>>4056716 Anerection(clinically:penile erectionorpenile tumescence) is a physiological phenomenon in which thepenisbecomes firmer, engorged and enlarged. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is often associated withsexual arousalorsexual attraction, although erections can also be spontaneous. The shape, angle and direction of an erection varies considerably in humans.
Physiologically, erection is triggered by theparasympatheticdivision of theautonomic nervous system(ANS), causingnitric oxide(avasodilator) levels to rise in thetrabeculararteriesandsmooth muscleof the penis. The arteriesdilatecausing thecorpora cavernosaof the penis (and to a lesser extent thecorpora spongiosum) to fill withblood; simultaneously theischiocavernosusandbulbospongiosusmuscles compress theveinsof 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, includingsexual stimulationandsexual arousal, and is therefore not entirely under conscious control. Erections during sleep or upon waking up are known asnocturnal penile tumescence(NPT). Absence of nocturnal erection is commonly used to distinguish between physical and psychological causes oferectile dysfunctionandimpotence.
A penis which is partly, but not fully, erect is sometimes known as a semi-erection (clinically:partial tumescence); a penis which is not erect is typically referred to as being flaccid, or soft.
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