The venous system consists of all the vessels (vena cava superior and inferior, veins, venules etc) that collect blood from all the capillaries and transport this back to the heart.
In the arterial system there is a pump (right and left heart) that pumps the blood through all the arteries to the capillaries, but there is not a ‘venous pump’ 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 the Venous Return.
B. Factors that promote Venous Return:
In contrast to the arteries, the veins have valves. These valves make sure that blood can only flow in one direction; towards the heart.
2. Muscle pump.
Probably the most important factor. When a skeletal muscle contracts (which is often), the veins inside that muscle will be squeezed. This will increase the blood pressure inside these veins and make the blood flow away from the high pressure. Because of the valves, the blood will flow towards the heart.
Since skeletal muscles mostly contract rhythmically (such as walking, running, cycling), this will induce rhythmic increase and decrease in venous blood pressure and rhythmic flow of venous blood towards the heart.
At the end of the capillaries, the hydrostatic (= blood) pressure is not zero but approximately 20-30 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.
4. Breathing (= Respiratory pump):
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, for example, in the abdomen. This will induce a flow of blood 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).
By the way; in contrast to all the other veins, there are no valves in the caval 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. Flow of the venous blood is then again directed by the valves 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.
C. Some additional notes :
Not all factors are equally important and the order of importance is pretty much the order discussed above. The muscle pump is the most important factor whereas the arterial pulse and the cardiac suction are the least important.
If the valves are deficient, venous return will become more difficult. If the valves become deficient in a particular area, than that area will have a problem in its venous return (swelling, oedema etc). This is for example the case in varicosities. This tends to occur in those people who, by profession, have to stand a long time (shopkeepers). 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
We often use our muscle pump, especially when standing, because then the blood has to flow back from the legs all the way to the heart. This is more difficult then when a person is lying flat (as in sleeping). During normal standing, we often use muscles without noticing it, to keep our muscle pump working. This can be done by walking (teachers in front of the class), or by shaking legs etc.
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 reason why this occurs more often during the summer is that the soldier, because of the high temperature outside, develops a high body temperature (especially if they have to wear thick ceremonial clothing or wear battle gear). To get the body temperature back to normal, the circulation in the skin must open (see Special Circulations). But this compromises even further the cardiac distribution, leaving less blood to flow to the brain. The result is fainting.
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 then 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.