Why does exercising causes the change in our body ( breathing/heart rate)

Explanation:

The biggest change in the body caused by regular exercise is to muscle tissue. ... At the cellular level, exercise triggers the release of substances that dilate the blood vessels in the muscle, letting in more oxygen, says Prof Bradford. These are short-term effects, but deeper changes come with regular exercise.

Exercising causes changes in our body because we're working our muscles so there's more going on with us compared to when we're just laying down. We need more oxygen to go to our muscles so we breathe harder and more rapidly. Our heart also beats faster because it needs to pump more blood to the muscles that are being worked.