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!

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 is 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.

What do you think happens to the speed of our breathing when we are resting?

When you rest, your cells are not working very hard, so they need less oxygen. They also produce less carbon dioxide. This means that you do not need to breathe as often as when you are 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.