboplatin, and lithium, which is used to treat bipolar disorder. Other causes include bacterial and viral infections, metabolic and toxic disorders, and hypocalcaemia.

Chronic nephritis: develops more slowly has many causes, including abnormal immune system reactions, bacterial kidney infection, drug hypersensitivity or exposure to a toxin. It can also be caused by radiation exposure, obstruction of the urinary tract, hypertension, sickle cell anemia and polycystic kidney disease (PKD), among other conditions.

Nephrosis: can be caused by kidney disease or it may be a complication of another disorder, particularly diabetes.

The risk factors include:

History of polycystic kidney disease.
Kidney disease or infection.
Drug allergies.
Hypertension.
Diabetes.
Sickle cell anemia.
Long-term dialysis.

Diagnosis

Common diagnostic tests for nephritis & Nephrosis include:-

Urinalysis to check kidney function.
Renal ultrasounds to check size and shape of the kidneys and to determine whether there are any blockages in the urinary tract.
Renal scan may also be done to measure blood flow thought the kidneys.
A biopsy may also be performed.

Treatment

The aim of all treatment is to reduce inflammation, limit damage to the kidney's, and support the body until kidney function is restored.

Medication – Nephritis caused by infection is treated with antibiotics. Both nephritis and nephrosis may be treated with anti-inflammatory drugs. Diuretics or special diets may be prescribed. Nephrosis can sometimes be suppressed using corticosteroids, including prednisone and cortisone.
Dialysis – If either condition progress to kidney failure, dialysis may be needed, which is a treatment that filters wastes from the blood.
Surgery – For kidney failure, a kidney transplant may be needed.

Tubular Function Tests

Tests of renal tubular function can define states of diuresis and antidiuretics and natriuresis and antinatriuresis. Under certain circumstances the tests can distinguish oliguria due to pre renal azotemia, which is reversed by restoration of hemodynamic status to normal, from that due to established acute renal failure, which persists despite restoration of normal renal blood flow. In the former, tubular conservation mechanisms are enhanced, and in the latter they are lost. However, in non oliguric renal dysfunction, which accounts for more than 50 percent of cases encountered clinically, the differences in tubular function are less distinct from prerenal azotemia. Diuretic therapy overcomes tubular conserving function. Thus, treatment with loop diuretics, osmotic diuretics, saline loading, or natriuretic vasodilators (low-dose dopamine, fenoldopam, prostaglandin E1, or ANP) may render tubular function tests uninterpretable.

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