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Cardiac index for the human being (cardiac output per square meter of surface area) at different ages. (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)

Instead, it is the various factors of the peripheral circulation that affect flow of blood into the heart from the veins, called venous return, that are the primary controllers.

The main reason peripheral factors are usually more important than the heart itself in controlling cardiac output is that the heart has a built-in mechanism that normally allows it to pump automatically whatever amount of blood that flows into the right atrium from the veins. This mechanism, called the Frank-Starling law of the heart, was discussed in Chapter 9. Basically, this law states that when increased quantities of blood flow into the heart, the increased blood stretches the walls of the heart chambers. As a result of the stretch, the cardiac muscle contracts with increased force, and this empties the extra blood that has entered from the systemic circulation. Therefore, the blood that flows into the heart is automatically pumped without delay into the aorta and flows again through the circulation.

Another important factor, discussed in Chapter 10, is that stretching the heart causes the heart to pump faster—at an increased heart rate. That is, stretch of the sinus node in the wall of the right atrium has a direct effect on the rhythmicity of the node itself to increase heart rate as much as 10 to 15 per cent. In addition, the stretched right atrium initiates a nervous reflex called the Bainbridge reflex, passing first to the vasomotor center of the brain and then back to the heart by way of the sympathetic nerves and vagi, also to increase the heart rate.

Effect of increasing levels of exercise to increase cardiac output (red solid line) and oxygen consumption (blue dashed line). (Redrawn from Guyton Ac, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)

Under most normal unstressful conditions, the cardiac output is controlled almost entirely by peripheral factors that determine venous return. However, we shall see later in the chapter that if the returning blood does become more than the heart can pump, then the heart becomes the limiting factor that determines cardiac output.

Cardiac Output Regulation Is the Sum of Blood Flow Regulation in All the Local Tissues of the Body—Tissue Metabolism Regulates Most Local Blood Flow

The venous return to the heart is the sum of all the local blood flows through all the individual tissue segments of the peripheral circulation. Therefore, it follows that cardiac output regulation is the sum of all the local blood flow regulations.

The mechanisms of local blood flow regulation were discussed in Chapter 17. In most tissues, blood flow increases mainly in proportion to each tissue's metabolism. For instance, local blood flow almost always increases when tissue oxygen consumption increases; this effect is demonstrated in Figure 20-2 for different levels of exercise. Note that at each increasing level of work output during exercise, the oxygen consumption and the cardiac output increase in parallel to each other.

To summarize, cardiac output is determined by the sum of all the various factors throughout the body that control local blood flow. All the local blood flows summate to form the venous return, and the heart automatically pumps this returning blood back into the arteries to flow around the system again.

Figure 20-3

Figure 20-4

Figure 20-3

Chronic effect of different levels of total peripheral resistance on cardiac output, showing a reciprocal relationship between total peripheral resistance and cardiac output. (Redrawn from Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB Saunders Co, 1980.)

Effect of Total Peripheral Resistance on the Long-Term Cardiac Output Level. Figure 20-3 is the same as Figure 19-5. It is repeated here to illustrate an extremely important principle in cardiac output control: Under most normal conditions, the long-term cardiac output level varies reciprocally with changes in total peripheral resistance. Note in Figure 20-3 that when the total peripheral resistance is exactly normal (at the 100 per cent mark in the figure), the cardiac output is also normal. Then, when the total peripheral resistance increases above normal, the cardiac output falls; conversely, when the total peripheral resistance decreases, the cardiac output increases. One can easily understand this by reconsidering one of the forms of Ohm's law, as expressed in Chapter 14:

Figure 20-4

Cardiac Output =

Arterial Pressure Total Peripheral Resistance

The meaning of this formula, and of Figure 20-3, is simply the following: Any time the long-term level of total peripheral resistance changes (but no other functions of the circulation change), the cardiac output changes quantitatively in exactly the opposite direction.

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Essentials of Human Physiology

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