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GE 345: Week 5The GI Tract |
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| Intro | Mouth | Stomach | Sm Int | Colon | | GI/Alimentary Tract provides water, electrolytes, and nutrients to the body:
General DesignLong tube consisting of several muscular and mucosal layers, and an inner epithelial lining. Each muscle layer is a branching lattice of smooth muscle bundles. An action potential anywhere in the muscle mass generally travels in all directions, causing widespread contraction.
Nervous ControlIntrinsic control: enteric nervous system. May be controlled by sympathetic or parasympathetic systems.
Enteric neurons secrete neurotransmitters, many of whose functions are not yet known. Acetylcholine typically sitmulates GI activity. Norepinephrine inhibits. Blood FlowPart of the splanchnic circulation. Includes blood flow through the gut, spleen, pancreas and liver. All blood though gut, spleen and pancreas flows immediately to the liver through the portal vein, where bacteria and other particles are removed, and nonfat, water souble nubrients absorbed from the gut are stored and processed, before the blood returns to the general circulation. Blood flow is related to level of local activity. During nutrient absorption blood flow increases. Possibly controlled by the hormones which influence gut activity, kinins (kallidin and bradykinin) which are powerful vasodilators, and decreased oxygen concentration in the gut from increased metabolic rate. Parasympathetic stimulation increases blood flow to the GI tract as it increases glandular secretions. Thought to be more from secretions than parasympathetic stimulation. Sympathetic stimulation causes intense vasoconstriction. Flow returns to normal after a few minutes via an autoregulatory escape mechanism -- ischemia triggers local vasodilator mechanisms. Movements in the GI TractThe tract moves to either propel or mix the contents.
SecretionsDigestive enzymes and mucus (lubricates and protects tract) are secreted throughout the alimentary tract. Digestive secretions are formed in response to the presence of food in the tract. In some parts of the tract, these secretions can vary based on the type of food present. Most secretory glands lie in the walls of the tract, although a few are outside. Secretory substances are synthesized in the ER and Golgi complexes, and discharged into the cytoplasm in secretory vesicles, which remains stored until extrusion through the cell's surface. DigestionProteins, carbohydrates and fats are all digested in a process called hydrolysis, in which hydrogen and hydroxyl ions derived from water split the molecules with the assistance of enzymes. In the normal human diet, carbohydrate sources are mainly the disaccharides sucrose (cane sugar) and lactose (in milk), and polysaccharides, commonly known as starches, which are found in most foods. Starches are first hydrolyzed to the disaccharide maltose, which, along with lactose and sucrose, is hydrolyzed into the monosaccharides glucose, galactose, and fructose in the small intestine. Triglyceride, a glycerol nucleus with three fatty acids, is the most common fat in the diet. Common diets also include some phospholipids, cholesterol and cholesterol esters. Phospholipids and cholesterol esters contain fatty acid, and are considered fats themselves. Cholesterol is a sterol compound with no fatty acid, but does exhibit some physical and chimical characteristics of fats, is derived from fats, and is metabolized similarly to fats, so is considered a fat from a dietary standpoint. During fat digestion, fatty acids are separated from glycerol. Dietary proteins are derived from meats and vegetables, and are digested in the stomach and upper part of the small intestine and broken down into their constituent amino acids. |