Erection

a human penis first flaccid, then erect.

An erection, clinically: penile erection or penile tumescence is a physiological phenomenon in which the penis becomes firm, engorged, and enlarged. Penile erection is the result of a complex interaction of psychological, neural, vascular, and endocrine factors, and is often associated with sexual arousal or sexual attraction, although erections can also be spontaneous. The shape, angle, and direction of an erection vary considerably between humans. The state of a penis which is partly, but not fully, erect is sometimes known as semi-erection; a penis that is not erect is typically referred to as being flaccid, or soft.

Physiology

Physiologically, an erection is required for a male to effect vaginal penetration or sexual intercourse and is triggered by the parasympathetic division of the autonomic nervous system, causing the levels of nitric oxide to rise in the trabecular arteries and smooth muscle of the penis. The arteries dilate causing the corpora cavernosa of the penis to fill with blood; simultaneously the ischiocavernosus and bulbospongiosus muscles compress the veins of the corpora cavernosa restricting the egress and circulation of this blood. The erection subsides when parasympathetic activity reduces to baseline.

Terminology

Clinically, erection is often known as "penile erection", and the state of being erect, and the process of erection, are described as "tumescence" or "penile tumescence". The term for the subsiding or cessation of an erection is "detumescence". Colloquially and in slang, erection is known by many informal terms. Commonly encountered English terms include 'stiffy', 'hard-on', 'boner' and 'woody'. There are several slang words, euphemisms, and synonyms for an erection in English and in other languages.

Human Penis Size

The length of the flaccid penis is not necessarily indicative of the length of the penis when it becomes erect, with some smaller flaccid penises growing much longer, and some larger flaccid penises growing comparatively less. 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. Though the size of a penis varies considerably between males, the average length of an erect human penis is 13.12 cm (5.17 inches), while the average circumference of an erect human penis is 11.66 cm (4.59 inches).

Spontaneous Erections

Spontaneous erections, also known as involuntary, random, or unwanted erections, are commonplace and a normal part of male physiology. Socially, such erections can be embarrassing if they happen in public or when undesired. Such erections can occur at any time of day, and if clothed may cause a bulge which can be disguised or hidden by wearing close-fitting underwear, a long shirt, or baggier clothes. Few slang terms associated with erect penis outline becoming visible from underneath clothing are mooseknuckle and man bulge.

Erectile Dysfunction

Erectile dysfunction is a sexual dysfunction characterized by the inability to develop and/or maintain an erection. The study of erectile dysfunction within medicine is known as andrology, a sub-field within urology. Erectile dysfunction occurs for a variety of reasons. Ultimately, the cause for erectile dysfunction is that not enough nitric oxide (NO) is released by the vascular endothelium of the branches of the perineal artery, a branch of the internal pudendal artery. Erectile dysfunction may occur due to physiological or psychological reasons, most of which are amenable to treatment.