valves of the venous system frequently become "incompetent" or sometimes even are destroyed. This is especially true when the veins have been overstretched by excess venous pressure lasting weeks or months, as occurs in pregnancy or when one stands most of the time. Stretching the veins increases their cross-sectional areas, but the leaflets of the valves do not increase in size. Therefore, the leaflets of the valves no longer close completely. When this develops, the pressure in the veins of the legs increases greatly because of failure of the venous pump; this further increases the sizes of the veins and finally destroys the function of the valves entirely. Thus, the person develops "varicose veins," which are characterized by large, bulbous protrusions of the veins beneath the skin of the entire leg, particularly the lower leg.
Whenever people with varicose veins stand for more than a few minutes, the venous and capillary pressures become very high, and leakage of fluid from the capillaries causes constant edema in the legs. The edema in turn prevents adequate diffusion of nutritional materials from the capillaries to the muscle and skin cells, so that the muscles become painful and weak, and the skin frequently becomes gangrenous and ulcerates. The best treatment for such a condition is continual elevation of the legs to a level at least as high as the heart. Tight binders on the legs also can be of considerable assistance in preventing the edema and its sequelae.
Clinical Estimation of Venous Pressure. The venous pressure often can be estimated by simply observing the degree of distention of the peripheral veins—especially of the neck veins. For instance, in the sitting position, the neck veins are never distended in the normal quietly resting person. However, when the right atrial pressure becomes increased to as much as +10 mm Hg, the lower veins of the neck begin to protrude; and at +15 mm Hg atrial pressure essentially all the veins in the neck become distended.
Direct Measurement of Venous Pressure and Right Atrial Pressure
Venous pressure can also be measured with ease by inserting a needle directly into a vein and connecting it to a pressure recorder. The only means by which right atrial pressure can be measured accurately is by inserting a catheter through the peripheral veins and into the right atrium. Pressures measured through such central venous catheters are used almost routinely in some types of hospitalized cardiac patients to provide constant assessment of heart pumping ability.
Pressure Reference Level for Measuring Venous and Other Circulatory Pressures
In discussions up to this point, we often have spoken of right atrial pressure as being 0 mm Hg and arterial pressure as being 100 mm Hg, but we have not stated the gravitational level in the circulatory system to which this pressure is referred. There is one point in the circulatory system at which gravitational pressure factors caused by changes in body position of a healthy person usually do not affect the pressure measurement by more than 1 to 2 mm Hg. This is at or near the level of the tri-cuspid valve, as shown by the crossed axes in Figure 15-12.Therefore, all circulatory pressure measurements discussed in this text are referred to this level, which is called the reference level for pressure measurement.
The reason for lack of gravitational effects at the tricuspid valve is that the heart automatically prevents
Natural reference point
Natural reference point
Reference point for circulatory pressure measurement (located near the tricuspid valve).
significant gravitational changes in pressure at this point in the following way:
If the pressure at the tricuspid valve rises slightly above normal, the right ventricle fills to a greater extent than usual, causing the heart to pump blood more rapidly and therefore to decrease the pressure at the tri-cuspid valve back toward the normal mean value. Conversely, if the pressure falls, the right ventricle fails to fill adequately, its pumping decreases, and blood dams up in the venous system until the pressure at the tricuspid level again rises to the normal value. In other words, the heart acts as a feedback regulator of pressure at the tricuspid valve.
When a person is lying on his or her back, the tricuspid valve is located at almost exactly 60 per cent of the chest thickness in front of the back.This is the zero pressure reference level for a person lying down.
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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.