CVI is a condition when the venous wall and/or venous valves within the arteries of the legs are not working properly, making it difficult for the blood to return to the heart from the legs. This causes pooling of the blood in the leg veins and this pooling is called stasis.
The veins return blood low in oxygen because it has been used up by the tissues of the body. This blood has to return from the legs against gravity. When we walk the calf and foot muscles contract with each step squeezing the thin walled veins. To prevent the blood moving downwards due to the effect of gravity we have one-way valves in the leg veins