The exact reason for the development of venous ulcers has not yet been clearly established but the main principle behind it is the stasis of blood at the periphery. Veins are these expansible and collapsible conduits in our body which carry blood from different parts to the heart. Blood in these vessels essentially needs to be driven against the strong forces of gravity in the lower limbs and hence, valves present along the course of the veins aid in the unidirectional flow towards the heart. This happens to be nature’s way of preventing the stasis of blood. However, once this protective mechanism is disrupted, it causes the pooling of fluid in the legs.

So what is causing the impaired circulation? It has been proposed that it is due to the following two main reasons:

1) Incompetence of the valves –
Faulty valves allow for the backflow of blood. More and more blood begins to accumulate in the expansible veins, leading to its stasis and an increase in pressure on the vessel walls. This condition is known as “ambulatory chronic venous hypertension” and is associated with the varicosity of veins.

2) Chronic venous hypertension could also arise in a “post-phebitic limb.”
To understand this, let’s first get acquainted with a condition known as Deep vein thrombosis (DVT). In this, a blood clot (thrombus) is formed in one or more deep veins of the body, usually in the legs. Post-phebitic limb consists of veins that have been partially recanalized following DVT which causes increased venous pressure, especially around the ankles (through perforators.)

So once sufficient pressure builds up in the venous system, proteins and fluid begin to leak out of the capillaries and occupy the spaces between the cells. The damage cascades further in the following manner:

Certain leaked substances are potentially toxic (eg: Haemosiderin from lysed RBCs, reactive oxygen species and various other inflammatory mediators from WBCs.) They irritate the skin, causing it to get inflamed. Along with the skin, even the subcutaneous tissue is affected. The body tries to heal the injury through fibrosis which basically is a mesh-like patch work. This impedes the supply of oxygen and nutrients to the tissue, depriving it of essential nutrition needed to remain viable. The devitalized tissue slowly breaks down, leaving behind massive, non-healing ulcers which are intially painful but eventually turn painless. They may even get secondarily infected, the chances of which are higher in the presence of co-morbidities like diabetes mellitus.

Wounds of the distal lower extremities arising from causes other than the ones directly related to venous insufficiency (e.g., scratch, bite, burn, or surgical incision) may ultimately fail to heal if underlying (often undiagnosed) venous disease is not properly addressed.