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Summary of Chapter 2 – Digestion and Absorption Food enters the mouth and travels down the esophagus and through the lower esophageal sphincter to the stomach, then through the pyloric sphincter to the small intestine, on through the ileocecal valve to the large intestine, past the appendix to the rectum, ending at the anus. The wavelike contractions of peristalsis and the periodic squeezing of segmentation keep things moving at a reasonable pace. To digest food, the salivary glands, stomach, pancreas, liver (via the gallbladder), and small intestine deliver fluids and digestive enzymes. The many folds and villi of the small intestine dramatically increase its surface area, facilitating nutrient absorption. Nutrients pass through the cells of the villi and enter either the blood (if they are water soluble or small fat fragments) or the lymph (if they are fat soluble). Nutrients leaving the digestive system via the blood are routed directly to the liver before being transported to the body’s cells. Those leaving via the lymphatic system eventually enter the vascular system but bypass the liver at first. Within the circulatory system, lipids travel bundled with proteins as lipoproteins. Different types of lipoproteins include chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Elevated blood concentrations of LDL are associated with a high risk of heart disease. Elevated HDL are associated with a low risk of heart disease. Fiber Carbohydrate Fat Protein Vitamins Minerals and Water The salivary glands secrete a watery fluid into the mouth to moisten the food. The salivary enzyme amylase begins digestion. Glands in the base of the tongue secrete a fat-digesting enzyme known as lingual lipase. Some hard fats begin to melt as they reach body temperature. Chewing and crushing moisten protein-rich foods and mix them with saliva to be swallowed. No action. The salivary glands add water to disperse and carry food. Stomach acid and enzymes start to digest salivary enzymes, halting starch digestion. To a small extent, stomach acid hydrolyzes maltose and sucrose. The acid-stable lingual lipase splits one bond of triglycerides to produce diglycerides and fatty acids. A gastric lipase accesses and hydrolyzes a very small amount of fat. Stomach acid uncoils protein strands and activates stomach enzymes. Intrinsic factor (see Chapter 8) attaches to vitamin B12. Stomach acid (HCl) acts on iron to reduce it, making it more absorbable. The stomach secretes enough watery fluid to turn a moist, chewed mass of solid food into liquid chyme. The pancreas produces enzymes and releases them through the pancreatic duct into the small intestine. Then enzymes on the surfaces of the small intestinal cells break disaccharides into monosaccharides, and the cells absorb them. Bile flows in from the liver and gallbladder (via the common bile duct). Pancreatic lipase flows in from the pancreas (via the pancreatic duct); broken down to monoglycerides, Broken down to amino acids and absorbed. Bile emulsifies fatsoluble vitamins and aids in their absorption with other fats. Watersoluble vitamins are absorbed. The small intestine, pancreas, and liver add enough fluid so that approximately 2 gallons are secreted into the intestine in a day. Many minerals are absorbed. Vitamin D aids in the absorption of calcium. Mouth The mechanical action of the mouth and teeth crushes and tears fiber in food and mixes it with saliva to moisten it for swallowing. Stomach Fiber is unchanged. Small intestine Fiber is unchanged. glycerol, fatty acids and absorbed. Large intestine (colon) Most fiber passes intact through the digestive tract to the large intestine. Here, bacterial enzymes digest some fiber. Fiber holds water; regulates bowel activity; and binds cholesterol and some minerals, carrying them out of the body as it is excreted with feces. Some fat and cholesterol, trapped in fiber, exit in feces. Bacteria produce vitamin K, which is absorbed. More minerals and most of the water are absorbed.