20.7: Small Intestine
- Page ID
- 53822
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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.
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.