Respiration is a process which takes place in all living organisms. It is the process which releases energy from food. Respiration is like combustion; it needs oxygen if it is to work efficiently.

We breathe oxygen in through our mouths and down to the lungs. Other animals have different breathing systems; for example, a fish has gills, and plants take in gases through their leaves and roots. In our body the oxygen travels from the lungs all over the body in the blood stream.

The amount of oxygen an animal needs will be very large if the animal is very active. This is because if the animal is active it needs a lot of energy. To get that energy from its food it needs a lot of oxygen too. Two examples which show this are in the opposite column.