The small intestine (small in diameter compared to the large intestine) is divided into three sections, as shown in Figure :
- The duodenum, about 25 cm (10 inches) long, receives chyme from the stomach through the pyloric sphincter. Ducts that empty into the duodenum deliver pancreatic juice and bile from the pancreas and liver, respectively.
- The jejunum, about 2.5 m (8 feet) long, is the middle section of the small intestine.
- The ileum, about 3.6 m (12 feet) long, is the last section of the small intestine. It ends with the ileocecal valve (sphincter), which regulates the movement of chyme into the large intestine and prevents backward movement of material from the large intestine.
The functions of the small intestine include the following:
- Mechanical digestion. Segmentation mixes the chyme with enzymes from the small intestine and pancreas. Bile from the liver separates fat into smaller fat globules. Peristalsis moves the chyme through the small intestine.
- Chemical digestion. Enzymes from the small intestine and pancreas break down all four groups of molecules found in food (polysaccharides, proteins, fats, and nucleic acids) into their component molecules.
- Absorption. The small intestine is the primary location in the GI tract for absorption of nutrients.
The components of carbohydrates, proteins, nucleic acids, and water‐soluble vitamins are absorbed by facilitated diffusion or active transport. They are then passed to blood capillaries.
- Vitamin B 12: Vitamin B 12combines with intrinsic factor (produced in the stomach) and is absorbed by receptor‐mediated endocytosis. It is then passed to the blood capillaries.
- Lipids and fat‐soluble vitamins: Because fat‐soluble vitamins and the components of lipids are insoluble in water, they are packaged and delivered to cells within water‐soluble clusters of bile salts called micelles. They are then absorbed by simple diffusion and, once inside the cells, mix with cholesterol and protein to form chylomicrons. The chylomicrons are then passed to the lymphatic capillaries. When the lymph eventually empties into the blood, the chylomicrons are broken down by lipoprotein lipase, and the breakdown products, fatty acids and glycerol, pass through blood capillary walls to be absorbed by various cells.
- Water and electrolytes: About 90 percent of the water in chyme is absorbed, as well as various electrolytes (ions), including Na +, K +, Cl –, nitrates, calcium, and iron.
Modifications of the mucosa for its various specialized functions in the small intestine include the following:
- The plicae circulares (circular folds) are permanent ridges in the mucosa that encircle the inside of the small intestine. The ridges force the food to spiral forward. The spiral motion helps mix the chyme with the digestive juices.
- Villi (singular, villus) are fingerlike projections that cover the surface of the mucosa, giving it a velvety appearance. They increase the surface area over which absorption and digestion occur. The spaces between adjacent villi lead to deep cavities at the bases of the villi called intestinal crypts ( crypts of Lieberkühn). Glands that empty into the cavities are called intestinal glands, and the secretions are collectively called intestinal juice.
- Microvilli are microscopic extensions of the outer surface of the absorptive cells that line each villus. Because of their brushlike appearance (microscopically), the microvilli facing the lumen form the brush border of the small intestine. Like the villi; the microvilli increase the surface area over which digestion and absorption take place.
The villi of the mucosa have the following characteristics:
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An outer epithelial layer (facing the lumen) consists of the following cell types:
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Absorptive cells, the primary cell type of the epithelial layer, synthesize digestive enzymes called brush border enzymes that become embedded in the plasma membranes around the microvilli. Various nutrients in the chyme that move over the microvilli are broken down by these brush border enzymes and subsequently absorbed. Refer to Table, earlier in this chapter, for a list of these enzymes.
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- Goblet cells, located throughout the epithelial layer, secrete mucus that helps protect the epithelial layer from digestion.
- Enteroendocrine cells secrete hormones into blood vessels that penetrate each villus.
- Paneth cells, located in the epithelial layer facing the intestinal crypts, secrete lysozyme, an enzyme that destroys bacteria.
- An inner core of lamina propria (connective tissues) contains blood capillaries and small lymphatic capillaries called lacteals.
The submucosa that underlies the mucosa of the small intestine bears the following modifications:
- Brunner's (duodenal) glands, found only in the submucosa of the duodenum, secrete an alkaline mucus that neutralizes the gastric acid in the incoming chyme.
- Peyer's patches (aggregated lymphatic nodules), found mostly in the submucosa of the ileum, are clusters of lymphatic nodules that provide a defensive barrier against bacteria.