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The venous system acts as a collecting system, returning blood from the capillary networks to the heart passively down a pressure gradient. The capillaries merge to form venules, which in turn unite to form larger, but fewer, veins which amalgamate finally into the venae cavae. The walls of veins consist of the same three layers as arteries, but the elastic muscle components are much less prominent; the walls in general are thinner and more distensible than those of arteries.
The vessels have a relatively large diameter (the vena cava is 2-3 cm in diameter) and thus offer low resistance to blood flow. Some veins, especially in the arms and legs, have internal folds of the endothelial lining that function as valves and allow blood to flow in one direction only, towards the heart. These valves can be damaged if over stretched by high venous pressures for long periods, for example during pregnancy or in people who stand for extended periods; the valves become incompetent, lose their function, and varicose veins develop. As a result of this, oedema and varicose ulcers can develop. A major part of the blood volume, approximately 60%, is contained within the venous system and for this reason veins are sometimes referred to as capacity vessels. The capacity of the venous system can be modified by altering the lumen size of the muscular venules and veins; the changes are mediated by altering the venomotor tone, that is, the degree of contraction of the smooth muscle in the tunica media. Venomotor tone is mainly under the control of the sympathetic nervous system. Changes in the venomotor tone can increase or decrease the capacity of the venous circulation and therefore can partially compensate for variations in the effective circulating blood volume.
VENOUS RETURN Venous blood flow occurs along relatively small pressure gradients and even small variations in resistance and vessel radius affect the return flow. The effect of gravity retards venous return: when upright, as the veins are distensible and due to the hydrostatic pressure of a column of blood in the veins below the level of the heart, blood tends to collect or pool in the feet and legs. When vertical, the leg veins take on a circular form which has a greater capacity; when horizontal the veins take on an elliptical shape with a lower capacity. Increased venomotor tone, reducing the diameter and hence capacity of the veins, helps to reduce venous pooling. Venous pooling is a useful term but it suggests stagnation which does not occur; venous pooling simply indicates that the veins accommodate a greater volume of blood. One can see the effect of gravity on the veins in the neck: when sitting or standing the neck veins above a level 5-10 cm higher than the heart are not prominent, but when lying down the veins distend. This is due to the fact that, in contrast to venous return from the feet, blood from the head returns to the heart aided by gravity when upright. However, the blood supply to the head has to overcome the effect of gravity; failure of this phenomenon can be observed when someone stands up too quickly after bending down and feels dizzy due to a temporary reduction in the effective pressure head delivering blood to the brain. It is vital that an adequate venous return to the heart is maintained at all times because the cardiac output depends on the venous return - in most instances the cardiac output equals the venous return. Thus, if the venous return falls, cardiac output and blood pressure may also drop. Several mechanisms exist to help maintain the venous return at all times. Increasing the venomotor tone is an important mechanism as it decreases the capacity of the venous system and so aids venous return. After a long period of bed rest when the body is not constantly being exposed to the force of gravity and the veins do not have to compensate, venomotor tone is reduced, and this method of reducing the effect of gravity is temporarily less efficient. This should be remembered when helping someone to get up after a period of bed rest. It is essential to move slowly and steadily and to support the person in case he or she becomes dizzy and faint. Venous return is also assisted by two systems sometimes referred to as the skeletal muscle pump and respiratory pump.  Contraction of the skeletal muscles,
especially in the limbs, squeezes the veins and this pushes blood in the extremities
towards the heart; back flow is prevented by the presence of numerous valves. There are
also many communicating channels which allow emptying of blood from the superficial limb
veins into the deep veins when rhythmic muscular contractions occur. Consequently, every
time a person moves his or her legs or tenses the muscles, a certain amount of blood is
pushed towards the heart. The more frequent and powerful such rhythmic contractions are,
the more efficient their action. (Sustained continuous muscle contractions, unlike
rhythmic contractions, impede blood flow due to the veins being continuously 'blocked'.)
The muscle pump mechanism is an efficient system: the venous pressure in the feet of
someone walking is of the order of 25 mmHg (3.3kPa), whereas in the feet of an individual
standing absolutely still it is of the order of 90 mmHg (12kPa). So when an individual
stands still for long periods of time, the muscle pump cannot operate and venous return is
decreased. This can result in people fainting due to an inadequate cerebral blood flow.
e.g. soldiers fainting on parade, people fainting in operating theatres after standing
still for long periods. Thus it is advisable to contract the muscles of the legs and
buttocks voluntarily to aid venous return if standing still for long periods.
To see an animation of this process click on the image
Respiration produces cyclical variations in intra pleural and intra thoracic pressure. With each inspiration, the pressure is lowered with the thorax and hence also within the right atrium of the heart; this increases the pressure gradient and aids blood flow back to the heart. Simultaneously, the descent of the diaphragm into the abdomen raises the intra-abdominal pressure and increases the gradient to the thorax, again favouring venous return. With expiration, the pressure gradients are reversed and blood tends to flow in the opposite direction; fortunately this tendency is prevented by the valves in the medium sized veins. Thus venous return is maintained by changes in venomotor tone, altering the capacity of the venous system, and by the skeletal muscle and respiratory pumps. Obviously it is also necessary to maintain an adequate circulating blood volume. If the blood volume is depleted for some reason, e.g. dehydration or haemorrhage, in the short term venoconstriction and vasoconstriction in the body's blood reservoirs, such as the skin, liver, lungs and spleen, can increase the effective circulating blood volume. However, the blood volume must be restored eventually by fluid replacement. The pressures in the central regions of the venous system directly reflect the blood volume; thus central venous pressure (CVP), or right atrial pressure, is a good indicator of blood volume, unlike arterial pressures which are reflexly regulated and controlled. [Human Biology Contents Page] [My Home Page] This page authored by John Ross. Please e-mail any comments or queries to johnross@cwcom.net or leave a message in the guest book. This page last updated on Wednesday, 30 June 1999 15:11 +0100 |
The image on the right
shows a cutaway section through a large vein
This Pie Chart shows the overall
distribution of the total blood in a healthy adult at any one time. As you can see by far
the largest proportion is in the veins. 