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20.7: Small Intestine

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    53822
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    Small Intestine

    The small intestine has three regions (in the order in which they occur in the alimentary canal): the duodenum, the jejunum, and the ileum. The duodenum is the shortest segment of the small intestine and receives chyme from the stomach where the gastric juices are neutralized and bile and digestive enzymes are injected into the small intestine at the hepatopancreatic ampulla. Secretions into the duodenum are not continuous and their release is controlled by the hepatopancreatic sphincter. The jejunum is longer than the duodenum and shorter than the ileum. The ileum carries cyme to the cecum, part of the large intestine (also known as the colon). Passage from the ileum to the cecum is controlled by the ileocecal valve.

    Diagram and cadaver images of small intestine

    Above: (Top left) The three regions of the small intestine: duodenum, jejunum, and ileum. (Top right and bottom) Cadaver images of small intestine, mesentery connecting the small intestine to the posterior body wall, and plicae circulares, aka circular folds, in the small intestine which increase surface area to maximize nutrient absorption.

    The small intestine continues chemical digestion and is the main organ of the alimentary canal responsible for nutrient absorption. The inner surface of the small intestine is structured to maximize surface area, since more surface area provides more opportunities for nutrient absorption. The tissue inside the small intestine is wrapped in large folds called plicae circulares (also known as circular folds) that increase the surface area of the small intestine. The plicae circulares are most prominent in the duodenum and progressively thin out and transition to the smoother wall of the distal ileum. Within each plica circulare are villi, finger-like folds in the tissue lining the inside of the small intestine that increase surface area further. Closely examining the cells lining the inside of small intestine, tiny finger-like extensions of the cell membranes of these cells can be seen. These are folds formed within the individual cells of the epithelium lining the inside of the small intestine and are called microvilli. These microvilli further increase surface area to maximize nutrient absorption in the small intestine.

    Histology of the small intestine. plicae circulares (circula folds), mesentery, submucosa, muscularis externa, serosa

    Above: Microscopic images of a cross section of small intestine (top) magnified by 10x and (bottom) magnified by 100x. Each plica has a core of submucosa that is overlain by all mucosal layers, including villi.

    Nutrients are absorbed through the mucosa and into blood vessels and lacteal capillaries (lymphatic vessels that absorb fats) in the tissue lining the inside of the small intestine. Blood vessels collecting nutrients from digesting materials in the small intestine enter the superior mesenteric vein, that drains into the hepatic portal vein to reach the liver. The liver adjust and balances the nutrient levels in the blood before the blood drains from the liver. Fats absorbed in the lacteal capillaries drain into lacteals, larger lymphatic vessels in the villi of the small intestine that collect digested fats. Lacteals carry the absorbed fats through the lymphatic system into the thoracic duct, which ultimately drains into the left subclavian vein where these absorbed fats enter the blood stream.

    villi and microvilli of the small intestine.

    Above: (Top) This innermost layer of the small intestine is the mucosa. The mucosa is composed of three layers: a simple columnar epithelium with microvilli and goblet cells; a lamina propria of loose connective tissue in which intestinal glands are located; and a thin muscularis mucosae of smooth muscle. Beneath the mucosa is the submucosa followed by the muscularis externa and the serosa. Tissue is magnified by 100x. (Bottom) Simple columnar epithelium with microvilli lines the small intestine, possessing both absorptive (enterocytes) and mucus-secreting goblet cells. Wandering lymphocytes and macrophages provide immune surveillance. Tissue is magnified by 1000x.


    This page titled 20.7: Small Intestine is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Rosanna Hartline.

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