Physiology of the venous system in the lower limbs
The main purpose of the venous system within the general circulation, is to carry oxygen-depleted blood rich in cell metabolism waste back to the heart. It is within 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 actions ensures venous return in the lower limbs:
Firstly, the presence of mobile anti-reflux valves and the resistance of the vein walls allowing the blood to move in one direction only : from the superficial to towards the deep venous system and from the feet to the heart.
Secondly, a pump mechanism which activates and maintains the blood flow through 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 superficial towards the deep system and from the most distal part of the body towards the heart.
The pump mechanism mainly results from a combination of different forces:
The stimulation of the venous system of the foot
The muscle pump, more specifically, the muscles of the calf (leading to alternate opening and closing of the valves): which is the main driving force behind the pump mechanism,
The beating of the heart and the negative pressure due to the phenomenon of aspiration from the abdominal cavity that occurs during deep breathing.
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 the 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 on venous flow leads to a relatively low residual pressure. Once the blood has flowed into the circulatory system and, especially, through the micro-vessels of the very extensive capillary network, the pressure induced by the beating of the heart is greatly diminished. When it leaves the capillaries, the residual pressure of the blood is only 15 mmHg due to a loss of pressure caused by the successive reduction in vessel diameter. However, this pressure is sufficient to create a positive pressure gradient in comparison with the low pressure present in the right side of the 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.
A combination of two main actions ensures venous return in the lower limbs: 