When we do activities like exercise and sport, we use our muscles. Muscles need oxygen to work.
We breathe oxygen into our lungs. The oxygen goes into our blood. The blood takes the oxygen into every part of the body. Our muscles use the oxygen. Our muscles make carbon dioxide. The blood takes the carbon dioxide to our lung. We breathe carbon dixide into the air.
When we do exercise, our muscles need a lot of oxygen. The heart work fast and it pumps the blood around the body. When we rest, the heart slows down.