Blood supply to a rat's testis, coloured scanning electron micrograph (SEM). The ideal temperature for the development of sperm cells is slightly lower than body temperature. The circulatory system of testes is adapted to ensure that the testes are kept slightly cooler. Blood arrives at the testis through an artery (red) and leaves through the network of veins (yellow). The network covers much of the artery, providing a large surface area for heat exchange. As the venous blood is slightly cooler, this system cools the arriving arterial blood. This is known as counter- current heat exchange. This is a resin cast. Magnification: x40 when printed 10cm high.