The erection of the penis, clitoris or a nipple is its enlarged and firm state. It depends on a complex interaction of psychological, neural, vascular and endocrine factors. The ability to maintain the erectile state is key to the reproductive system and many forms of life couldn't reproduce in a natural way without this ability. Penis erection A penis erection occurs when two tubular structures that run the length of the penis, the corpora cavernosa, become engorged with venous blood. This may… (More on Erection)