Breathing heavily after you have finished running a race is your body way of

Breathing heavily after you have finished running a race is your body way of repaying an oxygen debt.

Explanation:

Any kind of exercise that is performed makes our muscles work harder. As a result, more calories get burned by these muscles. To burn the extra calories, more supply of oxygen is required by the muscles. As you exercise harder, the breathing increases and become heavier to bring more oxygen to the lungs. From the lungs, the oxygen is carried to the muscles which are burning more calories.

You breathe heavily at the end of a race to get enough supply of oxygen.