The Relationship Between the Structure and Functions of the Digestive System and Digestive Tract Diseases

The human digestive system comprises organs such as the esophagus, stomach, intestines, liver, gallbladder and pancreas. Digestive system diseases refer to organic or functional diseases arising in these organs. Digestive tract diseases are extremely common; China has long had the saying "nine out of ten stomachs are diseased." The "stomach" in this folk proverb refers broadly; in fact the stomach trouble of the proverb includes diseases of multiple organs—the stomach and intestines, as well as the liver, gallbladder and pancreas. The burden caused by gastrointestinal and liver diseases accounts for one-tenth of all diseases. In China, gastric cancer and liver cancer are respectively the second and third leading causes of death among malignant tumors.

The main physiological function of the gastrointestinal tract is to take in, transport and digest food, absorb nutrients and excrete waste. After food enters the stomach from the mouth via the esophagus, it undergoes a series of digestive breakdown processes in the gastrointestinal tract, becoming small-molecule substances that are absorbed by the intestine, then processed by the liver into the various substances the body needs, which travel through the blood to the body's tissues to maintain normal life. The unabsorbed part of food or its ineffective residue forms feces and is excreted. If one or several links of the digestive tract go wrong, the body's ability to absorb nutrients and generate essential substances is impaired, affecting health.

The enzymatic reactions formed by the hydrolytic enzymes secreted by the pancreas and gastrointestinal glands, the bile secreted by the liver, and the intestinal flora are the main process by which food is broken down in the digestive tract and the essential substances of the body are generated. Absorption of digested nutrients requires structurally and functionally intact intestinal mucosal epithelial cells. Inadequate absorptive function of the intestinal mucosa and abnormal contraction of smooth muscle are the main pathological processes causing gastrointestinal disease. Congenital and acquired enzyme deficiencies, inflammatory and tumor lesions of the intestinal mucosa, bacterial overgrowth in the small intestine (blind-loop syndrome) which decomposes bile salts so they lose their fat-digesting action, and excessive resection of intestinal segments (short-bowel syndrome) which loses a large mucosal absorptive area, are the main causes of maldigestion and malabsorption.

The activity of the digestive tract is governed by the autonomic nervous system; sympathetic excitation can alter gastrointestinal motility, and damage to the vagus nerve can cause abnormal sensitivity of the gastroduodenum to distension. The hypothalamus is the subcortical center of the autonomic nervous system and an important link connecting the large intestine with lower centers. Hence digestive tract diseases are closely tied to emotion; the common saying that the stomach and intestines are emotional organs is not without reason. The digestive tract also has an enteric nervous system (ENS) that can function independently of the central nervous system; this is called the "brain of the gut." The ENS can directly receive various signals from within the gastrointestinal lumen; the neurotransmitters it secretes upon activation are polypeptide molecules such as substance P, opioid peptides, somatostatin and vasoactive intestinal peptide. The ENS has many reflex pathways and is also regulated by the central nervous system (the brain-gut axis); it plays an important role in regulating gastrointestinal motility, secretion, blood flow, and water and electrolyte transport. The integrity of, and coordination among, the central nervous system, autonomic nervous system and ENS is important for regulating gastrointestinal motility.

Various psychological factors, especially long-term high tension, can disturb the normal activity of higher nerves, causing disorder of the brain-gut axis, producing visceral hypersensitivity and thereby gastrointestinal functional disturbance.

Gastrointestinal hormones (also called gut peptides—small-molecule active substances and peptides derived from gastrointestinal endocrine and nerve cells, whose function is to transmit neural information) are indispensable to maintaining the normal physiological function of the digestive tract. Physiological function is maintained only when gut hormones are coordinated with one another and with the various gastrointestinal cells, tissues and organs; once this balance is broken, disease can result. Imbalance of gastrointestinal neurosecretion may underlie some symptom syndromes, such as irritable bowel syndrome and functional dyspepsia. In addition, the gut immune system plays an important role in systemic autoimmune diseases and the development of immune tolerance.

Gut-associated lymphoid tissue is part of the common mucosa-associated lymphoid tissue; it can recognize antigens entering the gastrointestinal tract, distinguishing which antigens should be ignored and which should provoke an immune response. Because the digestive tract opens directly to the outside and receives the greatest variety of external substances, its mucosa is exposed to pathogens, carcinogens and toxic substances relatively often; when immune and other defenses are weakened, infection, inflammation and injury readily occur. The high incidence of digestive-system tumors may also be related to this. Gastric cancer, esophageal cancer, liver cancer, colon cancer and pancreatic cancer are all common malignant tumors, accounting for a large proportion of all systemic malignant tumors. The gastrointestinal tract and liver contain abundant mononuclear macrophages, forming the immune protective barrier of the digestive tract and protecting it from invasion by external pathogenic factors; when this function is impaired, corresponding diseases appear. A normal gastrointestinal microecological environment is of great importance for maintaining health, resisting external microorganisms and preventing disease.

The liver is a key organ for the anabolism of carbohydrates, proteins, lipids and vitamins, operating through various complex enzymatic reactions; once hepatocytes are damaged and stop working, or enzymes are deficient, disease can result. The liver is also the body's detoxifying organ; it takes up, conjugates, transports, secretes and excretes bilirubin, and a block at any link can cause jaundice. The liver is the site of bile production; obstruction of bile-acid synthesis, transport, secretion or excretion from any cause can cause cholestatic liver disease and fat-soluble vitamin deficiency. Drug metabolism in the liver occurs mainly through a series of microsomal drug enzymes on the smooth endoplasmic reticulum of hepatocytes, chiefly cytochrome P450. The liver plays an important role in drug pharmacokinetics; conversely, drugs and their metabolites can also cause liver injury, leading to drug-induced liver disease.