If we should be moved to inquire why so small a fraction of its whole store of oxygen is given up by the blood to the tissues ordinarily, we shall find the answer in remembering that the demand of the body for oxygen is extremely variable; every increase in functional metabolism means an increase in the amount of fuel that is oxidized and therefore an increase in the amount of oxygen that is required. Actual measurements have shown that in very vigorous muscular exercise the oxygen consumption may be approximately ten times as great as in complete rest. In order that this very greatly increased metabolism may be carried on it must be possible for the blood to deliver approximately ten times as much oxygen to the active tissues as it delivers to them when quiet. There are just two ways in which this can be done; one is by a more complete decomposition of the oxyhemoglobin by which all its oxygen is set free; the other is by a more rapid movement of the blood. It is by a combination of these two that the oxygen requirements of the body in times of vigorous metabolism are taken care of. As has already been said, the heart rate is just about doubled in vigorous exercise. There has also been shown to be some increase in the amount of blood that it pumps with every beat. The result is that more than twice as much blood leaves the heart in a minute under these circumstances as in time of rest. The oxyhemoglobin is also completely decomposed when the tissues are active, and these two facts together are sufficient to account for the great increase in the oxygen supply.