In muscles. Working muscles consume more glycogen (glucose stored in the mitochondria of the cells), pick up more glucose from the blood and burn fat, blood-borne from the places of its accumulation. Enhanced metabolism requires additional oxygen and a greater flow of blood, otherwise the muscles get tired very quickly. All the necessary change in the organism produces brain. Glycogen originally comes from domestic sources of carbohydrates, but with prolonged activity ultimately produced from the accumulated fat. In the lungs. The depth and frequency of respiration immediately amplified. Normal ventilation for an adult is 10-12 liters of air per minute, during vigorous exercise may increase 10 times. Lungs of healthy nonsmoking men easily take from the air enough for any reasonable load of oxygen. The feeling of "lack of air" has no relation to the capacity of your lungs, just your heart and blood vessels can not carry enough oxygen to the muscles. You know it, remembering that endurance exercise strengthens the cardiovascular system, but not the lungs. In the blood vessels. Shortly after the muscles start to work with the load, the decomposition products diffuse into the surrounding tissue, causing an overall expansion of blood vessels, allowing increased blood flow. Degradation products stimulate the brain to return the team to increase blood flow to working muscles. During intense exercise the muscles can use the blood flow is 20 times greater than under normal conditions.