When a man is sexually stimulated by sight, thought, or touch, the brain sends signals that relax the smooth muscles around the arteries that supply blood to the spongy and cavernous bodies. The veins draining the bodies can't keep up, resulting in swelling. As the swelling reaches the limit of the penile skin, the penis becomes firm. The pressure of the spongy and cavernous bodies against the skin partially closes the veins, helping to maintain the erection. Erection continues until the signals from the brain stop, but erections are not consistent; waking and waning are normal, even during intercourse.
