Determinants of Venous Pressure

Determinants of Venous Pressure

The factors determining pressure in any elastic tube are the volume of fluid within it and the compliance of its walls.

Consequently, total blood volume is one important determinant of venous pressure because,as we will 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

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 nearly 100 mmHg.see this figure
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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 muscledecreasing 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 side of the heart.

Note the different effect of venous constriction compared to that of arterioles:

When arterioles constrict, it reduces forward flow through the systemic circuit,whereas constriction of veins increases forward flow.

Although the sympathetic nerves are the most important input, venous smooth muscle, like arteriolar smooth muscle, also responds to 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. 

skeletal muscle pump:

Duri ng 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 the skeletal muscle pump raises venous pressure locally, the valves permit blood flow only toward the heart and prevent flow back toward the tissues.see this figure

 respiratory pump:

During inspiration of air, the diaphragm descends, pushing on the abdominal contents and increasing 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.

Thus, venous return is enhanced during inspiration (expiration would reverse this effect if not for the venous valves).

The larger the inspiration, the greater the effect. Thus, breathing deeply and frequently, as in exercise, helps blood flow toward the heart.

You might get the (incorrect) impression from these descriptions that venous return and cardiac output are

independent entities.

However, any change in venous return almost immediately causes equivalent changes in cardiac output, largely through the Frank-Starling mechanism.

Venous return and cardiac output therefore must be identical except for very brief periods of time.

In summary,venous smooth muscle contraction, the skeletal muscle pump, and the respiratory pump all work to facilitate venous return and thereby enhance cardiac output by the same amount.

see this figure