During exercise the body reacts to the increased demand for energy. The heart rate, breathing rate and breath volume all increase to supply the muscles with more oxygenated blood.
If not enough oxygen is supplied, the muscles respire anaerobically. The incomplete oxidation of glucose causes a build up of lactic acid and creates an oxygen debt. During long periods of vigorous activity the muscles become fatigued and stop contracting efficiently.
Higher tier only: blood flowing through the muscles transports the lactic acid to the liver, where it is converted back into glucose. Oxygen debt is the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells.