Erection, in the sexual sense, is the hardening, enlarging and rising of the penis which often occurs in the sexually aroused male. Erection enables sexual intercourse and some other sexual activities, though it is not essential for all of them.
In addition to sexual arousal, erection can be caused by friction, retraction of the foreskin, or by the pressure of the filled urinary bladder. In healthy males, erections occur several times per night during the REM phases of sleep, and erections when waking up are common. These "unprovoked erections" are normal and a good sign that the erection pathways are functioning well. Erections already occur in infant boys, and in utero.
Physiologically, 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. The enlarged structure then exerts pressures on the exit vein, further reducing the outflow.
As blood flows in, the penis stiffens, its girth and length increases, and it rises to an angle that can vary from below horizontal to almost vertical. If present, the foreskin normally retracts and exposes the glans.
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. The sympathetic nervous system is also responsible for causing ejaculation, which explains why most males lose their erection after ejaculation.
When the erection mechanism fails to function properly, the result is impotence. Impotence in humans is treated with prosthetic devices, and with prescription drugs. Some prescription drugs are injected directly into the corpus cavernosum or used as urethral suppositories, directly causing erection even in the absence of sexual excitation, while others, such as Viagra, are taken orally and support erection that is due to sexual excitation.
Both an erection and the absence of an erection can be embarrassing, depending on circumstances.
An erection may be embarrassing:
- if the man does not want to reveal that he is sexually aroused
- if the erection is not caused by sexual arousal, but just by friction etc., and the man does not want to create the wrong impression that he is sexually aroused
- if the man is attempting to urinate and the penis will not point down.
- in a situation where an erection is inappropriate, such as in a classroom or a public place.
- if it is a teenager in a PE lesson, and he is naked in the showers
Not having an erection may be embarrassing:
- if a man is with a partner and it wrongly creates the impression that he does not find the other person attractive
- if a man is trying to have sex and cannot produce an erection
- if a man is trying to masturbate
- if a man is trying to impress his friends
- If a man is trying to impress a partner or potential partner
Also it may be disappointing for the partner because he or she finds an erection attractive, and disappointing for both for being limited in sexual activities.
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. Erections are also not encouraged in nude beaches and men are supposed to cover their erections until they go away.
Occasionally, circumcision causes the skin of the erect penis to be very tight and immobile on the shaft.
Erections may occur even after death, if the pressure within the penis increases for some reason, for example due to sinking fluids or the formation of gases of putrefaction. See death erection.
The clitoris of females also contains erectile tissue and may become erect during sexual arousal; the erection of nipples, however, is not due to erectile tissue.