A penile erection 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 physiological stimuli, also known as sexual arousal. 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. After a male has ejaculated during a sexual encounter or masturbation, his erection usually ends, but this may take time depending on the length and thickness of the penis.
Penile erection usually results from exposure to sexual stimulation from sexual arousal, and can also occur due to a full urinary bladder. In some males, erection can occur spontaneously at any time of day; it is known as nocturnal penile tumescence when occurring during REM sleep.
An erection results in swelling, hardening and enlargement of the penis, enabling sexual intercourse. The scrotum may also become tightened during an erection. Erection is not required for all sexual activities.
An erect penis can take on a number of different shapes and angles, ranging from a straight tube angled at a 45-90 degree angle, to a curvature to the left or right, up or down. A tightly curved penis, known as Peyronie's disease, is identified by a severe curve in the erect penis. This may cause physical and psychological effects for the affected individual, which could include erectile dysfunction or pain during erection. Treatments include oral medication (such as Vitamin E) or surgery, which is most often reserved as a last resort.
Generally, the size of an erect penis is fixed throughout post-pubescent life. Its size may be increased by surgery, although penile enlargement is controversial, and a majority of men were "not satisfied" with the results, according to one study.
Erectile dysfunction (also known as ED or '(male) impotence') is a sexual dysfunction characterized by the inability to develop or maintain an erection. It can occur due to both physiological and psychological reasons, most of which are amenable to treatment. Common physiological reasons include cardiovascular leakage and diabetes. Some drugs used to treat other conditions, such as lithium and paroxetine, may cause erectile dysfunction.
Erectile dysfunction, tied closely as it is to cultural notions of potency, success and masculinity, can have devastating psychological consequences including feelings of shame, loss or inadequacy; There is a strong culture of silence and inability to discuss the matter. In fact, around 1 in 10 men will experience recurring impotence problems at some point in their lives.
Swelling and enlargement may also occur during a clitoral erection but because a large proportion of the clitoris lies within the body and because of its smaller size, it is often not as obvious.
Nipple erection may result from three kinds of response. It happens in females during breast feeding. It is also an early part of the sexual response in females and males. Both of these are caused by the release of oxytocin. Nipple erection can also be caused by a tactile response to cold temperature in both males and females. The erection of nipples is not due to erectile tissue, but due to the contraction of smooth muscle under the control of the autonomic nervous system. It is more akin to a hair follicle standing on end than to a sexual erection.