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All cells in the body require a supply of water, electrolytes and nutrients. These must come from food, where they are present as carbohydrates, fats, proteins, etc. These complex molecules are broken down by the process of digestion and then absorbed into the blood stream. Digestion requires the food to be acted upon by enzymes and cofactors whose activity is facilitated by mechanical agitation of the gut contents. Absorption is a relatively slow process that requires controlled movement of material through the absorptive section of the gut. Gastrointestinal innervation includes the sympathetic and parasympathetic nerves. The gastrointestinal tract has a nervous system of its own called the enteric nervous system. Gastrointestinal smooth muscle is unique in that it has three types of membrane potential: resting potential, basic electrical rhythm, and action potential. There are a large number of endocrine cells secreting gut hormones in the gastrointestinal tract. These gut hormones are frequently peptides, and regulate the movements and secretions of the alimentary tract.
The stomach serves to store food, which we ingest faster then it can be digested. It also subjects food to physical and chemical disruption by attacks of acid and enzymes. It delivers the resulting chyme to the intestines at a rate they can handle. These functions require a complex pattern of motility. There are three patterns of motility in the stomach smooth muscle: tonic contraction, receptive relaxation, and peristalsis. Gastric emptying is when the gastric contents are slowly pushed in the duodenum. It is accelerated by gastric contents and gastrin, while it is inhibited by the enterogastric reflex and duodenal hormones. Gastric juice contains hydrochloric acid (HCl), pepsins, mucus, and intrinsic factor. HCl activates pepsinogens and stimulates secretin secretion. Pepsins partially hydrolyze protein. Mucus and bicarbonate secreted by mucus cells create the "mucus - bicarbonate barrier" that protects the mucosa from the caustic action of gastric acid and pepsins. Intrinsic factor binds to and protects vitamin B12 for absorption in the terminal ileum. Gastric secretions are stimulated by acetylcholine, gastrin, and histamine. While HCl, fat, and hypertonic solutions inhibit gastric secretions in the gastrointestinal tract. Gastric juice secretion is divided into three phases: The cephalic phase, the gastric phase, and the intestinal phase. The control of the cephalic phase is neurohumoral. The secretion of the gastric phase is regulated via local reflexes of intrinsic nerves, vagal reflexes and gastrin release. The regulation of the intestinal phase is predominantly by humoral factors.
The small intestine is the major site of digestion and absorption of nutrients. Patterns of motility of the small intestine are contraction, segmentation, and peristalsis. There are three major digestive juices in the small intestine; they are pancreatic juice, bile, and intestinal juices. The pancreatic juice, an important digestive juice, contains the required enzymes that are essential for the digestion of carbohydrates, proteins, and fats. The digestion of fat depends entirely on pancreatic lipase. The digestive enzymes of protein must be activated before functioning. The bicarbonate solution is secreted by duct cells, while enzymes are secreted by acinar cells. The secretion of pancreatic juice is regulated mainly by cholecystokinin (CCK) and secretin. The bile secreted by liver plays a vital role in the digestion and absorption of lipids. Bile is composed of bile salts, cholesterol, and lecithin. The enterohepatic bile circulation is regulated by neural and humoral factors. Among the factors that enhance bile secretion are secretin, CCK, gastrin, bile salt, and food rich in protein.
The processes of digestion yield a variety of small ions and molecules that must be absorbed, together with the large quantities of water and electrolytes. Most absorption takes place in the small intestine, with some further absorption of water and ions in the large intestine. The small intestine absorbs water, electrolytes such as Na+, K+, Cl-, sugars such as glucose, galactose and fructose, amino acids and dipeptides, vitamins and mineral and fats. Absorption from the small intestine is by diffusion, facilitated diffusion and active transport, activities enhanced by the structure of the mucosa. Carbohydrates, proteins are absorbed actively through intestinal epithelial cells along with Na+ after being digested into simple sugar, dipeptides, tripeptides and amino acids respectively. Among the fat digestion products, glycerin is absorbed along with simple sugar. Others are absorbed with the help of bile salts and transported mainly via the lymphatic system. Calcium is absorbed by diffusion into the intestinal epithelium and active transport across the basolateral membrane, all steps in the processes stimulated by 1,25-(OH)2-vitamin D3. Only ferrous iron is absorbed by a carrier protein.
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