Penis erectile tissue. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels of 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.