Your body is made up of tiny cells, each of which has a special job that keeps you alive. Every cell in your body needs a special gas called oxygen, that allows the cell to get energy from the food you eat. Oxygen in the air is brought into your lungs. That's when your blood picks it up and brings it to your cells.

Another gas, called carbon dioxide is a waste product of your cells. It is very dangerous if it builds up in your body. Your blood carries the carbon dioxide from your cells to your lungs, to let you breathe out all the bad gas!

Humans need a continuous supply of oxygen for cellular respiration, and they must get rid of excess carbon dioxide, the poisonous waste product of this process. When we exercise, our cells are working harder, and they need more oxygen. They also produce more carbon dioxide. That's why you breathe faster when you exercise!

You have probably noticed that when you're exercising, your heart beats more quickly. That's so that the blood can carry oxygen to your cells faster!

Your heart and lungs work together to make sure every cell in your body gets enough oxygen.

When you rest, your cells aren't working very hard, so they need less oxygen. They also produce less carbon dioxide. This means that you don't need to breathe as often as when you're moving around. Remember that because the heart and lungs work as a team, when your lungs are not working hard, your heart is not working hard either.

Gas exchange supports the supports this cellular respiration by constantly supplying oxygen and removing carbon dioxide. The oxygen we need is derived from the Earth's atmosphere, which is 21% oxygen. This oxygen in the air is exchanged in the body by the respiratory surface. In humans the alveoli in the lungs serve as the surface for gas exchange.