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An erection of the penis 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 stimuli. 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. Penile erection usually results from sexual
stimulation and/or arousal, but can also occur by such causes as a
full urinary bladder or spontaneously, most commonly during erotic
or wet dreams. An erection results in swelling and enlargement of
the penis or the swelling of the female counterpart to the penis,
clitoris. Erections enable sexual intercourse and other sexual
activities (sexual functions), though it is not essential for all
sexual activities.
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In addition to sexual arousal, erection can be caused by mechanical
stimulation, or by the pressure of the filled urinary bladder. They sometimes
already occur in infant boys, and in utero. Recently some scientists have
begun to doubt this very much (sources: see below). The question of why
it is common for men to have erections when waking up (colloquially
referred to as morning wood or morning glory) is still not settled; however,
one benefit is its prevention of young boys wetting the bed when waking
up with full urinary bladders.
In males, an erection is achieved by two mechanisms that play together:
increased inflow of blood into the vessels of erectile tissue, and decreased
outflow. The vessel system involved is known as the corpora cavernosa and
the corpus spongiosum. Muscles in the region relax, allowing more blood
to enter these sponge-like tissues. Contraction of other muscles reduce
the outflow of blood from the penis. The enlarged structure then exerts
pressures on the exit veins, further reducing the outflow.
As blood flows in, the penis stiffens, its girth and length increase and
it rises to an angle that can vary between individuals from below horizontal
to almost vertical.
Certain conditions (e.g. diabetes mellitus) result in erectile dysfunction,
a problem where penile erection is insufficient to achieve normal sexual
intercourse. In recent years, several drugs have been developed for treatment
of this condition.
If present, the foreskin normally retracts and exposes the glans. The skin
of the scrotum tightens, pulling the testicles in towards the base of the
penis.
Erection is caused by signals from the parasympathetic nervous system;
it is countered by the sympathetic nervous system which is mainly responsible
for the "fight-or-flight" response. This explains why under stressful
conditions, an erection is often difficult or impossible to achieve, and
sudden onset of stress can deprive one of erection. The sympathetic
nervous system is also responsible for causing ejaculation, which explains
why most males lose their erection after ejaculation.
Erection may occur spontaneously in the absence of any specific
sexual stimulation, particularly in adolescent males. All men who are physically
able to achieve an erection do so during sleep, typically several
times in a night. This is called nocturnal penile tumescence, and typically
happens during REM sleep. The average male has an erection for a
total of 100 minutes a night, and it is not uncommon for the erection
to be present when he wakes up. This is utilized by sexual health practitioners
to ascertain whether cases of erectile dysfunction are psychological
or physiological in nature. Patients presenting with impotence are fitted
with an elastic device worn around the penis which detects changes in girth
and relays the information to a computer for later analysis. Men who obtain
spontaneous erections in their sleep but not during waking hours
are therefore considered to suffer from some form of psychosomatic condition,
i.e. sexual anxiety disorder, which prevents them performing sexually.
Masturbation has no negative impact of erectile function
besides a temporary reduction in sex drive following orgasm and ejaculation
in the same way as sexual intercourse.
In some cases penile erection may occur even after death, if pressure within
the penis increases for some reason, for example due to sinking fluids or
the formation of gases of putrefaction. See death erection.
In females, the clitoris contains erectile tissue and becomes erect during
sexual arousal. The outer lips of the vulva also become engorged with blood
filling the vestibular bulbs, making them distinctly erect and redder in
color. Erection of the clitoris is exactly analogous to the erection of
the glans only less visible due to the relatively discreet anatomical positioning
of the clitoris; erection of the vestibular bulbs is analogous to the erection
of the penile shaft.
Erection of nipples, however, is not due to erectile tissue
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