Respiration: Question 9
Syllabus 12.3
A sprinter finishes a 200 m race. For several minutes afterwards, she continues to breathe heavily and her heart continues to beat rapidly, even though she has stopped running and is now standing still.
Which statement best explains why an oxygen debt occurs during the race and must be repaid in this way afterwards?
Show worked solution Hide worked solution
Worked solution
Step 1: Recall what happens during an all-out sprint
During a fast, intense sprint, the sprinter’s muscles need energy faster than aerobic respiration alone can supply it, even while she is breathing throughout. Her muscle cells therefore also respire anaerobically, breaking glucose down without oxygen and producing lactic acid, which builds up in the muscle tissue.
Step 2: Recall what an oxygen debt is
The lactic acid that has built up in her muscles must eventually be broken down by aerobic respiration, which needs oxygen. The oxygen debt is the extra amount of oxygen needed for this, over and above what is needed for resting activities. This is why her breathing and heart rate stay raised for several minutes after the race, even though she has stopped running.
Step 3: Rule out the other options
- Option B treats oxygen debt as simply refilling an oxygen store, rather than as the oxygen needed to break down accumulated lactic acid.
- Option C invents an unrelated cause, lung damage, that has no connection to the accepted explanation involving lactic acid.
- Option D confuses the role of oxygen in breaking down lactic acid with the separate process of exhaling carbon dioxide, which does not itself require extra oxygen.
Final answer
The oxygen debt occurs because anaerobic respiration during the sprint produced lactic acid, and extra oxygen is needed afterwards to break this lactic acid down, option A.