Digestion is the process of converting complex large food substances into smaller components. Complex food substances such as carbohydrates cannot be utilized by the organisms as such, hence it should be broken down into smaller components.
The food passes through a continuous canal which begins at the buccal cavity and ends at the anus. This canal can be divided into 6 compartments:
(1) the buccal cavity
(2) foodpipe or oesophagus
(4) small intestine
(5) large intestine ending in the rectum
(6) the anus.
All these parts together form the alimentary canal otherwise known as digestive tract.
The digestive tract and the associated glands together constitute the digestive system. Now, let us know what happens to the food in different parts of the digestive tract.
Once the food is broken down into smaller particles, it will be entering the digestive system. Let's understand how food is entering into each compartment of digestive tract.
(1) Mouth and buccal cavity:
Food is taken into body through mouth. Mouth secretes a fluid called saliva which has salivary glands. Salivary glands are helpful in breaking down the starch into sugar. Mouth, tongue and teeth are present in buccal cavity. All these are helpful in the process of ingestion.
Sometimes when you eat in a hurry, talk or laugh while eating, you may get hiccups or a choking sensation. This happens when food particles enter the windpipe. The windpipe carries air from the nostrils to the lungs. It runs adjacent to the food pipe. But inside the throat, air and food share a common passage. Then how is food prevented from entering the windpipe? During the act of swallowing a flap-like valve closes the passage of the windpipe and guides the food into the food pipe. If, by chance, food particles enter the windpipe, we feel choked, get hiccups or cough to get the food out of wind pipe.
(2) The food pipe/oesophagus:
The swallowed food passes into the foodpipe or oesophagus. It is present between stomach and chest. Food is pushed down by movement of the wall of the foodpipe. Actually this movement takes place throughout the alimentary canal and pushes the food downwards.
Stomach is a bag-like structure where the food is further digested. The food is churned. It is the widest part of the alimentary canal. It receives food from the food pipe at one end and opens into the small intestine at the other. The inner lining of the stomach secretes mucous, hydrochloric acid and digestive juices. The mucous protects the lining of the stomach. The acid kills many bacteria that enter along with the food from the mouth. The digestive juices break down the proteins into simpler substances.
Complex to simple substance: http://ncertbooks.prashanthellina.com/class_7.Science.Science/index.html
Components of digestive system:https://commons.wikimedia.org/wiki/File:Blausen_0316_DigestiveSystem.png