when we do activities like exercise and sports , we use our muscles. muscles need oxygen to work.
we breathe oxygen into our lungs. the oxygen goes into our blood takes the oxygen into every part of the body. Our muscle use the oxygen. our muscles make carbon dioxide. the blood takes the carbon dioxide to our lungs. we breathe carbon dioxide into the air.
when we do exercies, our muscles need a lot of oxygen. the heart works fast and pumps the blood around the body.
when we rest,
the heart slows down.