The small intestine is continuous with the stomach at the pyloric sphincter and leads into the large intestine at the ileocaecal valve. It is about 6 meters (20 ft.) long and lies in the abdominal cavity surrounded by the large intestine.
It consists of three parts, which are continuous with each other.
1. The duodenum is the first 25 cm (10 inches) of the small intestine and curves in a shape around the head of the pancreas. About half way along the concave surface of the duodenum, the bile duct and the pancreatic duct open together at a small papillae or opening called the ampoule of vater, which is guarded by the sphincter of Oddi.
2. The jejunum is the middle part of the small intestine and is about 2 meters (61/2 ft.) long.
3. The ileum is the terminal part, is about 3 meters (10ft.) long and ends at ileocaecal valve.
The walls of the small intestine are composed of the same four layers as the alimentary canal described earlier, except a few modification in the mucous membrane:
1. As in the stomach the mucous membrane is arranged in circular folds, to increase the surface area for secretion and absorption.
2. It has a velvety appearance due to the presence of minute projections called vilii, each containing a lymph vessel called lacteal and blood vessels.
3. It contains glands, which secretes intestinal juice.
Functions of the Small Intestine
The primary functions of the small intestine are digestion and absorption of ingested food by the body
Digestion in the small Intestine
When chyme passes into the duodenum it is mixed with the pancreatic juice and bile. Pancreatic juice, is a strongly alkaline fluid, produced in the pancreas, and contains three enzymes acting on three different food stuffs:
1. Trypsinogen is converted to active trypsin by enterokinase, an enzyme of the intestinal juice. So, trypsin converts peptones into peptides.
2. Amylase, converts all digestible polysaccharides (starches) not affected by ptyalin into disaccharides (sugars).
3. Lipase converts fats to fatty acid and glycerol. Bile is the fluid secreted by the liver, contains no enzyme but rich in alkaline salts, which help in the action of lipase to emulsify fats.
The intestinal juice or success enteric us is secreted by the glands of the small intestine is alkaline is nature and consists of the following enzymes.
1. Erepsin- converts peptides into amino acids.
2. Lipase- Completes the digestion of fats into fatty acid and glycerol.
3. Sucrose- converts cane sugar into simple sugars.
4. Lactose- converts lactose into simple sugars.
5. Maltose- converts maltose into simple sugars.
6. Enterokinase- changes inactive trypsinogen of pancreatic juice into active trypsin.
So, in the small intestine, the intestinal juice completes the digestion of all the foodstuffs which is ready for absorption.
Absorption of food stuffs
After the entire process of digestion gets completed, the end products of protein, fat and carbohydrate foods remains as amino acid, fatty acid and glycerol and as glucose respectively ready for absorption in the human body.
Absorption of food stuffs takes place in the small intestine through the villi. Villi are fingerlike projections, present in innumerable numbers in the mucous membrane of the small intestine. Each villus contains a large number of blood capillaries and lacteals. It has also tiny muscles that interact to facilitate absorption. The mucous membrane is also thrown in circular folds, to increase the surface area through which absorption takes place in the small intestine.
Glucose and amino acids are absorbed by the blood capillaries of the Villi, but fatty acids and glycerol are absorbed by the lacteals of the Villi, giving the lymph a milky appearance called Chyle.