4.10: Veins

Blood passing through capillaries flows into very small veins. The small veins join to form larger veins as they transport the blood back to the heart (Figure 4.1).

Veins are made up of the same three layers found in smaller arteries (Figure 4.9a) and (Figure 4.9c). The layers in veins are thinner and weaker, however, since venous blood pressure is much lower than arterial pressure; therefore, there is no need for thick strong walls in veins. Veins also tend to be somewhat larger in diameter than arteries in the same area of the body. This extra internal space, along with the greater ability of veins to expand outward, allows the veins to serve as a reservoir for storing blood.

Figure 4.9c Structure of a smaller vein. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission).

The inner layer provides smoothness to prevent blood clots, and the middle layer contains smooth muscle that regulates the diameter. When the muscle relaxes and the veins dilate, they can hold a considerable amount of blood. When the muscle contracts and constricts the veins, a great deal of blood is squeezed out and sent to the heart. These changes in diameter are useful in regulating blood pressure. For example, if blood pressure rises excessively, dilation allows the veins to store much of the extra blood from the arteries. The blood pressure will then return to normal. Conversely, if higher blood pressure is needed, the muscle layer contracts, squeezing more blood back to the heart. The heart immediately pumps this extra blood into the arteries, filling them further and increasing blood pressure and blood flow to the desired levels.

Since the blood pressure in veins is so low, blood flow tends to be sluggish. Gravity increases this tendency by pulling blood in veins below the heart downward, away from the heart. To prevent such backward flow, veins below the heart and in the arms contain valves. These valves consist of flaps of tissue extending from the walls of the veins into the blood (Figure 4.9d). The valves operate in the same way as do those in the heart.

Figure 4.9d Structure of a valve in a vein. (Sources of images below. Used with permission).

The movement of blood in veins is greatly aided by alternating contraction and relaxation of nearby muscles, such as occurs during exercise involving body movement. During contraction, muscles widen and press on neighboring veins, forcing blood to move along the veins. During relaxation, the muscles become thinner, allowing the veins to expand and fill with blood from below. Therefore, exercise promotes blood flow in veins.

Age Changes in Veins

Several age changes occur in veins, including accumulations of patchy thickenings in the inner layer and fibers in the middle layer and valves. However, these changes do not alter the functioning of veins because veins have such a large diameter to begin with that slight narrowing is unimportant. Veins have thin walls and are able to expand easily and compensate for narrowing, and there are often several veins draining blood from each area of the body, which can provide ample alternative routes for blood.

Diseases of Veins

Some disease changes in veins occur with increasing frequency and severity as age increases. One of the most common is varicose veins, which now ranks as the tenth leading chronic condition among people above age 64.

Varicose Veins

A varicose vein is a vein that has developed a much larger diameter than normal because blood has accumulated in the vein, stretching it outward. If the vein is stretched frequently and for prolonged periods, it loses its elasticity and remains permanently distended (Figure 4.16).

Figure 4.16 Varicose veins (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Varicose veins frequently develop in the legs. Conditions promoting their development in this area include standing still for long periods, sitting in a posture that reduces circulation, wearing tight clothing, and having certain diseases (e.g., congestive heart failure). Varicose veins are also found inside the abdomen; for example, cirrhosis of the liver is a common cause of varicose veins in the digestive system.

Varicose veins cause several problems. Affected veins close to the skin can be cosmetically undesirable because they appear as irregular bluish vessels. When veins remain engorged with blood for long periods or become inflamed, they can be very painful. They can even become sites of infection and, in extremely serious cases, sites of bleeding. Bleeding is the main problem when a person has varicose veins from cirrhosis. Very wide varicose veins also prevent the valves from stopping backward blood flow, since the valve flaps are too far apart to meet and blood slips back through the opening that remains between them. The blood backs up into the capillaries, slowing flow there. When this happens in the legs, swelling in the area below the varicose vein develops. Slow flow also prevents the capillaries from serving the needs of body cells, and the cells become weak and injured and may even die. Infection often adds to the resulting skin, nerve, and muscle problems.

Another undesirable result from varicose veins occurs because blood flow through these veins is fairly sluggish and the blood tends to clot. A stationary blood clot inside a vessel is called a thrombus. As in arteries, a thrombus in a vein can block blood flow. Frequently, blood flow propels the thrombus within the vein, in which case it is called a thromboembolus or simply an embolus. An embolus can cause serious problems when it moves to the heart and is pumped into the arteries, because as the arteries branch into narrower ones, the embolus will finally reach an artery through which it cannot pass and will block blood flow through that artery.

Almost all varicose veins develop in systemic veins such as those in the legs and the digestive system. Therefore, most emboli from varicose veins enter the right atrium and are pumped by the right ventricle into the pulmonary arteries. Such emboli are called pulmonary emboli.

A small pulmonary embolus causes death of the area of the lung normally serviced by the artery that has become blocked. If only a very small artery is blocked, the area that dies may be so small as to go unnoticed. However, repetition of this type of event or blockage of a larger pulmonary artery by a more substantial embolus may kill a considerable portion of the lung, significantly reducing the ability of the lung to serve the needs of the body. Dead lung tissue can become infected and form a pocket of pus called a pulmonary abscess. These infections and abscesses can make a person ill and can even be fatal.

A large pulmonary embolus can obstruct blood flow from the right ventricle to the lungs to such an extent that the right ventricle can no longer empty adequately and becomes overfull. This overfilling, coupled with the high pressure developed as the right ventricle attempts to pump blood through the artery. The result can be sudden death.

Since varicose veins cause such a variety of undesirable and serious consequences, slowing or preventing their formation can help maintain the quality and length of life. When possible, people who stand or sit for long periods of time should move about or change position frequently. When one is standing, alternately tensing and relaxing the leg muscles periodically can help pump blood out of the veins. Support stockings or tights that apply an even pressure over t veins. Elevating the legs for short periods allows accumulated blood to drain out of the veins. In addition, certain situations should be avoided. For example, sitting for long periods with the legs crossed or in a tightly bent position inhibits blood flow out of the veins. Clothing that is tight in the upper regions of the legs should be avoided for the same reason. Excessive habitual consumption of alcoholic beverages should be avoided because this is the most common cause of liver cirrhosis. Individuals who have a weak heart or are developing congestive heart failure should pay particular attention to these suggestions.

Hemorrhoids

One type of varicose vein is singled out here because of its location; it is found in the area of the anus and is called a hemorrhoid (Figure 4.13). Hemorrhoids may remain small for long periods, may enlarge slowly, or may become large in a short time. Some may reach the size of Ping‑Pong balls.

Figure 4.13 Hemorrhoids; two diagrams. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Like other types of varicose veins, hemorrhoids can be painful and may bleed, become infected, develop thrombi, and require surgery. These consequences can cause substantial disability.

Factors that promote the formation of hemorrhoids include chronic constipation, forced bowel movements, chronic cough, and cirrhosis of the liver. The first two factors are often found among disabled individuals and people whose occupations limit the availability of toilet facilities. Chronic cough is associated with smoking and other forms of air pollution, chronic bronchitis, and emphysema.

Several strategies can be used to decrease the chance of developing hemorrhoids. Adequate amounts of fiber and water in the diet help because these substances promote regular and relatively easy bowel movements, as does exercise. Adequate access to toilet facilities and timely use of those facilities are important. Smoking, breathing polluted air, and consuming alcohol habitually should be avoided.