We need energy to work and play. The more we work and play the more energy we need. Food gives us energy. Food is of use only when it is broken down to small pieces. This is done by the digestive system.
The food we eat consists of large particles, compared to the tiny cells that make up our body.
Digestion helps in breaking down the food into small particles called nutrients. These nutrients can be used by our body.  
The food that we eat must be chewed properly in the mouth. When we chew our food properly a digestive juice called saliva digests the food partly. This food then passes down the food pipe into the stomach.
The food is further digested in the stomach. The food then goes into the intestine. In the intestine the food gets mixed with other digestive juice. This completes digestion.
Digestive juice from the liver and pancreas also bring about digestion. The digested food is absorbed and is taken into the blood stream. The undigested food is removed from the body as solid waste in the form of faces through anus.
The circulatory system is made up of the heart, blood and the blood vessels. The heart pumps the blood and this blood is carried by the blood vessels to the various parts of the body.
The oxygen of the air and the digested food is carried by the blood to all the cells of our body.
Air is necessary for all living beings for survival. It is the oxygen in airs, which used by our body to burn food and provide the body with energy for its various activities. 
The inhaled air goes to the lungs through the windpipe. The lungs extract oxygen from the air and this oxygen is carried by blood to various parts of the body. The cells are in various parts of the body burn food using this oxygen, to generate energy for their various activities.
Waste substance like carbon dioxide and excess water are formed. Carbon dioxide and some water are breathed out. Excess water is removed from the body the body in the form of urine and sweat.

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