Question

How does cardiac output increase from 5 L/min at rest to 35 L/min during an incremental exercise where VO2 max is reached at the end?

Answer 1

We know that cardiac output is the multiplied value of stroke volume and heart rate. Stroke volume is the amount of blood the ejects out from the left ventricle during the contraction of the heart. Heart rate is the number of times heart contracts in a minute. Stroke volume is 70 ml in a healthy man. Heart rate is 70- 100 beats per minute in resting state. So if we take the value of 75 BPM the cardiac output becomes 5 litres per minute. When we do an incremental exercise the heart rate rises much higher and stroke volume increases to a small quantity. During exercise the body needs more oxygen supply. As there arrives a localised hypoxia situation in the tissues the autonomic system stimulates the heart to work fast to compensate the oxygen supply. As a result the heart rate elevated in a tremendous rate. As heart rate increases the cardiac output also increases. Along with this the contractile force of the heart also increases making the stroke volume increase in a small level. So this increment makes the cardiac output to reach 35 litres per minute. But the heart rate can't rate depending on the SA node firing. This is attained by the VO2max. It is the maximum oxygen utilising capacity of the tissues. If the tissues reach a level that it can't take more oxygen the heart rate remains at that particular level to keep the VO2 max