Small intestine is the longest part of the alimentary canal which is fitted into a compact space because of extensive coiling in human beings.
The length of the small intestine differs in various animals depending on the food they eat. Herbivores eating grass need a longer small intestine to allow the cellulose to be digested. Meat is easier to digest, hence carnivores like tigers have a shorter small intestine.
Digestion in small intestine : Small intestine is the longest part of the alimentary canal. It is the site of the complete digestion of carbohydrates, proteins and fat.
From the stomach, the food enters the small Intestine. Small intestine receives bile juice from liver and pancreatic juice from pancreas.
(1) Action of bile juice : The food coming from stomach is acidic and bile makes it alkaline for the pancreatic enzymes to act.
Bile salts break large fat globules present in food down into smaller globules. This process is called emulsification. This process increases the efficiency of enzyme action.
(2) Action of pancreatic juice : The pancreas secretes pancreatic juice which contains enzymes like trypsin for digesting proteins. pancreatic amylase for carbohydrates and lipase for breaking down emulsified fats.
(3) Action of intestinal juice : The wall of small intestine contain glands which secrete intestinal juice.
The enzymes present in it finally convert the proteins to amino acids, complex carbohydrates into glucose and fats into fatty acids and glycerol.
Absorption of digested food : Digested food is taken up by the walls of the intestine. The inner lining of the small intestine has numerous fingerlike projection called villi. Villi increase the surface area for absorption. The villi are richly supplied with blood vessels, which increase the efficiency of absorption of digested food.