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, sexual attraction or libido, although erections can also be spontaneous. The shape, angle, and direction of an erection vary considerably between humans.
This article is about penile erection. For the similar arousal in females, see clitoral erection. For other uses, see Erection (disambiguation).Erection
Physiologically, an erection is required for a male to effect penetration or sexual intercourse and is triggered by the parasympathetic division of the autonomic nervous system, causing the levels of nitric oxide (a vasodilator) to rise in the trabecular arteries 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.
Socio-sexual aspects
Social
Though an erection can have many causes, it's most commonly seen as an indicator of sexual arousal and is therefore considered taboo or inappropriate for a public setting in many societies. This taboo is lesser than that surrounding public sex but higher than that surrounding nudity. Erectile dysfunction is often considered a flaw, eliciting shame in individuals affected.
The penile plethysmograph, which measures erections, has been used by some governments and courts of law to measure sexual orientation. An unusual aversion to the erect penis is sometimes referred to as phallophobia.