After a period of vigorous exercise, a person continues to breathe rapidly even though they have stopped moving. Explain why the breathing rate remains high after vigorous exercise has ended.

During vigorous exercise the muscles use more oxygen than is supplied, so they switch to anaerobic respiration. This produces lactic acid, which accumulates in the muscle cells. The body must then repay an oxygen debt to remove the lactic acid. The lactic acid is carried to the liver where it is converted back to glucose (the Cori cycle) using oxygen. Therefore, even after exercise stops, the breathing rate remains high to provide the extra oxygen needed to oxidise the lactic acid and repay the debt.