To understand the various functions of the veins, it is first necessary to know something about pressure in the veins and what determines the pressure.
Blood from all the systemic veins flows into the right atrium of the heart; therefore, the pressure in the right atrium is called the central venous pressure.
Right atrial pressure is regulated by a balance between (1) the ability of the heart to pump blood out of the right atrium and ventricle into the lungs and (2) the tendency for blood to flow from the peripheral veins into the right atrium. If the right heart is pumping strongly, the right atrial pressure decreases. Conversely, weakness of the heart elevates the right atrial pressure. Also, any effect that causes rapid inflow of blood into the right atrium from the peripheral veins elevates the right atrial pressure. Some of the factors that can increase this venous return (and thereby increase the right atrial pressure) are (1) increased blood volume, (2) increased large vessel tone throughout the body with resultant increased peripheral venous pressures, and (3) dilatation of the arterioles, which decreases the peripheral resistance and allows rapid flow of blood from the arteries into the veins.
The same factors that regulate right atrial pressure also enter into the regulation of cardiac output because the amount of blood pumped by the heart depends on both the ability of the heart to pump and the tendency for blood to flow into the heart from the peripheral vessels. Therefore, we will discuss regulation of right atrial pressure in much more depth in Chapter 20 in connection with regulation of cardiac output.
The normal right atrial pressure is about 0 mm Hg, which is equal to the atmospheric pressure around the body. It can increase to 20 to 30 mm Hg under very abnormal conditions, such as (1) serious heart failure or (2) after massive transfusion of blood, which greatly increases the total blood volume and causes excessive quantities of blood to attempt to flow into the heart from the peripheral vessels.
The lower limit to the right atrial pressure is usually about -3 to -5 mm Hg below atmospheric pressure. This is also the pressure in the chest cavity that surrounds the heart. The right atrial pressure approaches these low values when the heart pumps with exceptional vigor or when blood flow into the heart from the peripheral vessels is greatly depressed, such as after severe hemorrhage.
Large veins have so little resistance to blood flow when they are distended that the resistance then is almost zero and is of almost no importance. However, as shown in Figure 15-9, most of the large veins that enter the thorax are compressed at many points by the surrounding tissues, so that blood flow is impeded at these points. For instance, the veins from the arms are compressed by their sharp angulations over the first rib. Also, the pressure in the neck veins often falls so low that the atmospheric pressure on the outside of the neck causes these veins to collapse. Finally, veins coursing through the abdomen are often compressed by different organs and by the intra-abdominal pressure, so that they usually are at least partially collapsed to an ovoid or slitlike state. For these reasons, the large veins do usually offer some resistance to blood flow, and because of this, the pressure in the more
peripheral small veins in a person lying down is usually +4 to +6 mm Hg greater than the right atrial pressure.
Effect of High Right Atrial Pressure on Peripheral Venous Pressure. When the right atrial pressure rises above its normal value of 0 mm Hg, blood begins to back up in the large veins. This enlarges the veins, and even the collapse points in the veins open up when the right atrial pressure rises above +4 to +6 mm Hg. Then, as the right atrial pressure rises still further, the additional increase causes a corresponding rise in peripheral venous pressure in the limbs and elsewhere. Because the heart must be weakened greatly to cause a rise in right atrial pressure as high as +4 to +6 mm Hg, one often finds that the peripheral venous pressure is not noticeably elevated even in the early stages of heart failure.
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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.