Determinants of Venous Pressure

The factors determining pressure in any elastic tube are, as we know, the volume of fluid within it and the compliance of its walls. Accordingly, total blood volume is one important determinant of venous pressure since, as we shall see, at any given moment most blood is in the veins. Also, the walls of veins are thinner and much more compliant than those of arteries. Thus, veins can accommodate large volumes of blood with a relatively small increase in internal pressure. Approximately 60 percent of the total blood volume is present in the systemic veins at any given moment (Figure 14-48), but the venous pressure averages less than 10 mmHg. (In contrast, the systemic arteries contain less than 15 percent of the blood, at a pressure of approximately 100 mmHg.)

Pulmonary circulation — 12%

Veins Venules

Veins Venules

Distribution Total Blood Volume

Arterioles and capillaries — 7%

FIGURE 14-48

Distribution of the total blood volume in different parts of the cardiovascular system.

Adapted from Guyton.

Arterioles and capillaries — 7%

FIGURE 14-48

Distribution of the total blood volume in different parts of the cardiovascular system.

Adapted from Guyton.

The walls of the veins contain smooth muscle innervated by sympathetic neurons. Stimulation of these neurons releases norepinephrine, which causes contraction of the venous smooth muscle, decreasing the diameter and compliance of the vessels and raising the pressure within them. Increased venous pressure then drives more blood out of the veins into the right heart. Although the sympathetic nerves are the most important input, venous smooth muscle, like arteriolar smooth muscle, is also influenced by hormonal and paracrine vasodilators and vasoconstrictors.

Two other mechanisms, in addition to contraction of venous smooth muscle, can increase venous pressure and facilitate venous return. These mechanisms are the skeletal-muscle pump and the respiratory pump. During skeletal-muscle contraction, the veins running through the muscle are partially compressed, which reduces their diameter and forces more blood back to the heart. Now we can describe a major function of the peripheral-vein valves: When

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

Valve open

Blood flows only toward

Contracted skeletal muscles

Valve closed

Valve open

Contracted skeletal muscles

Contracted Skeletal Muscle

Blood flows only toward

FIGURE 14-49

The skeletal-muscle pump. During muscle contraction, venous diameter decreases, and venous pressure rises. The resulting increase in blood flow can occur only toward the heart because the valves in the veins are forced closed by any backward flow. %

FIGURE 14-49

The skeletal-muscle pump. During muscle contraction, venous diameter decreases, and venous pressure rises. The resulting increase in blood flow can occur only toward the heart because the valves in the veins are forced closed by any backward flow. %

the skeletal-muscle pump raises venous pressure locally, the valves permit blood flow only toward the heart and prevent flow back toward the tissues (Figure 14-49).

The respiratory pump is somewhat more difficult to visualize. As will be described in Chapter 15, during inspiration of air, the diaphragm descends, pushes on the abdominal contents, and increases abdominal pressure. This pressure increase is transmitted passively to the intraabdominal veins. Simultaneously, the pressure in the thorax decreases, thereby decreasing the pressure in the intrathoracic veins and right atrium. The net effect of the pressure changes in the abdomen and thorax is to increase the pressure difference between the peripheral veins and the heart. Accordingly, venous return is enhanced during inspiration (expiration would reverse this effect if it were not for the venous valves). The larger the inspiration, the greater the effect. Thus, breathing deeply and frequently, as in exercise, helps blood flow from the peripheral veins to the heart.

One might get the (incorrect) impression from these descriptions that venous return and cardiac output are independent entities. However, any change in venous return, due say to the skeletal-muscle pump, almost immediately causes equivalent changes in cardiac output, largely through the operation of the Frank-Starling mechanism. Venous return and cardiac output therefore must be identical except for very brief periods of time.

In summary (Figure 14-50), the effects of venous smooth-muscle contraction, the skeletal-muscle pump, and the respiratory pump are to facilitate venous return and thereby to enhance cardiac output by the same amount.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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