Why do people and animals breath heavily after running a race?
To replenish oxygen levels.
After much physical exertion, lactic acid builds up in the body as well as carbon dioxide levels. Lactic acid build up can be prevented if the required oxygen flow to the muscles is met. During exercise, our body tends to use more oxygen than we can replenish. Hence after a workout, there is a need to reach our healthy bodily balance (heavy breathing to take in more oxygen).