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Figure 28 01 06.jpg
Three columns of erectile tissue make up most of the volume of the penis.
MeSH D010410
Anatomical terminology

Erection blood vessels
MeSH D010410
Anatomical terminology

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 varies considerably in humans.


Physiologically, erection is triggered by the parasympathetic division of the autonomic nervous system, causing the levels of nitric oxide (a vasodilator) to rise in the trabecular arteries and smooth muscle of the penis. The arteries dilate causing the corpora cavernosa of the penis (and to a lesser extent the corpus spongiosum) 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. Erection subsides when parasympathetic activity reduces to baseline.

As an autonomic nervous system response, an erection may result from a variety of stimuli, including sexual stimulation and sexual arousal, and is therefore not entirely under conscious control. Erections during sleep or upon waking up are known as nocturnal penile tumescence (NPT). Absence of nocturnal erection is commonly used to distinguish between physical and psychological causes of erectile dysfunction and impotence.

The state of a penis which is partly, but not fully, erect is sometimes known as semi-erection (clinically: partial tumescence); a penis which is not erect is typically referred to as being flaccid, or soft.


Composite image showing the development of a penile erection Erection Development V2.jpg
Composite image showing the development of a penile erection

An erection occurs when two tubular structures, called the corpora cavernosa, that run the length of the penis, become engorged with venous blood. This may result from any of various physiological stimuli, also known as sexual stimulation and 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.

The scrotum may, but not always, become tightened during erection. Generally, in uncircumcised males, the foreskin automatically and gradually retracts, exposing the glans, though some men may have to manually retract their foreskin.

Autonomic control

In the presence of mechanical stimulation, erection is initiated by the parasympathetic division of the autonomic nervous system 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 release acetylcholine, which in turn causes release of nitric oxide from endothelial cells in the trabecular arteries. [1] Nitric oxide diffuses to the smooth muscle of the arteries (called trabecular smooth muscle [2] ), acting as a vasodilating agent. [3] The arteries dilate, filling the corpus spongiosum and corpora cavernosa with blood. The ischiocavernosus and bulbospongiosus muscles also compress the veins of the corpora cavernosa, limiting the venous drainage of blood. [4] Erection subsides when parasympathetic stimulation is discontinued; baseline stimulation from the sympathetic division of the autonomic nervous system causes constriction of the penile arteries and cavernosal sinosoids, forcing blood out of the erectile tissue via erection-related veins which include one deep dorsal vein, a pair of cavernosal veins, and two pairs of para-arterial veins between Buck's fascia and the tunica albuginea. [5] [6] Erection rigidity is mechanically controlled by reduction blood flow via theses veins, and thereby building up the pressure of the corpus cavernosum and corpus spongiosum, an integral instructure, the distal ligament, buttresses the glans penis. [7]

After ejaculation or cessation of stimulation, erection usually subsides, but the time taken may vary depending on the length and thickness of the penis. [8]

Voluntary and involuntary control

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.[ citation needed ] The cortex may suppress erection, even in the presence of mechanical stimulation, as may other psychological, emotional, and environmental factors.[ citation needed ]

Nocturnal erection

The penis may become erect during sleep or be erect on waking up. Such an erection is medically known as nocturnal penile tumescence (informally: morning wood or morning glory). [9] [10] [11] [12]

Socio-sexual aspects


An erection is a common indicator of sexual arousal and is required for a male to effect vaginal penetration or sexual intercourse. An erection is necessary for natural insemination as well as for the harvesting of sperm for artificial insemination.

Erections are common for children and infants, and even occur before birth. [13] After reaching puberty, erections occur much more frequently. [14] The penile plethysmograph, which measures erections, has been used by some governments and courts of law to measure sexual orientation. An unusual aversion to the erect penis is sometimes referred to as phallophobia. [15]

Spontaneous or random erections

Visible erection through clothing.jpg
Erection visible beneath clothing

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. [14] Such erections can occur at any time of day, and if clothed may cause a bulge which (if required) can be disguised or hidden by wearing close-fitting underwear, a long shirt, or baggier clothes. [16]


An erect human penis. The pubic hair has been removed. Rasiertes erigiertes Glied.jpg
An erect human penis. The pubic hair has been removed.

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, while some larger flaccid penises growing comparatively less. [17] Generally, the size of an erect penis is fixed throughout post-pubescent life. Its size may be increased by surgery, [18] although penile enlargement is controversial, and a majority of men were "not satisfied" with the results, according to one study. [19]

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). [20]


Although many erect penises point upwards, it is common and normal for the erect penis to point nearly vertically upwards or nearly vertically downwards or even horizontally straight forward, all depending on the tension of the suspensory ligament that holds it in position. An erect penis can also take on a number of different shapes, ranging from a straight tube to a tube with a curvature up or down or to the left or right. An increase in penile curvature can be caused by Peyronie's disease. This may cause physical and psychological effects for the affected individual, which could include erectile dysfunction or pain during an erection. Treatments include oral medication (such as colchicine) or surgery, which is most often performed only as a last resort.

The following table shows how common various erection angles are for a standing male. In the table, zero degrees (0°) is pointing straight up against the abdomen, 90° is horizontal and pointing straight forward, and 180° is pointing straight down to the feet. An upward pointing angle is most common.

Occurrence of erection angles [21]
Angle (°)Percent of population

Medical conditions

Erectile dysfunction

Erectile dysfunction (also known as ED or "(male) impotence") is a sexual dysfunction characterized by the inability to develop and/or maintain an erection. [22] [23] The study of erectile dysfunction within medicine is known as andrology, a sub-field within urology. [24]

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. Common physiological reasons include diabetes, kidney disease, chronic alcoholism, multiple sclerosis, atherosclerosis, vascular disease -including arterial insufficiency and venogenic erectile dysfunction- [25] , and neurologic disease which collectively account for about 70% of ED cases. [3] Some drugs used to treat other conditions, such as lithium and paroxetine, may cause erectile dysfunction. [23] [26]

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. [27] There is a strong culture of silence and inability to discuss the matter. Around one in ten men experience recurring impotence problems at some point in their lives. [28]


Priapism is a painful condition in which the penis does not return to its flaccid state, despite the absence of both physical and psychological stimulation. Priapism lasting over four hours is a medical emergency.

In non-human animals

An erect horse penis Medimurski konj (vranac) na sajmu MESAP 2015.jpg
An erect horse penis

At the time of penetration, the canine penis is not erect, and only able to penetrate the female because it includes a narrow bone called the baculum, a feature of most placental mammals. After the male achieves penetration, he will often hold the female tighter and thrust faster, and it is during this time that the male's penis expands. Unlike human sexual intercourse, where the male penis commonly becomes erect before entering the female, canine copulation involves the male first penetrating the female, after which swelling of the penis to erection occurs. [29]

An elephant's penis is S-shaped when fully erect and has a Y-shaped orifice. [30]

Given the small amount of erectile tissue in a bull's penis, there is little enlargement after erection. The penis is quite rigid when non-erect, and becomes even more rigid during erection. Protrusion is not affected much by erection, but more by relaxation of the retractor penis muscle and straightening of the sigmoid flexure. [31] [32]

A male fossa's penis reaches to between his forelegs when erect. [33]

When not erect, a horse's penis is housed within the prepuce, 50 centimetres (20 in) long and 2.5 to 6 centimetres (0.98 to 2.36 in) in diameter with the distal end 15 to 20 centimetres (5.9 to 7.9 in). The retractor muscle contracts to retract the penis into the sheath and relaxes to allow the penis to extend from the sheath. [34] When erect, the penis doubles in length [35] and thickness and the glans increases by 3 to 4 times. [34] Erection and protrusion take place gradually, by the increasing tumescence of the erectile vascular tissue in the corpus cavernosum penis . [36] [37] Most stallions achieve erection within 2 minutes of contact with an estrus mare, and mount the estrus mare 5–10 seconds afterward. [38]

A bird penis is different in structure from mammal penises, being an erectile expansion of the cloacal wall and being erected by lymph, not blood. [39] The penis of the lake duck can reach about the same length as the animal himself when fully erect, but more commonly is about half the bird's length. [40] [41]


Clinically, erection is often known as "penile erection", and the state of being erect, and 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'. [42] There are several slang words, euphemisms and synonyms for an erection in English and in other languages. (See also The WikiSaurus entry.)

See also

Related Research Articles

Erectile dysfunction Human disease which results in trouble maintaining an erection

Erectile dysfunction (ED), also called impotence, is the type of sexual dysfunction in which the penis fails to become or stay erect during sexual activity. It is the most common sexual problem in men. Through its connection to self-image and to problems in sexual relationships, erectile dysfunction can cause psychological harm.

Priapism A medical condition where an erection lasts excessively long

Priapism is a condition in which a penis remains erect for hours in the absence of stimulation or after stimulation has ended. There are three types: ischemic (low-flow), nonischemic (high-flow), and recurrent ischemic (intermittent). Most cases are ischemic. Ischemic priapism is generally painful while nonischemic priapism is not. In ischemic priapism, most of the penis is hard; however, the glans penis is not. In nonischemic priapism, the entire penis is only somewhat hard. Very rarely, clitoral priapism occurs in women.

Erectile tissue is tissue in the body with numerous vascular spaces, or cavernous tissue, that may become engorged with blood. However, tissue that is devoid of or otherwise lacking erectile tissue may also be described as engorging with blood, often with regard to sexual arousal.

Corpus spongiosum penis

The corpus spongiosum is the mass of spongy tissue surrounding the male urethra within the penis. It is also called the corpus cavernosum urethrae in older texts.

Bulbospongiosus muscle

The bulbospongiosus muscle is one of the superficial muscles of the perineum. It has a slightly different origin, insertion and function in males and females. In males, it covers the bulb of the penis. In females, it covers the vestibular bulb.

Tumescence is the quality or state of being tumescent or swollen. Tumescence usually refers to the normal engorgement with blood of the erectile tissues, marking sexual excitation, and possible readiness for sexual activity. The tumescent sexual organ in men is the penis and in women is the clitoris and other parts of the genitalia like the vestibular bulbs.

PDE5 inhibitor

A phosphodiesterase type 5 inhibitor is a drug used to block the degradative action of cGMP-specific phosphodiesterase type 5 (PDE5) on cyclic GMP in the smooth muscle cells lining the blood vessels supplying various tissues. These drugs dilate the corpora cavernosa of the penis, facilitating erection with sexual stimulation, and are used in the treatment of erectile dysfunction (ED). Sildenafil was the first effective oral treatment available for ED. Because PDE5 is also present in the smooth muscle of the walls of the arterioles within the lungs, sildenafil and tadalafil dilates those vessels, and are FDA-approved for the treatment of pulmonary hypertension. Increasingly, the wider cardiovascular benefits of PDE5 inhibitors are being appreciated.

Bulb of vestibule

In female anatomy, the vestibular bulbs, bulbs of the vestibule or clitoral bulbs are two elongated masses of erectile tissue typically described as being situated on either side of the vaginal opening. They are united to each other in front by a narrow median band. Some research indicates that they do not surround the vaginal opening, and are more closely related to the clitoris than to the vestibule.

Deep shaft piercings are piercings which pass through the penile shaft. They are most commonly seen in the form of deeply placed ampallangs, apadravyas, and reverse shaft Prince Alberts. They are more rare piercings due to associated pain, difficulty, bleeding and long healing times. Common placement is directly behind the head of the penis, but they can be placed farther back.

Corpus cavernosum penis

A corpus cavernosum penis (singular) is one of a pair of sponge-like regions of erectile tissue, which contain most of the blood in the penis during an erection.

Dorsal artery of the penis

The Dorsal Artery of the Penis is a branch of the internal pudendal artery which ascends between the crus penis and the pubic symphysis, and, piercing the inferior fascia of the urogenital diaphragm, passes between the two layers of the suspensory ligament of the penis, and runs forward on the dorsum of the penis to the glans, where it divides into two branches, which supply the glans and prepuce. The dorsal arteries do give perforators to the corpora cavernosa, however their contribution to erectile function is inconsistent. Through retrograde flow they help supply the distal shaft skin. Additionally, they give branches to the circumflex arteries which supply the corpus spongiosum. The major clinical relevance is in the case of traumatic amputation of the penis, failure to perform re-anastomosis of the dorsal arteries leads to skin loss.

Corpus cavernosum of clitoris

The corpus cavernosum of clitoris is one of a pair of sponge-like regions of erectile tissue which contain most of the blood in the clitoris during clitoral erection. This is homologous to the corpus cavernosum penis in the male; the body of the clitoris contains erectile tissue in a pair of corpora cavernosa, with a recognizably similar structure.

Nocturnal penile tumescence is a spontaneous erection of the penis during sleep or when waking up. Along with nocturnal clitoral tumescence, it is also known as sleep-related erection. Men without physiological erectile dysfunction or severe depression experience nocturnal penile tumescence, usually three to five times during a period of sleep, typically during rapid eye movement sleep. Nocturnal penile tumescence is believed to contribute to penile health.

Venous leak, also called venogenic erectile dysfunction and penile venous insufficiency, is one category of vasculogenic impotence -a cause of erectile dysfunction in males. It affects all ages, being particularly awkward in young men. Much about venous leaks has not reached a consensus among the medical community, and many aspects of the condition, particularly its treatment strategies, are controversial. The prevalence of the condition is still unknown, although some sources claim it to be a common cause of erectile dysfunction.

Tunica albuginea (penis)

The tunica albuginea is the fibrous envelope that extends the length of the corpora cavernosa penis and corpus spongiosum penis. It is a bi-layered structure that includes an outer longitudinal layer and an inner circular layer. It consists of approximately 5% elastin, with the remainder mostly consisting of collagen.

Helicine arteries of penis

The helicine arteries of penis are arteries in the penis. They are found in the corpora cavernosa penis.

Human penis

The human penis is an external male intromittent organ that additionally serves as the urinal duct. The main parts are the root (radix); the body (corpus); and the epithelium of the penis including the shaft skin and the foreskin (prepuce) covering the glans penis. The body of the penis is made up of three columns of tissue: two corpora cavernosa on the dorsal side and corpus spongiosum between them on the ventral side. The human male urethra passes through the prostate gland, where it is joined by the ejaculatory duct, and then through the penis. The urethra traverses the corpus spongiosum, and its opening, the meatus, lies on the tip of the glans penis. It is a passage both for urination and ejaculation of semen

Body of penis

The body of the penis extends from the root to the ends of the corpora cavernosa penis, and in it these corpora cavernosa are intimately bound to one another with a dorsally fenestrated septum which becomes a complete one before penile crura.

Clitoral erection

Clitoral erection is a physiological phenomenon where the clitoris becomes enlarged and firm.

Penile ulltrasonography is medical ultrasonography of the penis. Ultrasound is an excellent method for the study of the penis, such as indicated in trauma, priapism, erectile dysfunction or suspected Peyronie's disease.


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