Human penis


In human anatomy, the penis is an external sex organ through which males ejaculate and urinate. Together with the testes and surrounding structures, the penis functions as part of the male reproductive system.
The main parts of the penis are the root, body, the epithelium, including the shaft skin, and the foreskin covering the glans. 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 urethra passes through the prostate gland, where it is joined by the ejaculatory ducts, and then passes through the corpus spongiosum. The external opening of the urethra at the tip of the glans is the urinary meatus.
An erection is the stiffening expansion and orthogonal reorientation of the penis, which occurs during sexual arousal. Erections can occur in non-sexual situations; spontaneous non-sexual erections frequently occur during adolescence and sleep. In its flaccid state, the penis is smaller, gives to pressure, and the glans is covered by the foreskin. In its fully erect state, the shaft becomes rigid and the glans becomes engorged but not rigid. An erect penis may be straight or curved and may point at an upward angle, a downward angle, or straight ahead., the average erect human penis is long and has a circumference of. Neither age nor size of the flaccid penis accurately predicts erectile length. There are also several common body modifications to the penis, including circumcision and piercings.
The glans penis is homologous to the clitoris in females.

Structure

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 on the dorsal side and one corpus spongiosum lies between them on the ventral side. These columns are surrounded by a fibrous layer of connective tissue called the tunica albuginea. The corpora cavernosa are innervated by lesser and greater cavernous nerves and form most of the penis containing blood vessels that fill with blood to help make an erection.
The crura are the proximal parts of the corpora cavernosa. The corpus spongiosum is an erectile tissue surrounding the urethra. The proximal parts of the corpus spongiosum form the bulb and the distal ends form the glans penis.
The enlarged and bulbous-shaped end of the corpus spongiosum forms the glans penis with two specific types of sinusoids, which supports the foreskin, a loose fold of skin that in adults can retract to expose the glans. The area on the underside of the glans, where the foreskin is attached, is called the frenulum. The rounded base of the glans is called the corona. The inner surface of the foreskin and corona is rich in sebaceous glands which secrete smegma. The structure of the penis is supported by the pelvic floor muscles.
The urethra, which is the last part of the urinary tract, traverses the corpus spongiosum and opens through the urinary meatus on the tip of the glans.
The penile raphe is the visible ridge between the lateral halves of the penis, found on the ventral or underside of the penis, running from the meatus and continuing as the perineal raphe across the scrotum and the perineum.
The human penis differs from those of most other mammals, as it has no baculum 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. It is a remnant of the baculum that has likely evolved due to change in mating practice.
The human penis cannot be withdrawn into the groin, and it is larger than average in the animal kingdom in proportion to body mass. The human penis is reciprocating from a cotton soft to a bony rigidity resulting from penile arterial flow varied between 2–3 to 60–80 mL/Min implies the most ideal milieu to apply Pascal's law in the entire human body; the overall structure is unique.

Size

Penile measurements vary, with studies that rely on self-measurement reporting a significantly higher average size than those which rely on measurements taken by health professionals. 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 long, while the average circumference of an erect human penis is 11.66 cm.
Among all primates, the human penis is the largest in girth, but is comparable to the chimpanzee penis and the penises of certain other primates in length. Penis size is affected by genetics, but also by environmental factors such as fertility medications and chemical/pollution exposure.

Normal variations

  • Pearly penile papules are raised bumps of somewhat paler color around the base of the glans, which typically develop in males aged 20 to 40. As of 1999, different studies had produced estimates of incidence ranging from 8 to 48 percent of all men. They may be mistaken for warts, but are not harmful or infectious and do not require treatment.
  • Fordyce's spots are small, raised, yellowish-white spots 1–2 mm in diameter that may appear on the penis, which again are common and not infectious.
  • Sebaceous prominences are raised bumps similar to Fordyce's spots on the shaft of the penis, located at the sebaceous glands and are normal.
  • Phimosis is an inability to retract the foreskin fully. It is normal and harmless in infancy and pre-pubescence, occurring in about 8% of boys at age 10. According to the British Medical Association, treatment does not need to be considered until age 19.
  • Curvature: few penises are completely straight, with curves commonly seen in all directions. Sometimes the curve is very prominent but it rarely inhibits sexual intercourse. Curvature as great as 30° is considered normal and medical treatment is rarely considered unless the angle exceeds 45°. Changes to the curvature of a penis may be caused by Peyronie's disease.

    Development

When the fetus is exposed to testosterone, the genital tubercle elongates and develops into the glans and shaft of the penis and the urogenital folds fuse to become the penile raphe. The urethra within the penis is developed from the urogenital sinus.

Growth in puberty

On entering puberty, the penis, scrotum and testicles will enlarge toward maturity. During the process, pubic hair grows above and around the penis. A large-scale study assessing penis size in thousands of 17- to 19-year-old males found no difference in average penis size between 17-year-olds and 19-year-olds. From this, it can be concluded that penile growth is typically complete not later than age 17, and possibly earlier.

Physiological functions

Urination

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. At the root of the penis lies the external sphincter muscle. This is a small sphincter of striated muscle tissue and is in healthy males, under voluntary control. Relaxing the urethral sphincter allows the urine in the upper urethra to enter the penis properly and thus empty the urinary bladder.
Physiologically, urination involves coordination between the central, autonomic, and somatic nervous systems. In infants, some elderly individuals, and those with neurological injury, urination may occur as an involuntary reflex. Brain centers that regulate urination include the pontine micturition center, periaqueductal gray, and the cerebral cortex. 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.

Voiding position

The distal section of the urethra allows a human male to direct the stream of urine by holding the penis. This flexibility allows the male to choose the posture in which to urinate. In cultures where more than a minimum of clothing is worn, the penis allows the male to urinate while standing without removing much of the clothing. It is customary for some boys and men to urinate in seated or crouched positions. The preferred position may be influenced by cultural or religious beliefs. Research on the medical superiority of either position exists, but the data are heterogenic. A meta-analysis summarizing the evidence found no superior position for young, healthy males. For elderly males with LUTS, however, the sitting position when compared to the standing position is differentiated by the following:
  • the post void residual volume was significantly decreased
  • the maximum urinary flow was increased
  • the voiding time was decreased
This urodynamic profile is related to a lower risk of urologic complications, such as cystitis and bladder stones.

Sexual stimulation and arousal

The penis incites sexual arousal when sexually stimulated, such as from mental stimuli, partnered activity, or masturbation, which can lead to orgasm.
The glans and the frenulum are erogenous zones of the penis. The glans has many nerve endings, which makes it the most sensitive. The most effective way to stimulate the penis is through oral stimulation, manual stimulation, or during sexual penetration. Frot is mutual penile stimulation between men.