An erection (clinically: penile erection or penile tumescence) is a

physiological Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...

phenomenon in which the

penis A penis (; : penises or penes) is a sex organ through which male and hermaphrodite animals expel semen during copulation (zoology), copulation, and through which male placental mammals and marsupials also Urination, urinate. The term ''pen ...

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 Sexual arousal (also known as sexual excitement) describes the Physiology, physiological and psychological responses in preparation for sexual intercourse or when exposed to Sexual stimulation, sexual stimuli. A number of physiological response ...

, sexual attraction or

libido In psychology, libido (; ) is psychic drive or energy, usually conceived of as sexual in nature, but sometimes conceived of as including other forms of desire. The term ''libido'' was originally developed by Sigmund Freud, the pioneering origin ...

, although erections can also be spontaneous. The shape, angle, and direction of an erection vary considerably between humans. Physiologically, an erection is required for a male to effect penetration or

sexual intercourse Sexual intercourse (also coitus or copulation) is a sexual activity typically involving the insertion of the Erection, erect male Human penis, penis inside the female vagina and followed by Pelvic thrust, thrusting motions for sexual pleasure ...

and is triggered by the

parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The autonomic nervous system is responsible for regulat ...

division of the

autonomic nervous system The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates viscera, internal organs, smooth muscle and glands. The autonomic nervo ...

, causing the levels of

nitric oxide Nitric oxide (nitrogen oxide, nitrogen monooxide, or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes den ...

vasodilator Vasodilation, also known as vasorelaxation, is the widening of blood vessels. It results from relaxation of smooth muscle cells within the vessel walls, in particular in the large veins, large arteries, and smaller arterioles. Blood vessel wal ...

) to rise in the trabecular

arteries An artery () is a blood vessel in humans and most other animals that takes oxygenated blood away from the heart in the systemic circulation to one or more parts of the body. Exceptions that carry deoxygenated blood are the pulmonary arteries in ...

and

smooth muscle Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...

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 Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood is com ...

; simultaneously the

ischiocavernosus The ischiocavernosus muscle (erectores penis ''or'' erector clitoridis in older texts) is a muscle just below the surface of the perineum, present in both men and women. Structure It arises by tendinous and fleshy fibers from the inner surface of ...

and bulbospongiosus muscles compress the

vein Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and feta ...

s 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 Sexual stimulation is anything that leads to sexual arousal or orgasm. This thing can be physical or of other senses, and is known as a stimulus. Sexual stimulation is a broad term, usually understood to mean physical touching of the genita ...

and

, and is therefore not entirely under conscious control. Erections during sleep or upon waking up are known as nocturnal penile tumescence (NPT), also known as "morning wood". Absence of nocturnal erection is commonly used to distinguish between physical and psychological causes of

erectile dysfunction Erectile dysfunction (ED), also referred to as impotence, is a form of sexual dysfunction in males characterized by the persistent or recurring inability to achieve or maintain a Human penis, penile erection with sufficient rigidity and durat ...

and

impotence Erectile dysfunction (ED), also referred to as impotence, is a form of sexual dysfunction in males characterized by the persistent or recurring inability to achieve or maintain a Human penis, penile erection with sufficient rigidity and durat ...

. 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.

Physiology

An erection is necessary for natural insemination as well as for the harvesting of sperm for

artificial insemination Artificial insemination is the deliberate introduction of sperm into a female's cervix or uterine cavity for the purpose of achieving a pregnancy through in vivo fertilization by means other than sexual intercourse. It is a fertility treatment ...

, and is common for children and infants. After reaching

puberty Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a female, the testicles i ...

, erections occur much more frequently. An erection occurs when two tubular structures, called the corpora cavernosa, that run the length of the penis, become engorged with

venous blood Venous blood is deoxygenated blood which travels from the peripheral blood vessels, through the venous system into the right atrium of the heart. Deoxygenated blood is then pumped by the right ventricle to the lungs via the pulmonary artery whi ...

. This may result from any of various

stimuli, also known as

and

. The corpus spongiosum is a single tubular structure located just below the corpora cavernosa, which contains the

urethra The urethra (: urethras or urethrae) is the tube that connects the urinary bladder to the urinary meatus, through which Placentalia, placental mammals Urination, urinate and Ejaculation, ejaculate. The external urethral sphincter is a striated ...

, through which

urine Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...

and

semen Semen, also known as seminal fluid, is a bodily fluid that contains spermatozoon, spermatozoa which is secreted by the male gonads (sexual glands) and other sexual organs of male or hermaphrodite, hermaphroditic animals. In humans and placen ...

pass during

urination Urination is the release of urine from the bladder through the urethra in Placentalia, placental mammals, or through the cloaca in other vertebrates. It is the urinary system's form of excretion. It is also known medically as micturition, v ...

and

ejaculation Ejaculation is the discharge of semen (the ''ejaculate''; normally containing sperm) from the penis through the urethra. It is the final stage and natural objective of male sexual stimulation, and an essential component of natural conception. ...

respectively. This may also become slightly engorged with blood, but less so than the corpora cavernosa. In some cases, the scrotum becomes tightened during an erection, and in most uncircumcised males, the

foreskin In male Human body, human anatomy, the foreskin, also known as the prepuce (), is the double-layered fold of Human skin, skin, Mucous membrane, mucosal and Muscle tissue, muscular tissue at the distal end of the human penis that covers the glans ...

automatically and gradually retracts throughout the various stages of erection, exposing the glans, though some individuals have to manually retract their foreskin.

Autonomic control

In the presence of mechanical stimulation, erection is initiated by the

division of the

with minimal input from the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

. Parasympathetic branches extend from the sacral plexus into the

supplying the erectile tissue; upon stimulation, these nerve branches release

acetylcholine Acetylcholine (ACh) is an organic compound that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

, which in turn causes the release of

from

endothelial cell The endothelium (: endothelia) is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and th ...

s in the trabecular arteries. Nitric oxide diffuses to the

of the arteries (called ''trabecular smooth muscle''), acting as a vasodilating agent. The arteries dilate, filling the ''corpus spongiosum'' and ''corpora cavernosa'' with blood. The

and bulbospongiosus muscles also compress the veins of the corpora cavernosa, limiting the venous drainage of blood. 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 through 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. Erection rigidity is mechanically controlled by reduction blood flow through 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. After

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

Voluntary and involuntary control

The

cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. It is the largest site of Neuron, neural integration in the central nervous system, and plays ...

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 spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

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

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'').

Socio-sexual aspects

Social

Though an erection can have many causes, it is most commonly seen as an indicator of

and is therefore considered taboo or inappropriate for a public setting in many societies. This taboo is lesser than that surrounding public sex but higher than that surrounding

nudity Nudity is the state of being in which a human is without clothing. While estimates vary, for the first 90,000 years of pre-history, anatomically modern humans were naked, having lost their body hair, living in hospitable climates, and not ...

. Erectile dysfunction is often considered a flaw, eliciting shame in individuals affected. The penile plethysmograph, which measures erections, has been used by some governments and

courts of law A court is an institution, often a government entity, with the authority to adjudicate legal disputes between parties and administer justice in civil, criminal, and administrative matters in accordance with the rule of law. Courts gene ...

to measure sexual orientation. An unusual aversion to the erect penis is sometimes referred to as phallophobia.

Spontaneous or random 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 (if required) can be disguised or hidden by wearing close-fitting underwear, a long shirt, or baggier clothes.

Size

The length of the flaccid penis is not 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. Though the size of a penis varies considerably between males, the average length of an erect

human penis In Human body, human anatomy, the penis (; : penises or penes; from the Latin ''pēnis'', initially 'tail') is an external sex organ (intromittent organ) through which males urination, urinate and ejaculation, ejaculate, as Penis, on other anim ...

is 13.12 cm (5.17 inches), while the average circumference of an erect human penis is 11.66 cm (4.59 inches).

Direction

Although many erect penises point upwards, it is common and normal for the erect penis to point nearly vertically upwards or horizontally straight forward or even nearly vertically downwards, 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 Peyronie's disease (PD) is a benign, acquired penile connective tissue disease characterized by the occurrence of fibrotic plaques within the Tunica albuginea (penis), tunica albuginea — the dense elastic covering of the Corpora cavernosa of pe ...

. This may cause physical and psychological effects for the affected individual, which could include

or pain during an erection. Treatments include oral medication (such as

colchicine Colchicine is a medication used to prevent and treat gout, to treat familial Mediterranean fever and Behçet's disease, and to reduce the risk of myocardial infarction. The American College of Rheumatology recommends colchicine, nonstero ...

) 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 and the average erection angle is 74.3 degrees. The penile curvature was measured same time. 63% men have straight penis. 22.2% men have upwards curvature and 14.8% men have downwards curvature.

Medical conditions

Erectile dysfunction

Erectile dysfunction (also known as ED or "(male) impotence") is a

sexual dysfunction Sexual dysfunction is difficulty experienced by an individual or partners during any stage of normal sexual activity, including physical pleasure, desire, preference, arousal, or orgasm. The World Health Organization defines sexual dysfunction ...

characterized by the inability to develop and/or maintain an erection. The study of erectile dysfunction within medicine is known as

andrology Andrology (from , ''anēr'', genitive , ''andros'' 'man' and , ''-logy, -logia'') is a name for the medicine, medical specialty that deals with male health, particularly relating to the problems of the male reproductive system and urology, urologi ...

, a sub-field within

urology Urology (from Ancient Greek, Greek wikt:οὖρον, οὖρον ''ouron'' "urine" and ''wiktionary:-logia, -logia'' "study of"), also known as genitourinary surgery, is the branch of medicine that focuses on surgical and medical diseases of t ...

. Erectile dysfunction may occur due to

psychological Psychology is the scientific study of mind and behavior. Its subject matter includes the behavior of humans and nonhumans, both consciousness, conscious and Unconscious mind, unconscious phenomena, and mental processes such as thoughts, feel ...

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, and neurologic disease which collectively account for about 70% of ED cases. Some drugs used to treat other conditions, such as

lithium Lithium (from , , ) is a chemical element; it has chemical symbol, symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it is the least dense metal and the ...

and

paroxetine Paroxetine ( ), sold under the brand name Paxil among others, is an Antidepressant, antidepressant medication of the selective serotonin reuptake inhibitor (SSRI) class used to treat major depressive disorder, obsessive-compulsive disorder, o ...

, may cause erectile dysfunction. Erectile dysfunction, tied closely as it is to cultural notions of potency,

success Success is the state or condition of meeting a defined range of expectations. It may be viewed as the opposite of failure. The criteria for success depend on context, and may be relative to a particular observer or belief system. One person mi ...

and

masculinity Masculinity (also called manhood or manliness) is a set of attributes, behaviors, and roles generally associated with men and boys. Masculinity can be theoretically understood as Social construction of gender, socially constructed, and there i ...

, can have devastating psychological consequences including feelings of

shame Shame is an unpleasant self-conscious emotion often associated with negative self-evaluation; motivation to quit; and feelings of pain, exposure, distrust, powerlessness, and worthlessness. Definition Shame is a discrete, basic emotion, d ...

, loss or inadequacy. 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.

Priapism

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.

Hard flaccid syndrome

Hard flaccid syndrome is a rare, chronic condition characterized by a flaccid penis that remains in a firm, semi-rigid or semi-erect state in the absence of sexual arousal.

Other animals

At the time of penetration, the

canine penis Canine reproduction is the process of sexual reproduction in domestic dogs, Wolf, wolves, coyotes and other Canis, canine species. Canine sexual anatomy and development Male reproductive system Erectile tissue As with all mammals, a dog's penis ...

is not erect, and only able to penetrate the female because it includes a narrow bone called the

baculum The baculum (: bacula), also known as the penis bone, penile bone, ''os penis'', ''os genitale'', or ''os priapi'', is a bone in the penis of many placental mammals. It is not present in humans, but is present in the penises of some primates, ...

, 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. An elephant's penis is S-shaped when fully erect and has a Y-shaped orifice. Given the small amount of erectile tissue in a

bull's penis A bull is an intact (i.e., not Castration, castrated) adult male of the species ''Bos taurus'' (cattle). More muscular and aggressive than the females of the same species (i.e. cows proper), bulls have long been an important symbol cattle in r ...

, 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. A male fossa's penis reaches to between his forelegs when erect. When not erect, a horse's penis is housed within the prepuce, long and in diameter with the distal end . The retractor muscle contracts to retract the penis into the sheath and relaxes to allow the penis to extend from the sheath. When erect, the penis doubles in length and thickness and the glans increases by 3 to . Erection and protrusion take place gradually, by the increasing tumescence of the erectile vascular tissue in the ''

corpus cavernosum penis A corpus cavernosum penis (singular) (from Latin, characterised by "cavities/ hollows" of the penis, : corpora cavernosa) is one of a pair of sponge-like regions of erectile tissue, which contain most of the blood in the penis of several animal ...

''. Most stallions achieve erection within 2 minutes of contact with an estrus mare, and mount the estrus mare 5–10 seconds afterward. A

bird penis A penis (; : penises or penes) is a sex organ through which male and hermaphrodite animals expel semen during copulation, and through which male placental mammals and marsupials also urinate. The term ''penis'' applies to many intromitten ...

is different in structure from

mammal penis A penis (; : penises or penes) is a sex organ through which male and hermaphrodite animals expel semen during copulation, and through which male placental mammals and marsupials also urinate. The term ''penis'' applies to many intromitt ...

es, being an erectile expansion of the cloacal wall and being erected by

lymph Lymph () is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to be recirculated. At the ori ...

, not blood. 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.

Terminology

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 Colloquialism (also called ''colloquial language'', ''colloquial speech'', ''everyday language'', or ''general parlance'') is the linguistic style used for casual and informal communication. It is the most common form of speech in conversation am ...

and in

slang A slang is a vocabulary (words, phrases, and linguistic usages) of an informal register, common in everyday conversation but avoided in formal writing and speech. It also often refers to the language exclusively used by the members of pa ...

, 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 (see also: The WikiSaurus entry).

References

{{Authority control Sexual arousal Andrology Men's health Penis Human penis