[4] Three main parts of the human penis include: The human penis is made up of three columns of erectile tissue: two corpora cavernosa lie next to each other (separated by a fibrous septum) on the dorsal side and one corpus spongiosum lies between them on the ventral side.

[10] The human penis differs from those of most other mammals, as it has no baculum (or erectile bone) and instead relies entirely on engorgement with blood to reach its erect state.

A distal ligament buttresses the glans penis and plays an integral role to the penile fibroskeleton, and the structure is called "os analog", a term coined by Geng Long Hsu in the Encyclopedia of Reproduction.

[12] The human penis cannot be withdrawn into the groin, and it is larger than average in the animal kingdom in proportion to body mass.

A 2015 systematic review of 15,521 men in which the subjects were measured by health professionals showed that the average length of an erect human penis is 13.12 cm (5.17 inches) long, while the average circumference of an erect human penis is 11.66 cm (4.59 inches).

When the fetus is exposed to testosterone, the genital tubercle elongates (primordial phallus) and develops into the glans and shaft of the penis and the urogenital folds fuse to become the penile raphe.

Males expel urine from the bladder through the urethra, which passes through the prostate where it is joined by the ejaculatory ducts, and then onward through the penis.

Relaxing the urethral sphincter allows the urine in the upper urethra to enter the penis properly and thus empty the urinary bladder.

During erection, these centers block the relaxation of the sphincter muscles, so as to act as a physiological separation of the excretory and reproductive function of the penis, and preventing urine from entering the upper portion of the urethra during ejaculation.

The distal section of the urethra allows a human male to direct the stream of urine by holding the penis.

Spontaneous erections frequently occur during adolescence due to friction with clothing, a full bladder or large intestine, hormone fluctuations, nervousness, and undressing in a nonsexual situation.

The primary physiological mechanism that brings about erection is the autonomic dilation of arteries supplying blood to the penis, which allows more blood to fill the three spongy erectile tissue chambers in the penis, the corpora cavernosa and corpus spongiosum, causing it to lengthen and stiffen.

Many penises are curved in right, left, upwards or downwards direction depending upon the tension of the suspensory ligament that holds it in position.

A series of muscular contractions delivers semen, containing male gametes known as sperm cells or spermatozoa, from the penis.

Fluids are added by the seminal vesicles and the vasa deferentia turn into the ejaculatory ducts, which join the urethra inside the prostate.

[31] The ischiocavernosus muscle helps to stabilize the penis during erection by compressing the crus and slowing the return of blood through the veins.

The human penis has been argued to have several evolutionary adaptations that maximise reproductive success and minimise sperm competition.

[34] Evolution has caused sexually selected adaptations to occur in penis size in order to maximise reproductive success and minimise sperm competition.

Another reason for this adaptation is that, due to the nature of the human posture, gravity creates vulnerability for semen loss.

Another evolutionary theory of penis size is female mate choice and its associations with social judgements in modern-day society.

These varied in height, body shape and flaccid penis size, with these aspects being examples of masculinity.

[43] Secondly, the male replaces the rival's sperm with his own, thereby increasing the probability of his fertilising the egg and successfully reproducing with the female.

[43] This research showed that, when combined with thrusting, the coronal ridge of the penis is able to remove the seminal fluid of a rival male from within the female reproductive tract.

[43] In many cultures, referring to the penis is considered taboo or vulgar, and a variety of slang words and euphemisms are used to talk about it.

In English, these include member, dick, cock, prick, johnson, dork, peter, pecker, manhood, stick, rod, third/middle leg, dong, willy, schlong, and todger.

Trans women who undergo sex reassignment surgery have their penis surgically modified into a vagina or clitoris via vaginoplasty or clitoroplasty respectively.

[56][57] Among the world's major medical organizations, there is a consensus that circumcision reduces heterosexual HIV infection rates in high-risk populations during penile-vaginal sex.

[67][68][69] Some organizations which perform research into, or conduct regeneration procedures, include the Wake Forest Institute for Regenerative Medicine and the United States Department of Defense.

[70] A man at 44 sustained an injury after an accident and his penis was severed; urination became difficult as his urethra was partly blocked.

In 2015, the world's first successful penis transplant took place in Cape Town, South Africa in a nine-hour operation performed by surgeons from Stellenbosch University and Tygerberg Hospital.