Skin is a complex organ that covers almost all of the body surface. It has many functions but, as far as thread veins are concerned, its role as a regulator of body heat is important.

The skin has a blood supply from many small arterioles - these are small blood vessels that take blood at higher pressure into the capillary networks.

The capillary networks are made up of a great many tiny vessels with very thin walls.

From these networks the blood flows back into venules - these are small veins that take the blood back to the larger veins and then ultimately the heart.

Blood flow in the capillary networks can be controlled by the body using both hormones and nerves (the sympathetic nervous system).

When your body is too hot, more blood is allowed into the capillary networks and so more heat is lost to the air around the body. When this happens, the skin goes pink or red (often called flushing or blushing).

When your body is too cold, blood is not allowed into the capillary networks and so heat is kept in the body and the skin acts as insulation - this makes the skin look very white.

Thread veins of the LEGS occur when the VENULES dilate (get bigger) and become big enough to be visible through the skin. Thread veins of the FACE are usually more related to increased flow in the feeding Arteriole end of the network.