Liver converts lactic acid back to glucose which needs oxygen.
When a period of exercise is over, lactic acid must be removed. The body's tolerance of lactic acid is limited.
Lactic acid is taken to the liver by the blood, and either:
These processes require oxygen. This is why, when the period of activity is over, a person’s breathing rate and heart rate do not return to normal straightaway.
The amount of oxygen required to remove the lactic acid, and replace the body's reserves of oxygen, is called the oxygen debt.
When someone who has been exercising pays back an oxygen debt, it can take from a few hours for normal exercise, to several days after a marathon.