Background

Veins are vessels that carry blood from the tissues back to the heart. They have walls which are made up of three layers, an inner endothelial layer, a middle muscular layer and an outer connective tissue layer. As veins do not carry blood under direct pressure from the heart the walls are much thinner than those of arteries. They contain valves formed from the inner lining spaced at intervals within the cavity and these help prevent blood flowing backwards and facilitate forward flow. When standing erect the veins draining the lower limbs have to carry blood upwards against the pressure of gravity. This is facilitated by the effect of contraction of surrounding muscles.

Veins run both close to the surface below the skin (superficial veins) and deeper in the tissue between the muscles (deep veins).

The walls of the veins have their own blood supply and also supplied by nerves controlled by the autonomic nervous system. Contraction of the smooth muscle within the middle layer can reduce the size of the lumen (the hollow part of the tube) to vary the flow. The tone (or rigidity) of the vessel wall is controlled by the nerve fibres.