The runner's muscles needed a lot of energy to run during the race. To obtain this energy, his blood first supplies his muscles with oxygen from the lungs. The muscles, through a process called respiration, convert the oxygen and stored sugars into energy. During the race, his muscles used oxygen at a faster rate than his body could supply them with, creating an oxygen debt. To pay back an oxygen debt, a person must breathe at a fast rate. Therefore, the runner was out of breath after the race to pay back his oxygen debt.
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