The superficial system lies above the muscle fascia in the subcutaneous tissues. The principal veins in the superficial system are the great saphenous vein (GSV) and the small saphenous vein (SSV).

except for the pulmonary vein) Allows diffusion of gases and nutrients from blood into the body cells Wall Thick, muscular Thinner Very thin, one cell thick Lumen Small Large Very small ...

It's called deep vein thrombosis, or DVT. All our veins have small valves powered by the muscles around them. They keep blood moving toward the heart, instead of flowing backward inside veins.

except for the pulmonary vein) Allows diffusion of gases and nutrients from blood into the body cells Wall Thick, muscular Thinner Very thin, one cell thick Lumen Small Large Very small ...