A weightlifter performs a series of heavy lifts in quick succession. During the first few lifts, the muscles contract using ATP from aerobic respiration. As the intensity increases, the muscles switch to anaerobic respiration and lactic acid begins to accumulate. After the session ends, the weightlifter continues to breathe rapidly for several minutes. Explain why lactic acid accumulates in the muscles during intense exercise and describe what happens to this lactic acid after the exercise has finished.

During intense exercise the oxygen supply to the working muscles is insufficient to meet the demand for ATP. Consequently the cells rely on anaerobic glycolysis, which converts glucose to pyruvate and then to lactic acid (lactate) because the electron transport chain cannot operate without oxygen. Lactic acid therefore accumulates in the muscle because it is produced faster than it can be removed or oxidised.

After the session ends the weight‑lifter continues to breathe rapidly. The increased ventilation supplies extra oxygen, which is used to repay the oxygen debt. The extra oxygen allows the mitochondria to oxidise the accumulated lactate back to pyruvate, which then enters the Krebs cycle to produce ATP. Lactate is also transported in the blood to the liver, where it is converted back to glucose via gluconeogenesis.

Thus lactic acid builds up during intense exercise because of anaerobic respiration and is removed after exercise by oxygen‑dependent oxidation and conversion in the liver.