Cardiovascular Response to Exercise

NOTE: Most of the material in the cardiorespiratory unit is well covered in your text. Review the appropriate chapters. The notes below include only material in additional to that found in the text.

Blood Pressure Response to Different Exercise

In order to increase oxygen availability to working muscles, a number of changes must occur within the cardiovascular system that result in increased blood flow as well as a redistribution of the blood. Increasing either HR or SV elevates Q. This is aided by both sympathetic and hormonal (catecholamine) stimulation which increases HR and contractility. During rest, most of the blood resides in the venous system. With the onset of exercise, there is constriction of the venous system which increases blood return to the heart and in turn increases SV as explained by the Frank-Starling principle.

At rest, little blood is delivered to non-working muscles because of arteriole constriction. As muscles increase their activity, energy and oxygen demands are increased which stimulate the local arterioles to dilate and the precapillary sphincters to relax allowing more blood and oxygen into the working muscle. Regulation of the arterioles and precapillary sphincters is through a combination of sympathetic neural control and local metabolic factors, respectively. Furthermore, more oxygen is release from the blood during periods of high energy demands as demonstrated by the O₂ difference between the arterial and venous blood [(a-v) O₂ diff].

Blood pressure is the force that drives blood through the circulatory system and is influenced by the cardiac output and total peripheral resistance (TPR).

Blood Pressure = Cardiac Output x Total Peripheral Resistance

Blood flow between two points is directly proportion to the driving force (pressure) between them. Any changes in Q or resistance to blood flow influences blood pressure as well as blood flow. In Example A in the figure below, resistance to blood flow would be greater in the smaller vessel, and would result in greater arterial blood pressure in Example B.

During rhythmic exercise, Q increases because of increased HR and SV. At the same time, total peripheral resistance decreases when the arterioles serving working muscles dilate which increases blood flow to the working muscles. Further, the greater the muscle mass being used during exercise, the greater the reduction in peripheral resistance. Thus, during rhythmic exercises there is less increase in BP involving the legs than with the arms due to greater vasodilatation in the larger leg muscles.

The type of exercise also influences BP. Activities such as running which are rhythmic in nature cause less increase in BP than activities that are static or involve slow muscular movements such as weight lifting. When muscles contract, they temporarily slow or even stop blood flow through the tissues until relaxation occurs. This temporarily increases blood resistance and pressure. While running, muscles alternately contract and relax; it is during the relaxation phase in which muscle blood flow is restored and resistance decreased. However, an individual performing a heavy, sustained contraction (e.g. isometric contraction) will have a greater rise in BP due to the higher resistance to blood flow by the contracting muscles.

These differences can be illustrated by the two examples below. In the first figure, subjects performed 5-min bouts of cycling exercise at increasing intensity. BP and HR were measured at rest and near the end of each bout. In the second example, subjects performed a "simulated" isometric exercise for 2 min. BP and HR were taken before, and at 1 min and 2 min.

You should be able to explain the answers to these two questions. 1) How do the BP responses compare between rhythmic and static exercise, and 2) how do the BP responses from arm exercise compare with leg exercise?