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Sunday, 26 August 2018

Role of small intestine in Digestion




Role of small intestine in Digestion


Small intestine is the main site of digestion in which most ingested food of mammals is digested and absorbed in the small intestine.

Size of small intestine
The size human small intestine is about 4 cm in diameter and 7 to 8 m in length  It is intermediate in length between the small intestines of typical carnivores and  small intestine of herbivores of similar size, and it reflects the human’s omnivorous eating habits.
 The length of the small intestine directly relates to the total surface area available for absorbing nutrients, as determined by the many circular folds and minute projections of the inner gut surface (figure a). On the circular folds, thousands of fingerlike projections called villi. Villi  project from each square centimeter of mucosa (figure b).

Columnar epithelial cells, contain numerous microvilli, cover both the circular folds and villi (figure
c). These small projections are so dense that the inner wall of the human small intestine has a total surface area of approximately
300 m2
the size of a tennis court.


Part of small intestine
Duodenum
The first part of the small intestine, called the duodenum, functions primarily in digestion. The duodenum consist of many digestive enzymes that intestinal glands in the duodenal mucosa secrete. The pancreas secretes other enzymes. In the duodenum, digestion of carbohydrates and proteins is completed, and most lipids are digested.

Jejunum or Ileum
 The next part is the jejunum, and the last part is the ileum. Both function in nutrient absorption. The jejunum and ileum absorb the end products of digestion such as, amino acids, simple sugars, fatty acids, glycerol, nucleotides, water.
Much of this absorption involves active transport and the sodium dependent ATPase pump.

 Sugars and amino acids are absorbed into the capillaries of the villi, whereas free fatty acids enter the epithelial cells of the villi and recombine with glycerol to form triglycerides. The triglycerides are coated with proteins to form
small droplets called
chylomicrons, which enter the lacteals of the villi.

 From the lacteals, the chylomicrons move into the lymphatics and eventually into the bloodstream for transport throughout the body. Besides absorbing organic molecules, the small intestine absorbs water and dissolved mineral ions. The small intestine absorbs about 9 liters of water per day, and the large intestine absorbs the rest.

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