Chemical digestion in the small Intestine and Absorption of Food

Chemical digestion in the small Intestine

When acid chyme passes into the small intestine, it is mixed with pancreatic juice, bile and intestinal juice.

Bile is not primarily a digestive juice because it contains no enzyme but it helps in the digestion of fats. The bile salts emulsifies fats and helps the pancreatic lipase to act and digest it easily. The pancreatic juice contains 3 powerful enzymes. They are:

Pancreatic amylase - Converts carbohydrates - amylase into simple sugars like glucose, fructose and galactose.

Trypsin & Chymotrypsin - Converts peptones into Polypeptides. In the beginning trypsin is present in the form of inactive trypsinogen and Chymotrypsinogen. This trypsinogen is converted into active trypsin by the action of enterokinase which is secreted in the small intestine.

Pancreatic lipase - Converts fats into fattyacids and glycerol.

After pancreatic digestion, the food which is now called chyme proceeds further in the intestine. Here it comes in contact with Succus entericus which is a juice produced by the small intestine. Succus entericus contains three enzymes. They are:

1.   Erepsin        - It converts polypeptides into amino acids.

Nucleotidases  - Converts nucleotide, into nucleosides.

Nucleosidases - Converts nucleosides into pentose, purine

and pyrimidine.

It also contains three sugar splitting enzymes called lactase, maltase and sucrase converting the respective sugars into simple sugars, mostly glucose. It also has lipase which acts on fats and converts them into fatty acids and glycerol.

The final products of digestion of the carbohydrates is glucose, for the proteins are amino acids and fats are fatty acids, and glycerol.

Absorption of Food

Absorption is the process by which water, minerals, vitamins and end products of digestion are absorbed through the mucosa of alimentary canal (especially the small intestines) into blood stream either directly or via lymphatic vessels.

In the stomach there is little absorption. Water, alcohol, glucose, and simple salts are absorbed to a certain degree. The main absorption occurs in small intestine especially in the lower (ileum) part, the upper part of the small intestine is mainly associated with the process of digestion.

The mucous membrane of small intestine is covered with minute fingerlike projections known as villi. About 50 lakhs of villi are found in small intestine. Each villus contains an arteriole, a venule, a capillary network and a lacteal (lymphatic vessel). Nutrients that diffuse through the epithelial cells which cover the villus are able to pass through the capillary walls and the lacteal and enters the blood.

About 90% of all absorption takes place throughout the length of the small intestine. The other 10% occurs in the stomach and large intestine. Both monosaccharides and amino acids are absorbed by a positive pressure gradient between the intestinal content and the blood as well as by an active process involving enzymatic reactions and transported in the blood stream to the liver via the hepatic portal system. The excess amount of glucose is converted into glycogen and stored in the liver, when need arises glycogen is converted into glucose and is utilized by the body.

Fatty acids and glycerol do not enter the blood stream immediately. They are absorbed by the lacteals. So these lymph ducts are seen as white and milky in appearance after a meal of fat. The mineral salts and water soluble vitamin B Complex and C are absorbed via portal blood.

Functions of the Large Intestine

In the large intestine the absorption of water continues until semisolid consistency of faeces is achieved. Mineral salts, vitamins and some drugs are also absorbed into the blood capillaries from the large intestine. The large intestine is heavily colonized by certain types of bacteria which synthesize vitamin K and Folic acid. Unabsorbed carbohydrate undergoes bacterial fermentation and produces gas. These gases pass out of the bowel as flatus. The large intestine exhibits mass movements.

Defaecation

This is the process of emptying the bowels or the passage of faeces. When a mass movement forces the contents of the sigmoid colon into the rectum the nerve endings in the anal walls are stimulated resulting in defaecation.