Digestion is the process of converting complex or large food substances into smaller components. An organisms cannot utilize a complex food substance such as carbohydrates. So, it needs to 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 six compartments:
(1) the buccal cavity
(2) food pipe or oesophagus
(3) stomach
(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 enters into each compartment of the digestive tract.
(1) Mouth and buccal cavity:
Food is taken into the body through the mouth. Mouth secretes a fluid called saliva, which has salivary glands. Salivary glands helps to break down the starch into sugar. Mouth, tongue and teeth are present in the buccal cavity. All these are helpful in the process of ingestion.
A Teeth:
Sometimes when you eat in a hurry, talk or laugh, you may get hiccups or a choking sensation. It 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 any chance, food particles enter the windpipe, we feel choked or get hiccups or cough.
(2) The food pipe/oesophagus:
The swallowed food passes through the food pipe or oesophagus. The food is being pushed down by the movement of the wall of the food pipe. Actually, this movement takes place throughout the alimentary canal and pushes the food downwards.  
(3) Stomach:
Stomach is a bag-like structure where the food is further digested. The food is churned here. 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:
Components of digestive system: