The structure of the small intestine and the process of digestion taking place there
The small intestine:
The small intestine is a folded tube extending from the pyloric region of the stomach to the ileocaecal valve where it joins the large intestine. It is about 5 meters long and is made up of three parts.
Duodenum Jejunum and Ileum:
The Duodenum is the first part of the small intestine. It is about 25 to 30 cms in length and lies in a C shaped fashion around the head of the pancreas. About half way along, the concave surface of the Duodenum, the bile duct and the pancreatic duct enter the duodenal cavity together as a small papilla called the ampulla of vater.
The Jejunum forms 2/5ths of the small intestine while the Ileum constitutes the rest of it. There is very little distinction between Jejunum and Ileum as the former imperceptibly merges into the later. Both the jejunum and the ileum lay coiled in the central and lower abdominal cavity below the liver and the stomach.
Structure of the Small Intestine:
The small intestine performs two functions. Completion of the process of chemical digestion and absorption of digested food. It also has the function of moving the undigested food into the large intestine.
The process of digestion of food in the small intestine: The presence of the acid chyme (food from the stomach) in the duodenum causes the mucosa to secrete mucous and intestinal juice containing the enzyme enters- okinase. At the same time the mucosa also releases into the blood stream the hormones secretin and cholecystokinin which are carried to the pancreas. Secretin stimulates the pancreas to secrete the pancreatic juice.
The pancreatic juice consists of amylase (for digesting carbohydrates), trypsinogen and chymotrypsinogen (for digestion of proteins), and lipase (for digestion of fats). The pancreatic juice enters the duodenum.
The enzymes trypsinogen and chymotrypsinogen are converted into their active state trypsin and chymotrypsin in the presence of enterokinase. The cholysystokinine hormone also stimulates the gall bladder to empty the bile which is essential for digestion of fat. The bile is emptied into the duodenum via the bile duct.
The small intestine also secrets digestive enzymes from the epithelial cells covering the villi. The food that is digested is absorbed through the epithelial surface of the villi.