Erection

An erection (clinically: penile erection or penile tumescence) is a physiological phenomenon in which the penis becomes firm, engorged, and enlarged. Penile erection is the result of a complex interaction of psychological, neural, vascular, and endocrine factors, and is often associated with sexual arousal or sexual attraction, although erections can also be spontaneous. The shape, angle, and direction of an erection varies considerably in humans.

Physiological phenomenon in which penis becomes firm
This article is about penile erection. For the similar arousal in females, see clitoral erection. For other uses, see Erection (disambiguation).

Erection

Three columns of erectile tissue make up most of the volume of the penis.
Identifiers
MeSH D010410
TE E1.0.0.0.0.0.8
Anatomical terminology

Erection blood vessels
Identifiers
MeSH D010410
TE E1.0.0.0.0.0.8
Anatomical terminology

Physiologically, erection is triggered by the parasympathetic division of the autonomic nervous system, causing the levels of nitric oxide (a vasodilator) to rise in the trabeculararteries and smooth muscle of the penis. The arteries dilate causing the corpora cavernosa of the penis (and to a lesser extent the corpus spongiosum) to fill with blood; simultaneously the ischiocavernosus and bulbospongiosus muscles compress the veins of the corpora cavernosa restricting the egress and circulation of this blood. Erection subsides when parasympathetic activity reduces to baseline.

As an autonomic nervous system response, an erection may result from a variety of stimuli, including sexual stimulation and sexual arousal, and is therefore not entirely under conscious control. Erections during sleep or upon waking up are known as nocturnal penile tumescence (NPT), also known as “morning wood”. Absence of nocturnal erection is commonly used to distinguish between physical and psychological causes of erectile dysfunction and impotence.

The state of a penis which is partly, but not fully, erect is sometimes known as semi-erection (clinically: partial tumescence); a penis which is not erect is typically referred to as being flaccid, or soft.

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Erection Stages
Side views and comparison of the stages of an uncircumcised (top) and a circumcised (bottom), human penis erection.

An erection occurs when two tubular structures, called the corpora cavernosa, that run the length of the penis, 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.

The scrotum may, but not always, become tightened during erection. Generally, in uncircumcised males, the foreskin automatically and gradually retracts, exposing the glans, though some men may have to manually retract their foreskin.

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