Exercise and cardiac output

Learn about cardiovascular reactions to exercise and heart volume during exercise and training.

Cardiovascular reactions to exercise

The function of the cardiovascular system is primarily to increase the supply of oxygen to the skeletal and heart muscle. VO2 during exercise is determined by heart production and oxygen recovery, or the aVO2 difference referred to as the thick equation. As you can see, heart volume plays a crucial role in determining VO2. A vital function of the cardiovascular system is to remove CO2 and heat from the contractile muscle. And finally, the most critical arterial blood pressure must be maintained to ensure an adequate abundance of essential organs, especially the brain. It is important to remember that average arterial pressure produces heart volume and total peripheral resistance. The regional blood flow and heart volume changes will interact to determine the average arterial pressure response to activity.

Changes in oxygen uptake

Suppose we start by looking at incremental training. In that case, we can examine the changes in oxygen uptake, heart volume, and various tissue blood flows as we go from rest through light training to heavy training and then to maximum training. As we have shown before, you can see that oxygen uptake increases in proportion to the intensity of exercise, and so does heart rate. There is a considerable increase in muscle blood flow. A maximum training, the active skeletal muscles reach 90% of available heart volume; the heart increases activity during exercise. There is, therefore, a slight increase in coronary blood flow. When we talk about heat and fluid balance, a meaningful way to remove heat during exercise is the evaporation of sweat that requires heat transfer from the body’s surface to the skin.

You will notice an increase in the skin’s blood flow during light to heavy training to facilitate this heat removal. However, as you move to higher intensities, there is a reduction in the skin’s blood flow as we approach maximum heart volume. And this sets up, if you will, a competition between muscle and skin and the other vital organs, heart, and brain that need to be controlled. We will talk a bit about that in the next module as well. Some of the vascular beds that may be less important during exercise may be vasoconstriction. So the splanchnic region and the kidneys will have less blood directed at them during practice, and their vascular beds are vasoconstrictive. Some studies have suggested that cerebral blood flow may increase slightly during exercise.

Cardiovascular response to exercise

Although the heart volume of a given VO2 is very similar between a passive person and an athletic person, heart rate and stroke volume responses are quite different. One of the adaptations to exercise training is a reduction in heart rate at a given submaximal training intensity. You can see this reduction in heart rate in the sports group. Maximum heart rate, if any, may be slightly lower in an athletic group or unchanged. Stroke volume will increase in both groups, and then it tends to flatten out at moderate training intensity. Some studies suggest that the stroke volume may continue to increase in athletic populations until it flattens out at higher training intensity. Part of the reason that the stroke volume decreases is that with an increase in heart rate, the diastolic filling time becomes limiting, and it may not be able to, or the heart can not be filled optimally at very high heart rates.

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What is considered cardiovascular exercise

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