ERECTION

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 could not reproduce in a natural way without this ability.

A penile erection occurs when two tubular structures that run the length of the penis, the corpora cavernosa, become engorged with venous blood. This may result from any of various physiological stimuli, also known as sexual arousal. The corpus spongiosum is a single tubular structure located just below the corpora cavernosa, which contains the urethra, through which urine and semen pass during urination and ejaculation, respectively. This may also become slightly engorged with blood, but less so than the corpora cavernosa. After a male has ejaculated during a sexual encounter or masturbation, his erection usually ends, but this may take time depending on the length and thickness of the penis.

Penile erection usually results from exposure to sexual stimulation from sexual arousal, and can also occur due to a full urinary bladder. In some males, erection can occur spontaneously at any time of day; it is known as nocturnal penile tumescence when occurring during REM sleep.

An erection results in swelling, hardening and enlargement of the penis, enabling sexual intercourse. The scrotum may also become tightened during an erection. Erection is not required for all sexual activities.