Erection

# Overview
The erection of the penis, clitoris or a nipple is its enlarged and firm state. It depends on a complex interaction of psychological, neural, vascular and endocrine factors.
The ability to maintain the erectile state is key to the reproductive system and many forms of life could not reproduce in a natural way without this ability.

# Penis erection
A penis 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 man has ejaculated during sexual encounter or masturbation, his erection usually ends, but this may take time depending on the length and thickness of the penis.
Penis erection usually results from exposure to sexual stimulation from sexual arousal, but can also occur by such causes as a full urinary bladder or spontaneously during the course of a day or at night, often during REM sleep (see "nocturnal penile tumescence"). An erection results in swelling, hardening and enlargement of the penis. Erection enables sexual intercourse and other sexual activities (sexual functions), though it is not essential for all sexual activities. An erection may also occur once woken up, called nocturnal penile tumescence. The scrotum may also become tightened during an erection.

## Autonomic control
In the presence of mechanical stimulation, erection is initiated by the parasympathetic division of the autonomic nervous system (ANS) with minimal input from the central nervous system.  Parasympathetic branches extend from the sacral plexus into the arteries supplying the erectile tissue; upon stimulation, these nerve branches initiate the release of nitric oxide, a vasodilating agent, in the target arteries.  The arteries dilate, filling the corpora spongiosum and cavernosa with blood.  Erection subsides when parasympathetic stimulation is discontinued; baseline stimulation from the sympathetic division of the ANS causes constriction of the penile arteries, forcing blood out of the erectile tissue.
The cerebral cortex can initiate erection in the absence of direct mechanical stimulation (in response to visual, auditory, olfactory, imagined, or tactile stimuli) acting through erectile centers in the lumbar and sacral regions of the spinal cord.  The cortex can suppress erection even in the presence of mechanical stimulation, as can other psychological, emotional, and environmental factors.
The opposite term is detumescence.

## Shape and size
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 (see image), 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 the procedure is controversial, and mostly the results are disappointing. Penile enlargement is a controversial subject. See Penis enlargement.

## Erectile dysfunction
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 also may cause erectile dysfunction in patients who received them such as lithium, paroxetine, etc.
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; often unnecessary since in most cases the matter can be helped. 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.
The study of erectile dysfunction within medicine is covered by andrology, a sub-field within urology.

# Clitoral erection
Clitoral erection is a part of sexual arousal in women. The clitoris is the anatomically homologous counterpart of the penis, and the physiological mechanism of its erection is similar.
Swelling and enlargement may also occur during a clitoral erection but because a large proportion of the clitoris is within the body and its drastically reduced size, it is often not as obvious.

# Nipple erection
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.