The angle of an erect penis varies from pointing downwards, upwards or sideways, and the penis may also bend.

An erection, or penile erection, is a physiological phenomenon where the penis becomes firmer, engorged and enlarged. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is usually, though not exclusively, associated with sexual arousal or sexual attraction. The angle of an erect penis varies from pointing downwards, upwards or sideways^{[clarification needed]}, and the penis may also bend.

Erections during sleep or when waking up are known as nocturnal penile tumescence (NPT). A penis which is partly, but not fully, erect is sometimes known as a semi-erection; a penis which is not erect is typically referred to as being flaccid, or soft.

Contents   [hide]  1 Terminology 2 Physiology 2.1 During sexual activity 2.2 Autonomic control 3 Shape and size 4 Spontaneous or random erections 5 Medical conditions 5.1 Erectile dysfunction 5.2 Priapism 6 In non-human animals 7 See also 8 References

Terminology

Erections have many informal and slang terms, including 'stiffy', 'hard-on', 'boner' and 'woody'.^[1] There are several slang words, euphemisms and synonyoms for an erection in English and in other languages. See the WikiSaurus entry for a list of alternative terms.

Physiology

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 ejaculationrespectively. This may also become slightly engorged with blood, but less so than the corpora cavernosa.

During sexual activity

A penile erection is an indicator of sexual arousal and is required for a male to effect vaginal penetration and sexual intercourse. The scrotum may, but not always, become tightened during an erection. Generally, the foreskinautomatically and gradually retracts, exposing the glans, though some males may have to manually retract their foreskin.

After a male has ejaculated during a sexual encounter or masturbation, the erection usually ends, but this may take time depending on the length and thickness of the penis.^[2]

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 thesacral 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.^[3] Nitric oxide diffuses to the smooth muscle of the arteries (called trabecular smooth muscle^[4]), acting as a vasodilating agent. The arteries dilate, filling the corpora spongiosum and cavernosa with blood. The ischiocavernosus andbulbospongiosus muscles also compress the veins of the corpora cavernosa, limiting the venous drainage of blood.^[5] 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.^[6] 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 term that is opposite to erection is detumescence.

Shape and size

Main article: Human penis size

Wikimedia Commons has media related to: Erect human penis

An erect human penis

The length of the flaccid penis does not necessarily correspond to the length of the penis when it becomes erect; some smaller flaccid penises grow much longer, while some larger flaccid penises grow comparatively less.^[7] Generally, the size of an erect penis is fixed throughout post-pubescent life. Its size may be increased by surgery,^[8] although penile enlargement is controversial, and a majority of men were "not satisfied" with the results, according to one study.^[9]

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 reserved 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 degrees is horizontal and pointing straight forward, while 180 degrees would be pointing straight down to the feet. An upward pointing angle is most common.

Occurrence of erection angles^[10]

Angle (°)	Percent of population
0–30	5
30–60	30
60–85	31
85–95	10
95–120	20
120–180	5

Spontaneous or random erections

Spontaneous erections are also known as involuntary, random or unwanted erections and are normal. Such erections can be embarrassing if they happen in public, such as a classroom or living room.^[11][12] Erections can occur spontaneously at any time of day, and if clothed may cause a bulge or "hump". This can be disguised or hidden by wearing close-fitting underwear, a long shirt and baggier clothes.^[13]

The penis can regularly get erect during sleep and men or boys often wake up with an erection. Such an erection is medically known as nocturnal penile tumescence and informally known as morning wood or morning glory.^{[14][15][16][17]} Once a boy reaches his teenage years, erections occur much more frequently due to puberty.^[18] Male erections are common for children and infants, and even occur before birth.^[19]

Medical conditions

Erectile dysfunction

Main article: 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.^[20][21] The study of erectile dysfunction within medicine is known as andrology, a sub-field within urology.^[22]

Erectile dysfunction can occur due to both physiological and psychological reasons, most of which are amenable to treatment. Common physiological reasons include diabetes, kidney disease, chronic alcoholism, multiple sclerosis, atherosclerosis, vascular disease, and neurologic disease which collectively account for about 70 percent of ED cases. Some drugs used to treat other conditions, such as lithium and paroxetine, may cause erectile dysfunction.^[21][23]

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;^[24] There is a strong culture of silence and inability to discuss the matter. In fact, around one in ten men will experience recurring impotence problems at some point in their lives.^[25]

Priapism

Priapism is a medical condition which could possibly be painful, and is a prolonged erection at least four hours long, which does not return to its flaccid state, despite the absence of both physical and psychological stimulation.

In non-human animals

This section requires expansion.(November 2012)

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.^[26]

An elephant's penis is S-shaped when fully erect and has a Y-shaped orifice.^[27]

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.^[28][29]

A male fossa's penis reaches to between his forelegs when erect.^[30]

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 2.4 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. When erect, the penis doubles in length and thickness and the glans increases by 3 to 4 times.^[31] Erection and protrusion take place gradually, by the increasing tumescence of the erectile vascular tissue in the corpus cavernosum penis.^[32]

See also

Sexuality portal

• Clitoral erection
• Death erection
• Erectores pilorum
• Issues in social nudity
• Nipple erection
• Nocturnal penile tumescence
• Priapism
• Sexual function

References

1. ^ Urge: Hot Secrets For Great Sex retrieved 27 February 2012
2. ^ Harris, Robie H. (et al.), It's Perfectly Normal: Changing Bodies, Growing Up, Sex And Sexual Health. Boston, 1994. (ISBN 1-56402-199-8)
3. ^ wiley.com > Viagra function image Retrieved on Mars 11, 2010
4. ^ APDVS > 31. Anatomy and Physiology of Normal ErectionRetrieved on Mars 11, 2010
5. ^ Moore, Keith; Anne Agur (2007). Essential Clinical Anatomy, Third Edition. Lippincott Williams & Wilkins. p. 265. ISBN 0-7817-6274-X.
6. ^ Drake, Richard, Wayne Vogl and Adam Mitchell, Grey's Anatomy for Students. Philadelphia, 2004. (ISBN 0-443-06612-4)
7. ^ "Penis Size FAQ & Bibliography". Kinsey Institute. 2009.
8. ^ Li CY, Kayes O, Kell PD, Christopher N, Minhas S, Ralph DJ (2006). "Penile suspensory ligament division for penile augmentation: indications and results". Eur. Urol. 49 (4): 729–733.doi:10.1016/j.eururo.2006.01.020. PMID 16473458.
9. ^ "Most Men Unsatisfied With Penis Enlargement Results". Fox News. 2006-02-16. Retrieved 2008-08-17.
10. ^ Sparling J (1997). "Penile erections: shape, angle, and length".Journal of Sex & Marital Therapy 23 (3): 195–207.doi:10.1080/00926239708403924. PMID 9292834.
11. ^ The "What's Happening to My Body?" Book for Boys Lynda Madaras
12. ^ The What's Happening to My Body Book for Girls Lynda Madaras
13. ^ Making Sense of Sex Sarah Attwood
14. ^ Morning Erections: Sizemed retrieved 28 February 2012
15. ^ After Prostate Cancer: A What-Comes-Next Guide to a Safe and Informed recovery: p.48
16. ^ Listen To Your Hormones, Abraham Harvey Kryger - 2004. p.32
17. ^ Sexuality now: embracing diversity Janell L. Carroll
18. ^ The "What's Happening to My Body?" Book for Boys Lynda Madaras; retrieved 11 February 2012
19. ^ erections in babies retrieved 11 February 2012
20. ^ Milsten, Richard (et al.), The Sexual Male. Problems And Solutions. London, 2000. (ISBN 0-393-32127-4)
21. ^ ^a b Sadeghipour H, Ghasemi M, Ebrahimi F, Dehpour AR (2007). "Effect of lithium on endothelium-dependent and neurogenic relaxation of rat corpus cavernosum: role of nitric oxide pathway".Nitric Oxide 16 (1): 54–63. doi:10.1016/j.niox.2006.05.004.PMID 16828320.
22. ^ Williams, Warwick, It's Up To You: Overcoming Erection Problems. London, 1989. (ISBN 0-7225-1915-X)
23. ^ Sadeghipour H, Ghasemi M, Nobakht M, Ebrahimi F, Dehpour AR (2007). "Effect of chronic lithium administration on endothelium-dependent relaxation of rat corpus cavernosum: the role of nitric oxide and cyclooxygenase pathways". BJU Int. 99 (1): 177–182.doi:10.1111/j.1464-410X.2006.06530.x. PMID 17034495.
24. ^ Tanagho, Emil A. (et al.), Smith's General Urology. London, 2000. (ISBN 0-8385-8607-4)
25. ^ NHS Direct – Health encyclopaedia -Erectile dysfunction
26. ^ "Semen Collection from Dogs". Arbl.cvmbs.colostate.edu. 2002-09-14. Retrieved 2012-01-29.
27. ^ Shoshani, p. 80.
28. ^ Functional Anatomy and Physiology of Domestic Animals - William O. Reece - Google Boeken. Books.google.com. Retrieved 2012-12-02.
29. ^ Modern Livestock and Poultry Production - James R. Gillespie, Frank B. Flanders - Google Boeken. Books.google.com. Retrieved 2012-12-02.
30. ^ Köhncke, M.; Leonhardt, K. (1986). "Cryptoprocta ferox" (PDF).Mammalian Species (254): 1–5. Retrieved 19 May 2010.
31. ^ "The Stallion: Breeding Soundness Examination & Reproductive Anatomy". University of Wisconsin-Madison. Archived from the original on 2007-07-16. Retrieved 7th July 2007.
32. ^ Sarkar, A. (2003). Sexual Behaviour In Animals. Discovery Publishing House. ISBN 978-81-7141-746-9.