C. General Functional Features: The main functions of the digestive tract are the absorption of nutrients and water and the excretion of wastes and toxins.
1. Digestion. Enzymatic degradation of foods is a prerequisite for absorption; enzymes act mainly at food surfaces. Chewing exposes more surface area. Lip, cheek, and tongue muscles help position food between the teeth. Saliva dissolves water-soluble particles and contains enzymes that attack carbohydrates. Taste buds check for contaminants toxins, and nutrients. The tongue moves chewed food back into the oral pharynx and closes the epiglottis to protect the airway. The esophagus adds mucus to reduce friction, but mainly moves material to the stomach. Glands in the stomach wall add acid (HCI), a protease (pepsin), and mucus to the mixture (now called chyme). Smooth muscles in the stomach wall mix and pulverize the chyme and move it to the small intestine (duodenum), where pancreatic enzymes and bile are added. The enzymes hydrolyze nutrients to an absorbable form. The detergent action of bile disperses water-insoluble lipid into tiny droplets, increasing the surface area available to pancreatic lipases. The lining epithelial cells (enterocytes) of the small intestine have additional enzymes on their luminal surfaces to complete the hydrolysis of certain nutrients.
2. Absorption. This primary function of the digestive tract occurs mainly in the intestines: the small intestines absorb nutrients, and the large intestines absorb water. To maximize the absorptive surface, the small intestine's lining has multiple permanent folds including plicae circulares and villi. Intestines are lined by absorptive cells (enterocytes) whose apical microvilli further increase the surface area. These cells absorb and transfer amino acids and sugars to capillaries in the lamina propria, whose blood carries them to the liver for further process ing. Enterocytes assemble chylomicrons from absorbed lipids and transfer them to lymphatic capillaries (lacteals) in the lamina propria. From here, lipids reach the blood through the lymphatic vascular system.
3. Excretion. Metabolic wastes are excreted by the liver as bile and emptied into the duodenal lumen by the bile duct. Smooth muscles in the walls of the small intestine move undigested material and waste products to the large intestine (colon). Here, more mucus is added and most of the water is extracted. This concentrates and solidifies the intestinal contents, forming feces. This material is further dehydrated and stored in the rectum and finally expelled through the anal canal.
4. Endocrine function. Individual cells with characteristics of the diffuse neuroendocrine system are scattered among the epithelial cells lining the tract's mucosal glands and crypts. These enteroendocrine cells were formerly called argentaffin, ar gyrophilic, and enterochromaffin cells because of their affinity for stains containing silver and chromium. They secrete hormones and amines leg, serotonin, secretin, gastrin, somatostatin, cholecystokinin, glucagon) that regulate such local gastrointestinal functions as gut motility and the secretion of acid, enzymes, and hormones by other cell types. 5. Innervation. Distributed along and in the walls of the tract are the myenteric (Auerbach's) and submucosal (Meissner's) autonomic nerve plexuses. These include postsynaptic sympathetic fibers, pre- and postsynaptic parasympathetic fibers, parasympathetic ganglion cell bodies, and some visceral sensory fibers. After voluntary swallowing, these autonomic plexuses coordinate peristaisis-wavelike contractions of the muscularis externa that propel ingested material through the tract. They also control the independent activity of the muscularis mucosa, which maintains contact between the mucosa and the contents of the tract and help empty mucosal glands. These plexuses also modulate the secretory activity of certain DNES-like cells. In general, sympathetic action inhibits gut motility and parasympathetic action has the opposite effect. 6. Blood supply. Mesenteric branches of the abdominal aorta branch further in the mesenteries to form a series of arcades. Small arteries penetrate the tract walls to feed capillaries of the lamina propria. Amino acids, sugars, small fatty acids, and any toxins absorbed in the intestine thus travel directly to the liver to be metabolized, stored, or detoxified before reaching the general circulation. 7. Protection. The extensive absorptive surface of the digestive tract increases the risk of infection. The risk is reduced by immunoreactive cells--including IgA-secreting plasma cells--in the lamina propria and submucosa. Other defenses include lysozyme secreted by Paneth's cells, digestive enzymes in the lumen, the layer of mucus covering the epithelium, and the tight junctions between absorptive cells. Toxic substances that do reach the blood are carried directly to the liver for detoxification in the SER of the hepatocytes.