erection

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, often 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.

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.

The opposite term is detumescence.

Mechanism

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 due to the local production of Nitric oxide, 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.

The foreskin sometimes retracts and exposes the glans. (This depends on many factors, such as flaccid and erect lengths, the tightness of the frenar band in the foreskin, and the circumference of the corona of the glans.)

An 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^{citation needed]}, 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 males, an erection makes urination difficult or impossible.

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. Erection of the clitoris is exactly analogous to the erection of the penis only less visible due to the relatively discreet anatomical positioning of the clitoris.

Erection of nipples in both sexes, however, is not due to erectile tissue.

Culture

In several countries, movies and magazines available to juveniles may not depict erections; such depictions are often taken as one criterion to distinguish between soft and hard pornography.

Slang

As with nearly any aspect of the human body that is involved in sex, there are many slang words for an erection. Many of these are noted in the bathroom humor article.

For slang terms for the "erection", see WikiSaurus:erection — the WikiSaurus list of synonyms and slang words for erections in many languages.

Bibliography

• Harris, Robie H. (et al.), It's Perfectly Normal: Changing Bodies, Growing Up, Sex And Sexual Health. Boston, 1994. (ISBN 1-56402-199-8)

• Milsten, Richard (et al.), The Sexual Male. Problems And Solutions. London, 2000. (ISBN 0-393-32127-4)

• Tanagho, Emil A. (et al.), Smith's General Urology. London, 2000. (ISBN 0-8385-8607-4)

• Williams, Warwick, It's Up To You: Overcoming Erection Problems. London, 1989. (ISBN 0-7225-1915-X)

See also

• Priapism
• Foreskin
• Sexual function

External links

• Photographs of several different erections by size, shape, angle, etc. from a photographic website
• Sexual Medicine Society of North America's website: SexHealthMatters.org
• Patient Queries on Erection Answers from Physicians