A rescue worker completes a short period of vigorous stair climbing. During recovery, blood carries lactic acid away from the muscles. Some time later, a blood test shows that the worker's lactic acid level has returned to what it was before the climb.
What does this result show?
The muscles are still respiring anaerobically, so the oxygen debt is growing.
The lactic acid has been breathed out, so no extra oxygen was needed.
The liver is storing the lactic acid ready for the next period of exercise.
The oxygen debt has been repaid, so breathing can return to the resting rate.
92 exam-style questions on AQA GCSE Biology 4.2 Respiration, covering 4.2.1 Aerobic and anaerobic respiration, 4.2.2 Response to exercise, and 4.2.3 Metabolism. Each one has a worked solution and a mark scheme showing where the marks go.