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Purpose: The function of the veins is to transport blood from the capillaries back to the heart. This is the case for both the systemic (large) circulation as for the pulmonary circulation.
In the arterial system there is a pump (the heart) that pumps the blood through all the arteries to the capillaries. However, there is no “venous pump” in the venous system to pump the blood back to the heart.
So, the problem is; how does the venous blood return to the heart? There are several factors that help in this and together they take care of what we call the Venous Return.
These are the 6 factors that help/promote the blood to flow back to the heart:
Not all factors have the same importance. The first is the most important, no. 5 and 6 are the least important.
Probably the most important factor. When a skeletal muscle contract (which is often), the veins inside and between the muscles will be squeezed. This will increase the blood pressure inside the veins and make the blood flow away from the high pressure. Because of the valves, the blood will only flow in one direction, towards the heart.
Since skeletal muscles mostly contract rhythmically (such as walking, running, cycling), this will induce a rhythmic blood pressure in the veins and therefore a rhythmic flow of venous blood towards the heart.
Another factor in the venous return is the capillary pressure. At the end of the capillaries, the hydrostatic (= blood) pressure is not zero but approximately 20 mm Hg. As the pressure in the big veins is much lower, close to 0 mmHg, there is therefore a pressure gradient from capillaries to veins which will cause blood flow through the veins to the heart. That helps!
When a person inhales, the pressure inside the chest gets lower (becomes more negative). At that moment, the pressure in the chest is lower than in the abdomen. This will induce a flow of blood in the large abdominal vein (vena cava inferior) towards the chest.
And, when a person exhales, the pressure in the chest is higher than in the abdomen, and that will stop the blood flow (note that the blood will not flow back because it is blocked by the valves in the femoral veins).
In the body, the arteries and veins often run parallel to each other. Therefore, when an arterial pulse propagates down the artery, this will cause a local expansion (bulging) and this swelling will push against the veins. This will increase the local pressure in the veins. This pressure increase will push the venous blood towards the heart.
This is a very small effect that is caused by the movement of the heart during its contraction. When the heart contracts, it ejects blood into the aorta and into the pulmonary heart. This ejection is quite a force that will cause the ventricles to move away from these major arteries (action = reaction; here blood ejection is the action and the movement away is the reaction).
Anatomically however, the atria are located between the root of the major vessels and the ventricles. The major vessels are fixed in the chest (actually in the mediastinum), so, as the ventricles move away during ejection, the atria will be stretched. This will cause a lower pressure in the atria and will help venous blood flow, from the vena cava’s and the pulmonary veins, into the atria.
If the valves are deficient, venous return will become more difficult. If the valves become deficient in a particular area, then that area will have a problem in its venous return (swelling, oedema etc). This is for example the case with varicosities. This tends to occur in those people who, by profession, have to stand a long time (shopkeepers, teachers). Over the years, the valves will slowly deteriorate, expand, and blood will pool in those areas. These are visible as ugly swellings under the skin: varicosities.
In some situations, people are told to stand absolute immobile. Soldiers for example in a parade. It is then not uncommon for a soldier to faint (this often happens during the summer, then you see pictures of such a poor fellow lying flat on the parade grounds). The reason why this person fainted is because he has to stand absolute motionless, he is not allowed to move a single muscle, his venous return became too low, which in turn decreased his cardiac output, therefore not enough blood flowed to his brain, hence he fainted!
The nice thing about fainting and collapsing on the floor is that this behaviour often solves the problem. Before collapsing, the venous return was too much reduced because it had to pump upwards to the heart. This is about 1 meter higher than the legs. If one is collapsed and lying flat on the ground, then the legs are at the same level as the heart and it is much easier for the blood to flow back to the heart and to the brain. You can actually help by lifting the legs when the fainted person is on the floor to increase blood flow from the legs to the heart.
And this is my sketch of the veins in the pulmonary system.
Noteworthy here is that, usually, two large veins run from each lung to the heart, to the right atrium.
However, there are some anatomical variations between individuals. For example, in some people, the two veins from the right lung for example, merge into a single vessel before entering the right atrium.
