The small intestine is composed of three parts called, duodenum, jejunum, and ileum. Duodenum is about 20-25 cm long. The chyme passed from the stomach into the duodenum. The acidity of the chyme stimulates the release of secretion of the pancreas, liver, and duodenal cells.
It is a large gland. Its exocrine tissues secrete a juice called pancreatic juice. The pancreatic juice is transferred from the pancreas to the duodenum by the pancreatic duct. The pancreatic juice contains sodium bicarbonate (NaHCO3). It partly neutralizes the chyme coming from the stomach.
The pancreatic juice has many enzymes. These enzymes digest the different components of food like carbohydrates, fats, and proteins.
These enzymes are:
- Amylase: It is also called amylopsin. It digests starch into maltose.
- Lipase: It is a fat-digesting enzyme. It hydrolyses fats into fatty acids and glycerol.
- Trypsin: It is secreted in an inactive form called trypsinogen. The trypsinogen is activated by enterokinase. Enterokinase is an enzyme. It ¡s secreted by the lining of the duodenum. Trypsin splits proteins into peptones and polypeptides.
The liver secretes bile. The bile may be temporarily stored in the gall bladder. It is released into the duodenum through the bile duct. The bile is a green watery fluid. It contains no enzyme. Its green color is due to the bile pigments.
These bile pigments are formed by the breakdown of hemoglobin in the liver. The bile also contains bile salts. The bile salts act on fats and emulsify them. Emulsification means the breakdown of fats into small globules. So fats are easily digested by water-soluble lipase.
Sometimes, bile pigments are prevented from leaving the digestive tract. So they may accumulate in the blood and cause a condition called jaundice.
The liver secretes cholesterol. Sometimes, this cholesterol precipitates in the gall bladder to produce gallstones. This may block the release of bile.
The liver converts the toxic substances ammonia into less toxic compound urea. Ammonia is produced as a waste product during amino acid metabolism. The urea is transported to the kidney and the kidney excretes it outside the body.
Jejunum and ileum
The jejunum is the second portion of the small intestine. It extends from the duodenum to the ileum. It is about 2.4 meters in length. So it is about two-fifth of the small intestine. The lower three-fifth of the small intestine forms the ileum.
The digestion of food is completed in the jejunum and ileum. The wall of the jejunum and ileum secretes intestinal juice. The intestinal juice has many enzymes for digestion. These enzymes act on food and form final products.
Absorption of Food
Nearly all the absorption of the products of digestion takes place in the ileum.
The internal surface of the ileum has many folds. These folds are numerous finger-like outgrowths called villi. These villi give a velvety appearance to the inner wall of the ileum. Each villus is composed of three parts:
- It is richly supplied with blood capillaries.
- It has a vessel called a lacteal. The lacteal is a part of the lymphatic system.
- It ¡s covered with a covering of epithelial cells.
An electron microscope shows that the epithelial cells of villi have numerous closely packed cylindrical processes called microvilli.
The total area for absorption is increased due to these enfolding villi and microvilli. The absorption of different compounds takes place by the following methods:
- Sugars and Amino Acids: The blood in the capillaries of villi absorb sugars and amino acids by diffusion or active transport.
- Fats: Some of the fatty acids and glycerol are also absorbed by the blood. However, a large number of fatty acids and glycerol enter the epithelial cells of villi. In these cells, fatty acids and glycerol recombine to form fats. These fats then enter into lacteals. Some proteins are also present in lacteals. These proteins combine with the fats molecules to form lipo-protein droplets.
These droplets are transferred into the blood vessels through the thoracic lymph duct. Later, these lipo-proteins are hydrolyzed by blood plasma enzymes. They then enter the body cells. These fats may use in respiration or stored as fat in the liver or in the muscles the under the skin.
Transport of food through the ileum
The intestinal contents are pushed along the alimentary canal by normal peristaltic activity. There is an ileocolic sphincter at the end of the ileum. This sphincter opens from time to time and allows a small amount of residue food to enter into the large intestine.
Many humans lack the enzymes for the digestion of lactose in milk. So they develop intestinal diarrhea from consuming milk products. The epithelia cells of villi are constantly shed into the intestine. There is rapid cell division in crypts. These cells replace the cells.