The Anatomy of the Penis

The penis is used for penetrative sexual activity and urination

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The human penis has a reproductive function and urinary function. It contains the urethra, which allows for the passage of urine. It also has erectile and ejaculatory functions which make it possible for males to engage in sexual intercourse.

The penis consists of several parts, the most obvious of which are the shaft, the glans (head), and the foreskin. Removal of the foreskin for religious and/or medical purposes is referred to as circumcision.

Anatomy

The penis is centrally located on the front aspect of the body at the base of the pelvis. The scrotum, containing the testes, lies below the penis and is a separate structure.

There are several major structures of the penis:

  • Urethra: The penile urethra runs through the center of the penis and up to the bladder. Urine is released through this tube. Ejaculate also is released from the urethra.
  • Foreskin: The prepuce, or foreskin, is a shroud of skin that covers the head of the penis. It is this skin that is removed during circumcision.
  • Glans: The glans, or head of the penis, is a mucosal surface in uncircumcised men. In circumcised men, the glans is made of dry, non-mucosal skin.
  • Corpus cavernosa: The corpus cavernosa are two columns of tissue that run along the sides of the penis. When filled with blood, they lead to a penile erection. The corpus cavernosa are also sometimes referred to as erectile bodies.
  • Corpus spongiosum: The corpus spongiosum is the third column of erectile tissue that encloses the urethra and ends at the glans. It lies along the underside of the penis.

All of the structures of the penis are well supplied by blood vessels. It is the flow of blood into and out of the penis that provides the erectile function. When there is blood trapped in the erectile bodies of the penis, it is firm enough for sexual penetration. When the blood drains, it returns to the flaccid state.

The shaft of the penis, which consists of the urethra, and the three erectile bodies, is wrapped in fascia and skin. At the base of the penis are the suspensory ligaments, which support its position on the pelvis.

A systematic review of penis size found that the average flaccid length of the unstretched penis is 9 centimeters (cm), or 3.5 inches. However, there is substantial variation in penis size, and penis length tends to be correlated with overall height.

Function

The primary functions of the penis are urinary and sexual. The sexual function of the penis can be further broken down into two stages: erectile and ejaculatory.

Urinary Function

The urinary function of the penis is achieved by the passage of the urethra from the bladder, through the penis, to the exterior of the body. Obstruction of urinary flow in older men may be caused by the enlargement of the prostate gland, putting direct pressure on the urethra.

Erectile Function

The erectile function of the penis is controlled by the flow of blood into and out of the three erectile bodies—two corpus cavernosa and the corpus spongiosum. As such, erectile function is strongly related to the health of the cardiovascular system.

Medications for erectile dysfunction work by making it easier for the penile blood vessels to dilate and the penis to become and remain engorged.

Ejaculation occurs through the penis but is not solely a function of the penis. First, the vas deferens contract to move sperm towards the base of the penis. At the same time, the prostate gland and seminal vesicles release the secretions that make the bulk of the volume of semen.

Then, at the time of ejaculation, muscle contractions at the base of the penis (called the periurethral muscles) force semen out of the tip of the penis. The flow of the semen is uni-directional and regulated by muscle contractions around the base of the urethra.

If periurethral muscle contractions are incomplete, a man may experience retrograde ejaculation, a condition in which ejaculatory fluids move backward into the prostate gland instead of out of the body.

Penile Malformations

There are several congenital causes of penile malformations. Some occur when the production of hormones that influence penile and urethral growth are impaired during fetal development.

Hypospadias

Hypospadias is diagnosed when the opening of the urethra is on the base of the penis rather than at the tip. It is one of the most common congenital abnormalities in human males.

Around one-third of all hypospadias cases occurs in the context of other congenital disorders. This may indicate that the child has a disorder of sexual differentiation.

Surgical correction of hypospadias is generally recommended during infancy or early childhood; however, there are risks of long-term complications, including difficulty urinating and an increased risk of urinary tract infections.

Chordee

Chordee refers to a congenital curvature of the penis, in other words, a penis that is curved from the time of birth. It is more common in boys with hypospadias, but it can also occur on its own.

Between 4% and 10% of males experience some degree of penile curvature. That number may as be as high as 25% for males with hypospadias or a similar condition known as epispadias.

Aphalia

Congenital absence of the penis, or aphalia, is an extremely rare condition. It usually occurs in the context of other congenital abnormalities. Congenital absence of the foreskin is also possible but similarly rare.

According to research, aphalia affects one of every 30 million live births. So far, less than 100 cases have been reported.

Micropenis

Micropenis is defined as a stretched penis length that is less than 2.5 standard deviations of the mean. It is thought to occur in 1.5 out of 10,000 male children in the United States.

The size of the penis reflects the hormonal environment during fetal development and is often associated with disorders of sexual differentiation. It is also commonly associated with functional or structural problems of the testes.

Trapped penis, or inconspicuous penis, is different from micropenis. This occurs when the penis appears smaller than it actually is. Trapped penis may be caused by issues with the foreskin, a thick pad of pelvic fat, or other conditions that keep the penis' length obscured. It is often seen in some men with extreme obesity.

Associated Conditions

There are a number of medical conditions that can affect the penis. They can be broadly divided into the following categories:

Conditions Affecting Erectile Function

Erectile dysfunction occurs when a person can not get or maintain an erection sufficient for sexual intercourse. Erectile dysfunction may be caused by medical reasons, including medications. It may also be the result of stress or relationship problems.

Peyronie's disease is a curvature of the penis that is not congenital. Sometimes, Peyronie's disease is the result of an injury during sex. However, other times it is caused by inflammatory processes that are not well understood. If the symptoms of Peyronie's disease are severe, they can affect erectile function.

Priapism refers to an erection that lasts more than four hours. It is not always the result of sexual arousal and can be quite painful. Priapism occurs when blood becomes trapped in the penis and is unable to drain. It can occur in individuals of any age and is more common in those with certain medical conditions, such as sickle cell disease.

Inflammatory or Infectious Conditions

The penis is a possible site of infection for many sexually transmitted diseases (STDs). The external skin of the penis is susceptible to infections such as genital herpes and human papillomavirus (HPV). The urethra is a possible site of infections by other STDs such as gonorrhea and chlamydia.

Inflammatory or infectious conditions affecting specific parts of the penis include:

  • Balanitis. This refers to inflammation of the head of the penis (and sometimes the foreskin, often caused by sexually transmitted diseases. However, it can also be caused by auto-immune conditions and other forms of chronic inflammation.
  • Urethritis: This refers to inflammation or infection of the urethra. It is often, but not always, caused by a sexually transmitted infection in men.
  • Urethral stricture: This refers to the abnormal narrowing of the urethra, making it difficult to urinate. This may be the result of infection, inflammation, or other processes.

Not all issues with urination are caused by issues with the urethra or penis. They may also be caused by the prostate and/or bladder.

Conditions Affecting the Foreskin

Men and boys who have a foreskin may experience problems with either retracting the foreskin from the head of the penis or where the foreskin gets stuck in the retracted position.

The first condition, phimosis, is not an emergency, particularly before puberty. Many young men can not fully retract their foreskin until after puberty. The second, called paraphimosis, occurs when the foreskin gets stuck in the retracted position.

Paraphimosis is considered a medical emergency. This is because the stuck foreskin to restrict blood supply to the head of the penis and cause tissue necrosis (death).

Penile Malignancies

Penile cancer is very rare. However, it can be an aggressive form of cancer. It originates in the cells of the penis. Penile cancer is diagnosed in fewer than one man per 100,000 each year and accounts for fewer than 1% of cancers in men in the United States.

Urethral cancer is similarly rare. It originates in either the proximal or distal end of the urethra.

Diagnosis

The type of tests used to diagnose conditions of the penis depends on the symptoms being diagnosed.

Erectile Problems

Testing for erectile dysfunction and/or Peyronie's disease often involves taking a sexual and medical history.

In addition, your doctor may order imaging studies of your penis. This may include either an ultrasound or magnetic resonance imaging (MRI). Penile imaging is often done while the penis is erect. To get an erection in the doctor's office, the doctor will usually give a vasodilator injection that causes blood to flow into the penis.

You may also be asked to use one of several methods to determine whether you are getting erections at night. This is referred to as nocturnal tumescence testing.

Infectious or Inflammatory Conditions

Issues with penile discharge, pain, or inflammation may require testing for sexually transmitted diseases or other infections. This can include urine tests. It may also include a visual examination or urethral swabs to check for STDs and other infections.

Blood tests may also be used to look for signs of infection or inflammation that affect penis health. Biopsies may be used to identify any lumps or lesions.

Urinary Problems

Problems with urination usually involve a urinalysis to check for abnormalities in your urine. In older men especially, a prostate-specific antigen (PSA) test may be ordered to check for an enlarged prostate and prostatic inflammation. A digital rectal exam (DRE) may also be performed.

Issues with urination may require you to undergo a voiding cystourethrogram or VCU. This involves an image being taken while you urinate. The VCU allows the doctor to identify any blockages or restrictions in your urinary stream.

VCU is more commonly used in children than adults. It is also more often used after surgery affecting the penile urethra, including phalloplasty.

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