Venous Physiology

Mechanisms of venous return in orthostatic or supine


a) The "vis a tergo"

This is what remains of the propulsive force of left ventricular systolic, after having passed through the damping of the capillary bed. This mechanism was much more valued in ancient times to today.

b) The pulsation of the arteries paravenous:

The arterial beats can be transmitted to neighboring veins, thereby boosting blood toward the heart through the existence of the valves. Currently this facility is not given importance.

c) The "vis a fronte"

This is a suction force caused by cardiac and respiratory muscles:

- Heart muscle: the heart is a suction pump and impeller. This contraction, as known to the ancient (Erasistratus) is unimportant.

- Respiratory muscle: the rise of the diaphragm during breathing movements cause depression and abdominal and facilitates the return of venous blood by increasing the pressure gradient between the veins and right atrium. The most recent works have not provided any evidence of this action.

d) The independent tone of the venous wall:

Overall, the alpha receptors are excitatory and determine the contraction of muscle fibers, while the beta receptors are inhibitory and determine the relaxation vasodilation.

The existence in the same organ of different receivers with motor or inhibitory effect and the dominance of some over others explain why the same pharmacological agent can have opposite effects depending on the dose and circumstances.

Recent studies have shown that there are differences of tone influenced by circadian rhythms and other biorhythms most important, such as the changing of the seasons.

These mechanisms alone are not sufficient to ensure a satisfactory return of venous blood. The ongoing changes everything.

Mechanisms of venous return during walking


a) The plantar venous compression.

This squeezing action of the sole venous Lejars sangrehacia drives up, reinforcing the "vis a tergo."

b) "The calf muscle pump."

The function of this pump is crucial: in fact, muscle contractions are the engine without which the valves would not work at all.

The fascial covering of the muscles of the calf is inextensible. During contraction, increasing the volume of these muscles, the deep veins are compressed, which is squeezed like a sponge. The blood is then directed in all directions, but only channels centripetal direction by the action of the valves.

During muscle relaxation phase, there is instead a depressive action on the deep veins. The superficial veins are emptied then the network through the deep perforators.

c) Valves

In orthostatic or supine, the valves have no effect (or virtually none). Muscle contractions with valvular coaptation determine what valves become operational elements. But for valves, the blood driven by the march would be directed in all directions.

 

 
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