Both the artery and vein that supply blood to the testicle begin in the region of the kidney and therefore are very long. There are two ways that abnormalities in this system can cause fertility problems. First, as the vein passes through the groin, it branches and surrounds the artery.
Venous blood is cooler than arterial blood.
As a result, the warm arterial blood passes some of its heat off to the veins as it passes through the branched veins. By the time the blood reaches the testicle, it has cooled to the temperature of venous blood, allowing the arterial blood entering the testicle to be cooler than arterial blood in the rest of the body. If these veins are dialated, they don't work correctly, causing the arterial blood to arrive at the testicles without being cooled. Second, in a man with normal anatomy, the veins carry the used blood away quickly and efficiently. In a patient with a varicocele however, the testicular veins are large and dilated, and unable to cool the artery or to carry the venous blood away.