During exercise the muscles need more energy, so the body responds to the extra demand. The heart rate, the breathing rate and the breath volume all increase. This supplies the muscles with more oxygenated blood, so they can respire aerobically faster.
If the blood cannot deliver enough oxygen, the muscles also 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, which means they stop contracting efficiently.
Higher tier only: blood flowing through the muscles carries the lactic acid to the liver, where it is converted back into glucose. The oxygen debt is the extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells. This is why you keep breathing hard for a while after you stop.