Structure and functionThe gastrointestinal system comprises the hollow organs from mouth to anus that form the gastrointestinal tract, the pancreas, which mainly secretes digestive juices into the small intestine, and the liver and biliary system, which perform vital metabolic functions in addition to their contribution to digestion and absorption of nutrients.
The intestinal tractA hollow tubular structure into which nutrient-rich food is coerced, and from which wastes are expelled, is found in the most primitive multicellular organisms, from the hydra onwards. In humans, the tract is highly specialized throughout, both structurally and functionally. The mouth and teeth are the ﬁrst structures in this tract and are connected by a powerful muscular tube, the oesophagus, to the stomach. The stomach stores food after meals and is the site where major digestive processes commence. The small intestine is the main digestive and absorptive surface. The large intestine acts mainly as a reservoir for food waste and allows reabsorption of water from the mainly liquid material leaving the small intestine. It is not essential for life and, paradoxically, is affected by a number of common, serious diseases, such as inﬂammatory bowel disease and colorectal cancer.
The pancreasDigestive enzymes are produced in many parts of the gastrointestinal tract, including the mouth (salivary glands) and small intestine (enterocytes), although the exocrine pancreas is the most prodigious producer of digestive enzymes. Pancreatic failure causes malabsorption, which can be reversed by artiﬁcial enzyme supplements.
The liver and biliary systemWithout the liver, survival is measured in hours, and no artiﬁcial system has yet been devised to substitute for hepatic function. The liver is the largest solid organ in the body and its essential functions include regulation of protein, fat and carbohydrate metabolism, synthesis of plasma proteins, ketones and lipoproteins, and detoxiﬁcation and excretion. Via the hepatic portal circulation it receives and ﬁlters the entire venous drainage of the spleen, gastrointestinal tract and pancreas. Through the production of bile, it is also essential for digestion and absorption, particularly of dietary fats and fat-soluble vitamins.
Integrated functionThe gastrointestinal system is controlled by both intrinsic and extrinsic neuronal and endocrine mechanisms. Enteric nerves and endocrine cells are particularly important in coordinating motility, digestion and absorption, and in regulating feeding and overall nutrition, including the control of body weight. The gastrointestinal system presents a huge surface area that has to be protected against injury, particularly from microbial pathogens that are ingested with food and from the large population of commensal bacteria that populate the intestine. The mucosal immune system is critically important in regulating how the intestine responds to these challenges, providing protection and not reacting inappropriately to normal components of the diet.
Diseases and disordersNausea, vomiting, diarrhoea and constipation are common symptoms and their basic pathophysiology illustrates important aspects of gastrointestinal function.
Gastrointestinal symptoms are frequently not associated with any discernible pathological abnormality. These medically unexplained symptoms are often labelled functional disorders and, as our understanding of gastrointestinal physiology becomes more sophisticated, we may discover new explanations and treatments that are more effective.
Gastrointestinal system infections are common and are associated with signiﬁcant morbidity and mortality worldwide. They range from self-limiting food poisoning to life-threatening local and systemic infections. Even peptic ulceration is most frequently caused by infection, with the Helicobacter pylori bacterium.
For some major diseases, such as inﬂammatory bowel disease, the aetiological agent has not been identiﬁed, despite rapidly advancing genetic and molecular research. Conversely, coeliac disease, another serious and common gastrointestinal inﬂammatory disease, is caused by a well-characterized immune response to wheat-derived proteins.
Colon cancer is a major cause of cancer-related death and our molecular and cellular understanding of its pathogenesis, and the pathophysiology of other gastrointestinal, pancreatic and liver tumours, is rapidly increasing.
Liver damage is often caused by infections or drugs and may be acute or chronic. Acute liver disease can rapidly progress to liver failure, or can resolve, either spontaneously or with appropriate treatment. Chronic liver disease may cause cirrhosis, which is characterized by a variety of signs and symptoms and changes throughout the body, including the effects of hepatic portal venous hypertension.
The gastrointestinal system is essential to nutrition, and disordered nutrition is a major issue worldwide – both through undernutrition and starvation and through overnutrition, which causes obesity, possibly the single most important modern health problem in the afﬂuent world.
Diagnosis and treatmentClinical assessment, including a focused history and examination, is the foundation of diagnosis. In addition, the gastrointestinal system can be investigated by endoscopy, radiology and speciﬁc functional tests. Endoscopy and radiology may also be used therapeutically, and pharmacotherapy and surgery for gastrointestinal disorders exploit many unique features of the structure and function of the system.
OVERVIEW OF HUMAN DIGESTIVE SYSTEM