Blood Flow during Exercise - Circulation
Whenever we exercise or run we notice that we begin to breathe faster, the heart beats much faster than the usual average of 70 beats per minute. We know that the body at this time needs more oxygen and the heart must supply it to the muscles. There can be two ways of supplying this additional oxygen. The cardiac output can be increased or the amount of oxygen delivered by each volume of blood is increased. The arterial blood pumped out of the heart is already fully saturated but venous blood normally contains more than half the oxygen present in the arterial blood. Therefore, if more oxygen is extracted from the venous blood it can be supplied to the muscles. The total muscle of a lean person uses about 50 ml of O2 per minute which is supplied by about 1 litre of blood.
Arterial blood contains 200 ml of O2/litre and venous blood contains 150 ml of O2/litre. The oxygen extraction therefore, is only 25%. During heavy exercise, blood flow to muscles may be 20 litres per minute or even higher in atheletes and the oxygen extraction increases to 80-90%. In other words, during heavy exercise almost all the oxygen may be removed from the venous blood. The cardiac output can also be increased to deliver more oxygen. As said earlier, cardiac output can be increased by either increasing stroke volume or heart rate or both. At rest the human heart rate is 70 beats per minute and the stroke volume is 70 ml (from each side) giving a total cardiac output of 5 litres/minute. During exercise the cardiac output is increased about five-folds or more. Most of the increase is due to increase in pulse rate which may become 200 strokes per minute. Stroke volume may also increase beyond 100 ml. In a well-trained athelete the oxygen consumption may be increased as much as 100 times but this becomes possible more due to a three-fold increase in oxygen extraction from the blood.