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Figure 41.17 The stomach and its secretions Esophagus Cardiac orifice Stomach 5 µm Pyloric sphincter Interior surface of stomach. The interior surface of the stomach wall is highly folded and dotted with pits leading into tubular gastric glands. Small intestine Folds of epithelial tissue Epithelium 3 Pepsinogen 2 HCl Gastric gland. The gastric glands have three types of cells that secrete different components of the gastric juice: mucus cells, chief cells, and parietal cells. Pepsin (active enzyme) 1 2 HCl converts pepsinogen to pepsin. Mucus cells secrete mucus, which lubricates and protects the cells lining the stomach. 3 Pepsin then activates more pepsinogen, starting a chain reaction. Pepsin begins the chemical digestion of proteins. Chief cells secrete pepsinogen, an inactive form of the digestive enzyme pepsin. Parietal cell Parietal cells secrete hydrochloric acid (HCl). 1 Pepsinogen and HCI are secreted into the lumen of the stomach. Chief cell Figure 41.19 The duodenum Liver Bile Gallbladder Stomach Intestinal juice Acid chyme Pancreatic juice Pancreas Duodenum of small intestine Figure 41.20 Protease activation Pancreas Membrane-bound enteropeptidase Inactive trypsinogen Other inactive proteases Lumen of duodenum Trypsin Active proteases Figure 41.24 Digestion and absorption of fats Fat globule 1 Large fat globules are emulsified by bile salts in the duodenum. Bile salts 2 Digestion of fat by the pancreatic Fat droplets coated with bile salts Micelles made up of fatty acids, monoglycerides, and bile salts enzyme lipase yields free fatty acids and monoglycerides, which then form micelles. 3 Fatty acids and mono- glycerides leave micelles and enter epithelial cells by diffusion. 4 Chylomicrons containing fatty Epithelial cells of small intestine Lacteal substances are transported out of the epithelial cells and into lacteals, where they are carried away from the intestine by lymph. Figure 41.21 Flowchart of enzymatic digestion in the human digestive system Protein digestion Carbohydrate digestion Nucleic acid digestion Fat digestion Polysaccharides Disaccharides Oral cavity,(starch, glycogen) (sucrose, lactose) pharynx, Salivary amylase esophagus Smaller polysaccharides, maltose Proteins Pepsin Stomach Small polypeptides Polysaccharides Lumen of small intestine Pancreatic amylases Maltose and other disaccharides Polypeptides Pancreatic trypsin and chymotrypsin (These proteases cleave bonds adjacent to certain amino acids.) Smaller polypeptides DNA, RNA Pancreatic nucleases Disaccharidases Dipeptidases, carboxypeptidase, and aminopeptidase (These proteases split off one amino acid at a time, working from opposite ends of a polypeptide.) Monosaccharides Amino acids bile salts prevents small droplets from coalescing into larger globules, increasing exposure to lipase.) Pancreatic lipase Amino acids Small peptides Bile salts Fat droplets (A coating of Nucleotides Pancreatic carboxypeptidase Epithelium of small intestine (brush border) Fat globules (Insoluble in water, fats aggregate as globules.) Glycerol, fatty acids, glycerides Nucleotidases Nucleosides Nucleosidases and phosphatases Nitrogenous bases, sugars, phosphates Figure 41.23 The structure of the small intestine Vein carrying blood to hepatic portal vessel Microvilli (brush border) Blood capillaries Epithelial cells Muscle layers Villi Epithelial cells Large circular folds Lacteal Key Nutrient absorption Intestinal wall Villi Lymph vessel Figure 41.22 Hormonal control of digestion Enterogastrone secreted by the duodenum inhibits peristalsis and acid secretion by the stomach, thereby slowing digestion when acid chyme rich in fats enters the duodenum. Liver Enterogastrone Gallbladder Gastrin CCK Stomach Amino acids or fatty acids in the duodenum trigger the release of cholecystokinin (CCK), which stimulates the release of digestive enzymes from the pancreas and bile from the gallbladder. Pancreas Gastrin from the stomach recirculates via the bloodstream back to the stomach, where it stimulates the production of gastric juices. Secretin Duodenum CCK Key Stimulation Inhibition Secreted by the duodenum, secretin stimulates the pancreas to release sodium bicarbonate, which neutralizes acid chyme from the stomach.
