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Small intestine structure and muscular contractions

gafacom image result - Small intestine structure

The small intestine, consisting of the duodenum, jejunum and ileum, is the site where digestion is completed and nutrients are absorbed.  The small intestine is the  t portion of the GI tract between the pyloric sphincter of the stomach and the iliocecal valve that opens into the large intestine. It is positioned in the central and lower part of  the abdominal cavity and is supported except  for the first portion by the mesentary. The fan shaped mesentary  permits movement of the small intestine but prevents it from becoming kinked or twisted.

Enclosed within the mesentary are blood vessels, nerves and lymphatic vessels that supply the intestinal walls. The small intestine is about 12 feet long (3 m) in a living person but will measure twice that length in a cadaver due to relaxation of the muscular wall. It is called “small” due to its small diameter relative to the large intestine. The small intestine is the body’s major digestive organ and the main site of nutrient absorption. It contains digestive enzymes which aid in the final breakdown and absorption of  food.
Regions of the small intestine

1.    Duodenum is a fixed C shaped tube measuring 10 inches from the pyloric sphincter of the stomach to the duodenojejunal flexure.  It receives bile secretions from the liver and gall bladder  and pancreatic secretions from the pancreatic duct.

2.    Jejunum extends from the duodenum to the ileum, is approximately 3 feet long. It has a larger lumen and more internal folds than the ileum.

3.    Ileum (not to be confused with the ilium of the os coxae) makes up the remaining 67 feet of the small intestine. It empties into  the cecum of the large intestine through
the ileocecal valve. Lymph nodes called mesentary patches are abundant in the walls of the ileum.
Structural modifications of the Small Intestine

Digested food are absorbed across the lining of the intestinal mucosa. Absorption occurs mainly in the jejunum, although some occurs in the duodenum and ileum also. Absorption is aided  by structures that increase the surface area of the intestine.

1.    Plicae circulares are large macroscopic folds of mucosa
2.    Villi are finger-like macroscopic folds of the mucosa that  project into the lumen
3.    Microvilli are microscopic projections formed by the folding of epithelial cell membranes.

The villi have specialized goblet cells which secrete mucous. Additionally,  the center of the villi contain capillaries  and lymphatic vessels called lacteals. Proteins and carbohydrates are enter the capillaries and fatty acids  enter the lacteals.
Muscular contractions of the Small Intestine

Contractions of the longitudinal and circular muscles of the small intestine produce 3 distinct types of movement: rhythmic segementation, pendular movement and peristalsis.
1.    Rhythmic segmentations are local contractions of the circular muscle layer. They occur at the rate of  12-16 per minute in regions containing chyme. Rhythmic segmentations churn the chyme with digestive juices and bring it into contact with the mucosa. During these contractions,  the vigorous motion of the villi stirs  the chyme and facilitates absorption.

2.    Pendular movements primarily occur in the longitudinal  muscle layer. In this motion, a constrictive wave moves along a segment of the intestine and then reverses and moves in the opposite direction, moving the chyme back and forth. Pendular movements also mix the chyme but do not seen to have a particular frequency.

3.    Peristalsis is responsible for the propulsion of chyme through the small intestine. These wave-like contractions are weak and relatively short, occuring  at a frequency of about 15-18 per minute. Chyme requires about 3-10 hours to travel the length of the small intestine. Both muscle layers are involved in peristalsis.


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