Thermographic footage of a human male penis in the flaccid and erect states. When flaccid the penis is relatively cool, as it is largely external and cooled by the surrounding air. When erect and full of blood it is much warmer, as seen by the white colour. An erection is produced by the admission of venous blood into spongy tissues called the corpora cavernosa. Muscles at the base of the penis contract to prevent the backward flow of blood, leading to the penis remaining engorged.