Cardiac output is the amount of blood pumped from your heart to the rest of your body. When you exercise, your body can require up to 18 times more oxygen than when you are at rest. In order to satisfy your body's increased need for oxygen when you exercise, your heart makes dramatic changes in its function.
Heart Rate
Your heart is able to increase its output of blood by up to 600 percent when you exercise if you are a trained athlete. It accomplishes this in a couple of ways. First, it speeds up when you exercise. Most people can increase their heart rate to a maximum rate calculated by the following equation:
208 - 0.7 X age in years
You should be able to get within 15 beats a minute of this calculated heart rate when you exercise if you have a normal healthy heart.
Stroke Volume
Secondly, your heart is able to increase its stroke volume. Stroke volume is the amount of blood your heart is able to push out in one heart beat. Epinephrine increases in your body when you exercise, sending signals to the muscle cells in your heart to contract more vigorously. This contributes to increasing the stroke volume of your heart when you exercise. Total cardiac output is calculated by multiplying your heart rate by your stroke volume.
Increasing Stroke Volume
In severe exercise, the stroke volume also increases when more blood flows into the heart. More blood flows to the heart during exercise primarily because the veins returning the blood constrict and narrow during exertion. Envision large pipes transporting a cup of water sluggishly. Squeezing the same amount of water into smaller, narrow pipes will cause the water to flow much more quickly, assuming all other conditions are equal. Constriction of the veins also results in more of your blood at any given time traveling in the arterial system. This means more blood being carried away from the heart and toward your working muscles. In contrast, many of the tiny arterioles carrying blood into the muscles relax and dilate with activity. This reduces resistance to the flow of blood leaving the heart and allows for an increase in stroke volume. This reduced resistance to flow is a major contributor to increased stroke volume. (Cardiovascular Physiology)
Blood Pressure
The dilation of the small arterioles bringing blood to the exercising muscles tends to decrease blood pressure, but the increase in cardiac output tends to increase blood pressure. The blend of these two opposing forces results in the change in blood pressure you see during your exercise routine. The systolic, or top number, goes up while the bottom, or diastolic number is prevented from rising to a similar degree.
Anaerobic Exercise
Keep in mind that these changes in your heart are relevant only to aerobic exercise. If you are a weight lifter, for instance, you engage in anaerobic exercise. The combined effects of changes in heart rate, stroke volume, and dilation of small arteries may be very different for this type of exercise.
References
- Cecil Textbook of Medicine ed Lee Goldman, et al.; 2000
- Harrison's Principles of Internal Medicine ed Isselbacher, et al.; 1994
- Cardiovascular Physiology Robert Berne et al.; 1986
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