The main purpose of the venous system of the general circulation is to carry oxygen-depleted blood rich in cell metabolism waste back to the heart.
It is in the legs that the stresses are the greatest and the specific characteristics of the venous system are the most important, since the venous system must move blood against the force of gravity in the standing position .
A combination of two main phenomena ensures venous return in the lower limbs:
- Firstly, the presence of mobile anti-reflux valves and the tonus of veins wall allowing the blood to move in one direction only : from the surface towards the deep system and from the foot towards the heart.
- Secondly, a pump mechanism triggering and maintaining blood flow in the veins.
The anti-reflux valves allow fluid to circulate in one direction only, making it possible to maintain the normal direction of venous blood flow, even in the absence of pressure – or in the event of negative pressure – and thereby prevent backflow of the blood.
Normal blood flow is directed from the surface towards the deep system and from the most distal part towards the heart.
Contraction of the calf muscles can produce more than 200 mmHg of pressure, which propels the blood from the venules and venous sinusoids located in the muscles to the deep veins.
Healthy valves, present throughout the venous system (deep, perforating and superficial) open and close alternately to the rhythm of these muscle contractions.
On muscle contraction, the increase in pressure in the deep veins causes them to empty. Venous blood can only be propelled in one direction thanks to the anti-reflux valves, which prevent any backflow:
- the deep system valves open, allowing venous blood to flow upwards,
- the valves of the perforating veins close, preventing the venous blood from flowing back into the superficial system.
When the muscles relax, the pressure falls in the veins of the deep system. This then fills up again with venous blood from the most distal part in the lower limb and from the superficial system. This is made possible by opening of the perforating vein valves and simultaneous closing of the deep system valves.
At rest, venous return is regulated mainly by the heart pump and respiration
The effect of the heart pump leads to a relatively low venous flow residual pressure since the pressure induced by heart beats is greatly reduced after the blood has flowed into the circulation system and, especially, into the micro-vessels of the very extensive capillary network. When it leaves the capillaries, the residual pressure of the blood is only 15 mmHg due to a loss of pressure induced by the successive reduction in vessel diameter. However, this pressure is sufficient to create a positive pressure gradient in comparison with the low pressure in the right heart.
At rest, in addition to the effect of this residual pressure, respiration also plays a role in venous return: when we inhale, the diaphragm descends, leading to an increase in intra-abdominal pressure, which slows down blood flow. Conversely, when we exhale, there is a reduction in intra-abdominal pressure, which increases blood flow.
