An erection of the penis, clitoris or a nipple is its enlarged and firm state. It is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is usually, though not exclusively, associated with sexual arousal.

The maintenance of the erect penis is required for ejaculation, an important aspect of reproduction, and many forms of life could not reproduce in a natural way without this ability.

Penile erection

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 stimulation and 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 can also occur due to a full urinary bladder. In some males, erection can occur spontaneously at any time of day, and 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.

Autonomic control

In the presence of mechanical stimulation, erection is initiated by the parasympathetic division of the autonomic nervous system (ANS) with minimal input from the central nervous system. Parasympathetic branches extend from the sacral plexus into the arteries supplying the erectile tissue; upon stimulation, these nerve branches initiate the release of nitric oxide, a vasodilating agent, in the target arteries. The arteries dilate, filling the corpora spongiosum and cavernosa with blood. Erection subsides when parasympathetic stimulation is discontinued; baseline stimulation from the sympathetic division of the ANS causes constriction of the penile arteries, forcing blood out of the erectile tissue. The cerebral cortex can initiate erection in the absence of direct mechanical stimulation (in response to visual, auditory, olfactory, imagined, or tactile stimuli) acting through erectile centers in the lumbar and sacral regions of the spinal cord. The cortex can suppress erection even in the presence of mechanical stimulation, as can other psychological, emotional, and environmental factors. The opposite term is detumescence.