A student runs a 400 m race. During the race, their muscles begin to respire anaerobically as well as aerobically. After the race, the student continues to breathe heavily for several minutes. Explain why the student's muscles switch to anaerobic respiration during the race, and why the student continues to breathe heavily after finishing.

During intense exercise the demand for ATP rises sharply and the rate at which oxygen can be delivered to the working muscles is limited by the cardiovascular system. Consequently the oxygen supply becomes insufficient to meet the energy demand. To keep producing ATP the muscles therefore switch to anaerobic respiration, which does not require oxygen. Anaerobic respiration generates ATP from glucose but produces lactic acid as a by‑product. The accumulation of lactic acid creates an oxygen debt – the body must later use oxygen to oxidise the lactic acid back to pyruvate and recover the lost energy. After the race the student’s breathing remains heavy because the body is taking in extra oxygen to break down the lactic acid and repay the oxygen debt.