Blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of the blood vessels of the testes. This network of vessels infiltrate the testes' tissues, supplying them with blood. Gases and nutrients are exchanged between the blood and surrounding tissue via the permeable walls of capillaries. The testes are the male reproductive glands. They produced spermatozoa (sperm) and secrete male sex hormones (androgens). The cast was made by injecting resin into the blood vessels, followed by chemical digestion of the surrounding tissue.