Why can blue veins be seen on legs?
One of the main tasks of the human circulation is to transport oxygen and nutrients to body organs and peripheries such as the arms and legs. The structures involved can be broadly divided into arteries, capillaries and veins.
Blood is propelled through the arteries by pressure produced from the beating heart; that pressure is further maintained by the elastic nature of healthy arteries and their smaller branches (arterioles). Blood then passes through the capillary bed; a network of tiny capillaries within tissues (such as muscle and skin in the legs and feet). The capillaries have extremely thin walls (membranes) which allow oxygen and nutrients to pass into and supply the tissues. Blood returns to the heart via the low pressure veins with walls that are much thinner but tend to be larger in diameter than arteries.
This is why it is common to be able to see veins under the skin as the thin walls are more transparent to the blood they contain; it is rare to see an artery through the surface of the skin.
The colour of the blood varies with the amount of oxygen it contains. Oxygenated blood in arteries is bright red in colour whereas deoxygenated blood within the veins is a much darker and more blue colour. As a vascular surgeon I perform operations and procedures on both arteries and veins; I can identify the nature of an injured vessel by the colour of the blood.
Healthy leg veins
The contraction of muscles within the feet and the calves propel the blood back up through the veins towards the heart against gravity. A series of one-way valves allows the blood to travel in the correct direction.
The muscle compartments of the leg are surrounded by thick fibrous tissue called deep fascia. The venous system of the leg is broadly divided into two parts. The deep veins lie deep to this fascia and are in close proximity to the main arteries. The superficial veins are situated outside the deep fascia.
The long saphenous vein travels from the ankle up the inside of the calf and join the deep femoral veins at the groin.
The short saphenous vein travels from the outside of the ankle at the back of the calf and drain into the deep popliteal vein behind the knee.
The anterior thigh vein travels from the front and outside areas of the shin and thigh to drain into the deep femoral vessels at the groin.
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