A varicocele is a dilated (enlarged) group of veins in the scrotum. Just like some people have dilated or "varicose" veins in their legs, some men have dilated veins in their scrotum. The dilated veins are filled with excess blood.
Every part of the body needs oxygen. Blood carries oxygen to various parts of the body through arteries. Arteries have muscular walls that pump the blood away from the heart. Veins are the channels that bring the blood back to the heart, and they do not have thick, muscular walls. Their walls are fairly thin, and the blood in them moves more passively. It is pushed back toward the heart through the pressure of the blood being pumped away from the heart and the movement of the muscles and other structures surrounding the veins pushing against the vein walls.
Interspersed along the veins are a series of one-way valves. These valves allow the blood to flow toward the heart but stop the blood from "slipping" backwards. If these valves are not working properly, blood will flow backwards and, because of gravity, the blood will collect in the most dependent (lowest) part of the pathway. In people who have large varicose veins in their legs, the valves are not functioning properly and the blood is actually pooling in their legs. Because of this, they will find that after standing for long periods of time, the blood has collected in their legs, giving them a heavy, dragging feeling. Men with varicoceles may notice a heavy, dragging, aching feeling in the scrotum ("ball sack") at the end of the day. There too, the valves in the veins that drain the blood from the testicles (the internal-spermatic veins) are not functioning properly and allow the blood to collect.
