Explain the process of digestion of food in mouth, stomach and small intestine in human body.

Digestion in mouth:

1. Digestion of food begins in the mouth.
   
2. Saliva present in mouth contains a digestive enzyme, called salivary amylase, which breaks down starch into sugar.

Digestion in stomach:

1. Stomach stores and mixes the food received from the oesophagus with gastric juices.
   
2. The main components of gastric juice are hydrochloric acid, mucus and pepsinogen.
3. Hydrochloric acid dissolves bits of food and creates an acidic medium.
   
4. In this medium, pepsinogen is converted to pepsin, which is a protein-digesting enzyme.
   
5. Mucus protects the inner lining of the stomach from the action of HCl.

Digestion in small intestine:

1. Small intestine is the site for complete digestion of carbohydrates, proteins and fats.
   
2. It produces intestinal juice from the glands present in its wall.
   
3. The intestinal juice helps in further digestion of food.
   
4. Small intestine also obtains digestive juices from liver and pancreas that helps in mixing of food.
5. The liver produces bile juice that causes emulsification of fats and the pancreas produces pancreatic juice for digesting proteins and emulsified fats.
6. This digested food is finally absorbed through the intestinal walls.