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Chapter: Medical Physiology: Cardiac Output, Venous Return, and Their Regulation

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Normal Values for Cardiac Output at Rest and During Activity

Cardiac output varies widely with the level of activity of the body. The follow-ing factors, among others, directly affect cardiac output: (1) the basic level of body metabolism, (2) whether the person is exercising, (3) the person’s age, and (4) size of the body.

Normal Values for Cardiac Output at Rest and During Activity

Cardiac output varies widely with the level of activity of the body. The follow-ing factors, among others, directly affect cardiac output: (1) the basic level of body metabolism, (2) whether the person is exercising, (3) the person’s age, and (4) size of the body.

        For younghealthy men, resting cardiac output averages about 5.6 L/min. For women, this value is about 4.9 L/min. When one considers the factor of age as well—because with increasing age, body activity diminishes—the average cardiac output for the resting adult, in round numbers, is often stated to be almost exactly 5 L/min.

Cardiac Index

Experiments have shown that the cardiac output increases approximately in pro-portion to the surface area of the body.Therefore, cardiac output is frequently stated in terms of the cardiac index, which is the cardiac output per square meter of bodysurface area.The normal human being weighing 70 kilograms has a body surfacearea of about 1.7 square meters, which means that the normal average cardiac index for adults is about 3 L/min/m2 of body surface area.

Effect of Age on Cardiac Output. Figure 20–1 shows the cardiac output, expressedas cardiac index, at different ages. Rising rapidly to a level greater than 4 L/min/m2 at age 10 years, the cardiac index declines to about 2.4 L/min/m2 at age 80 years. We will see later that the cardiac output is regu-lated throughout life almost directly in proportion to the overall bodily meta-bolic activity. Therefore, the declining cardiac index is indicative of declining activity with age.



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