Venous return is the term that we used to describe ‘returning back of the blood into the right atrium via the venous system. Being a more passive type of activity than the much forceful cardiac ejection of arterial blood, it depends on several other body functions to maintain its flow. Thus, this article hopes to describe some of these factors which will affect the venous return at different levels.
First of all, when looking at the anatomy of the venous system, each of these veins, particularly the larger veins in the legs would be having a valve system which would only allow blood to flow in one direction and prevents the blood from returning back to its previous place. Thus, it effectively negates the effect of the gravity and facilitates whatever the mechanism tries and propels the venous blood towards the heart. At the same time, many of these peripheral large veins would be located deep within the leg musculature and thus would be influenced by the surrounding muscles when they contract as in the case of walking, running, jumping…etc. With this brief description about the arrangement of peripheral venous systems, especially regarding the arrangement in the legs, let us now take a look at what factors would facilitate the venous return.
Muscle pump : As described above, when the muscles of the lower limb contracts, it will exert pressure on the veins that are located within the musculature and this would propel the blood to a higher level. Due to the presence of one way valves, the blood would not return back when the muscles relax. This mechanism would be deranged in persons who are immobilized due to certain reasons or in case the valves are incompetent, which can therefore give rise to signs such as leg edema.
Respiratory pump : Another very important factors which affects the venous return would be the pressure changes within the thoracic cavity. As we all known, the lungs, the heart, the great vessels…etc would be lying in the thoracic cavity surrounded by the pleura. The intra-pleural pressure have shown a relationship with the venous return as the negative pressure within the pleural space as in the case of inspiration have shown to facilitate expansion of the cardiac chambers and the vasculature in the thoracic cavity. The net effect based on the frank starling laws would be an increase pressure gradient between the cardiac chambers and the returning venous blood and therefore an increased venous return. Although this should be the other way round when a person is in expiration, the mechanisms are such that the venous return would be maintained even though to a lesser amount than that when the inspiration is taking place.
Apart from the above mentioned factors, there are several other factors which can affect a persons venous return and among them, the amount of fluid maintained as the intra-vascular volume, the efficiency of the heart in attracting venous blood, gravitational forces…etc are few to name.