Penis blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels of the penis. The blood vessels seen here are the small arteries and veins, and capillaries, that bring blood to and from the skin and outer tissues of the penis. At the centre of the penis, below these blood vessels, is the corpus cavernosum region of the penis. This is one of two columns of erectile tissue that fill with blood during arousal, causing the penis to swell and lengthen. The penis is the male copulatory organ, which in mammals is also used for urination. The cast was made by injecting resin into the blood vessels, followed by chemical digestion of the surrounding tissues.