The Small Saphenous Vein (SSV) is the second main superficial vein in the leg (the main superficial vein in the leg being called the Great Saphenous Vein [GSV]). It is called a "truncal" vein as it is one of the long straight veins in the leg. The great saphenous vein is the other truncal vein in the leg.
The Small Saphenous Vein has a major role in draining blood from the foot and from the skin and fat of the back of the lower leg (calf).
Like all of the veins in the leg, the Small Saphenous Vein transmits the venous blood from the extremity of the leg towards the heart. All living tissues need to be given nutrition by arterial blood. This is pumped into the tissues, both deep and superficial, via arteries and then capillaries. When the blood has given its oxygen and nutrition to the tissues, it picks up carbon dioxide and other waste products to be transported away in venules and then the veins. Thus venous blood is what removes the cells waste products, called the products of cellular respiration.
As all tissues in the leg need a blood supply, whether we think of skin, fat, muscle and even bone, we have to think of the way that venous blood gets back to the heart. The simple way of thinking of this is that the veins have to help the blood to travel "inwards" and" upwards". The deep veins of the leg are in the muscle which provide the muscle pump. When the muscles contract, blood in the deep veins is squirted under high pressure up through the veins of the pelvis and abdomen and to the heart.
Venous blood in the outside of the foot, and in the skin and fat of the lower leg at the back and on the lateral side, drain via small veins into the Small Saphenous Vein. It is the Small Saphenous Vein that pumps this venous blood up the back of the leg and deposits it into the deep veins system, ready to be pumped on up to the heart.
As with all veins in the legs, in order to be able to function properly, it has to have valves. These are simple bicuspid or 2 leaf valves that let blood pass up the vein during movement, but close and stop it falling back down the vein by gravity at rest.