An individual's blood pressure is controlled on a second to second basis by a servo-mechanism known as the baroreflex. Located in the major artery of the neck (carotid artery) and in the aorta (just as it leaves the heart) are a series of pressure sensitive cells known as baroreceptors. These baroreceptors detect the level of blood pressure and relay information to the cardiovascular control centre in the brain. Here the measured pressure is compared to the preset value that the body normally experiences.

If there is an error between the measured and required pressure, certain responses occur to correct the difference. For example, at rest whilst sitting quietly, average blood pressure is 120 mmHg systolic and 80 mmHg diastolic. If the individual then stands, blood pressure drops due to the influence of gravity. This reduction in pressure is detected by the baroreceptors and heart rate and cardiac output are elevated such that blood pressure can be returned to its normally value.

During exercise the baroreflex remains active in controlling the level of blood pressure. However, whereas at rest pressure is maintained around the values mentioned above, during exercise the required pressure is reset to a much greater value. This is necessary as a higher pressure helps drive blood flow around the body. The level to which blood pressure is reset during exercise is dependent upon the work intensity performed.

During exercise there are dramatic changes in the cardiovascular system. Perhaps the greatest change occurs in blood flow to exercising muscle which, under maximal conditions can increase up to 35 fold. Such a large increase in blood flow has severe consequences for blood pressure regulation, as the opening of this large circulation can result in a sudden and substantial drop in pressure.

There is an everyday analogy that is very comparable to this situation. Imagine that you are taking a shower upstairs in your bathroom when someone elsewhere in the house turns a tap on. This results is a loss of pressure in the shower, as two circulations are now open. The answer to this problem is to increase the capacity of the pump so that the pressure can be corrected. This is exactly what happens during exercise.

At the start of exercise the level of required pressure is set to a higher value in the control centre in the brain. At this time the baroreceptors detect and relay the measured pressure to the brain, but because of the large increase in muscle blood flow this measured pressure is well below that new required level. In order to correct the difference, nerve impulses are sent to the heart to increase both heart rate, stroke volume and cardiac output and therefore, allow blood pressure to be maintained at the necessary level.

Thus, the linear increases in heart rate and cardiac output that are observed during dynamic exercise can be explained by the need to maintain blood pressure about a new set-point, rather than solely being explained by the need to increase muscle blood flow to allow delivery of nutrients for metabolism.