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Digestion in small intestine

Digestion in the Small Intestine Although chemical digestion of some nutrients begins in the oral cavity or stomach, most enzymatic hydrolysis of the macromolecules from food occurs in the small intestine. The small intestine is the alimentary canal’s longest compartment—over 6 m (20 feet) long in humans! Its name refers to its small diameter, compared with that of the large intestine. The first 25 cm (10 inches) or so of the small intestine forms the duodenum. It is here that chyme from the stomach mixes with digestive juices from the pancreas, liver, and gallbladder, as well as from gland cells of the intestinal wall itself. As you will see in Concept 41.5, hormones released by the stomach and duodenum control the digestive secretions into the alimentary canal. Pancreatic Secretions The pancreas aids chemical digestion by producing an alkaline solution rich in bicarbonate as well as several enzymes. The bicarbonate neutralizes the acidity of chyme and acts as a buffer. Among the pancreatic enzymes are trypsin and chymotrypsin, proteases secreted into the duodenum in inactive forms. In a chain reaction similar to the activation of pepsin, they are activated when safely located in the lumen of the duodenum. Bile Production by the Liver Digestion of fats and other lipids begins in the small intestine and relies on the production of bile, a mixture of substances that is made in the liver. Bile contains bile salts, which act as emulsifiers (detergents) that aid in digestion and absorption of lipids. Bile is stored and concentrated in the gallbladder. Bile production is integral to one of the other vital functions of the liver: the destruction of red blood cells that are no longer fully functional. In producing bile, the liver incorporates some pigments that are by-products of red blood cell disassembly. These bile pigments are then eliminated from the body with the feces. In some liver or blood disorders, bile pigments accumulate in the skin, resulting in a characteristic yellowing called jaundice. Secretions of the Small Intestine The epithelial lining of the duodenum is the source of several digestive enzymes. Some are secreted into the lumen of the duodenum, whereas others are bound to the surface of epithelial cells. While enzymatic hydrolysis proceeds, peristalsis moves the mixture of chyme and digestive juices along the small intestine. Most digestion is completed in the duodenum. The remaining regions of the small intestine, called the jejunum and ileum, Absorption in the Small Intestine Most of this absorption occurs at the highly folded surface of the small intestine, as illustrated in . Large folds in the lining encircle the intestine and are studded with fingerlike projections called villi. In turn, each epithelial cell of a villus has on its apical surface many microscopic projections, or microvilli, that are exposed to the intestinal lumen. The many side-by-side microvilli give cells of the intestinal epithelium a brush-like appearance that is reflected in the name brush border. Together, the folds, villi, and microvilli of the small intestine have a surface area of 200–300 m2, roughly the size of a tennis court. This enormous surface area is an evolutionary adaptation that greatly increases the rate of nutrient absorption. Depending on the nutrient, transport across the epithelial cells can be passive or activ. The sugar fructose, for example, moves by facilitated diffusion down its concentration gradient from the lumen of the small intestine into the epithelial cells. From there, fructose exits the basal surface and is absorbed into microscopic blood vessels, or capillaries, at the core of each villus. Other nutrients, including amino acids, small peptides, vitamins, and most glucose molecules, are pumped against concentration gradients into the epithelial cells of the villus. The capillaries and veins that carry nutrient-rich blood away from the villi converge into the hepatic portal vein, a blood vessel that leads directly to the liver. From the liver, blood travels to the heart and then to other tissues and organs. This arrangement serves two major functions. First, it allows the liver to regulate the distribution of nutrients to the rest of the body. Because the liver can interconvert many organic molecules, blood that leaves the liver may have a very different nutrient balance than the blood that entered. Second, the arrangement allows the liver to remove toxic substances before the blood circulates broadly. The liver is the primary site for the detoxification of many organic molecules, including drugs, that are foreign to the body. #Digestion #DigestionAndAbsorptionInSmallIntestine