Venous Blood Pressure

Venous pressure is a general term that represents the average blood pressure within the venous compartment. A more specific term, central venous pressure, describes the blood pressure in the thoracic vena cava near the right atrium. This pressure is important because it determines the filling pressure of the right ventricle, and thereby determines ventricular stroke volume through the Frank-Starling mechanism as discussed in Chapter 4.

Venous Blood Volume and Compliance

Several factors influence central venous pressure: cardiac output, respiratory activity, contraction of skeletal muscles (particularly leg and abdominal muscles), sympathetic vasoconstrictor tone, and gravitational forces. All of these factors ultimately change central venous pressure (APV) by changing either venous blood volume (AVV) or venous compliance (CV) as described by Equation 5-10.

Equation 5-10 is a rearrangement of the equation used to define compliance, in which compliance (in this case venous compliance) equals a change in venous volume divided by the change in venous pressure that occurs with the change in volume (see Compliance in Chapter 4 on the CD). Therefore, an increase in venous volume increases venous pressure by an amount determined by the compliance of the veins. Furthermore, a decrease in venous compliance, as occurs during sympathetic activation of veins, increases venous pressure.

The relationship described by Equation 5-10 can be depicted graphically as shown in Figure 5-10, in which venous blood volume is plotted against venous blood pressure. The different curves represent different states of venous tone, and the slope of a tangent line at any point on the curve represents the compliance. Looking at a single curve, it is evident v

Venous Compliance

FIGURE 5-10 Compliance curves for a vein. Venous compliance (the slope of line tangent to a point on the curve) is very high at low pressures because veins collapse. As pressure increases, the vein assumes a more circular cross-section and its walls become stretched; this reduces compliance (decreases slope). Point A is the control pressure and volume. Point B is the pressure and volume resulting from increased tone (decreased compliance) brought about, for example, by sympathetic stimulation of the vein.

FIGURE 5-10 Compliance curves for a vein. Venous compliance (the slope of line tangent to a point on the curve) is very high at low pressures because veins collapse. As pressure increases, the vein assumes a more circular cross-section and its walls become stretched; this reduces compliance (decreases slope). Point A is the control pressure and volume. Point B is the pressure and volume resulting from increased tone (decreased compliance) brought about, for example, by sympathetic stimulation of the vein.

that an increase in venous volume will increase venous pressure. The amount by which the pressure increases for a given change in volume depends on the slope of the relationship between the volume and pressure (i.e., the compliance). As with arterial vessels (see Fig. 5-4), the relationship between venous volume and pressure is not linear (see Fig. 5-10). The slope of the compliance curve (AV/AP) is greater at low pressures and volumes than at higher pressures and volumes. The reason for this is that at very low pressures, a large vein collapses. As the pressure increases, the collapsed vein assumes a more cylindrical shape with a circular cross-section. Until a cylindrical shape is attained, the walls of the vein are not stretched appreciably. Therefore, small changes in pressure can result in a large change in volume by changes in vessel geometry rather than by stretching the vessel wall. At higher pressures, when the vein is cylindrical in shape, increased pressure can increase the volume only by stretching the vessel wall, which is resisted by the structure and composition of the wall (particularly by collagen, smooth muscle, and elastin components). Therefore, at higher volumes and pressures, the change in volume for a given change in pressure (i.e., compliance) is less.

The smooth muscle within veins is ordinarily under some degree of tonic contraction. Like arteries and arterioles, a major factor determining venous smooth muscle contraction is sympathetic adrenergic stimulation, which occurs under basal conditions. Changes in sympathetic activity can increase or decrease the contraction of venous smooth muscle, thereby altering venous tone. When this occurs, a change in the volume-pressure relationship (or compliance curve) occurs, as depicted in Figure 5-10. For example, increased sympathetic activation will shift the compliance curve down and to the right, decreasing its slope (compliance) at any given volume (from point A to B in Fig. 5-10). This right-ward diagonal shift in the venous compliance curve results in a decrease in venous volume and an increase in venous pressure. Drugs that reduce venous tone (e.g., nitrodilators) will decrease venous pressure while increasing venous volume by shifting the compliance curve to the left.

The previous discussion emphasized that venous pressure can be altered by changes in venous blood volume or in venous compliance. These changes can be brought about by the factors or conditions summarized in Table 5-2. Central venous pressure is increased by:

1. A decrease in cardiac output. This can result from decreased heart rate (e.g., brady-cardia associated with atrioventricular [AV] nodal block) or stroke volume (e.g., in ventricular failure), which results in blood backing up into the venous circulation (increased venous volume) as less blood is pumped into the arterial circulation. The resultant increase in thoracic blood volume increases central venous pressure.

2. An increase in total blood volume. This occurs in renal failure or with activation of the renin-angiotensin-aldosterone system

TABLE 5-2 FACTORS INCREASING CENTRAL VENOUS PRESSURE (CVP),

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Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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