Urethra

The urethra (pl.: urethras or urethrae) is the tube that carries urine from the urinary bladder to the outside of the body through the penis or vulva in placental mammals. In males, it carries semen through the penis during ejaculation.

The external urethral sphincter is a striated muscle that allows voluntary control over urination. The internal sphincter, formed by the involuntary smooth muscles lining the bladder neck and urethra, is innervated by the sympathetic division of the autonomic nervous system and is found both in males and females.

The urethra is a fibrous and muscular tube which connects the urinary bladder to the external urethral meatus. Its length differs between the sexes, because it passes through the penis in males.

In the human male, the urethra is on average 18 to 20 centimeters (7 to 8 inches) long and opens at the end of the external urethral meatus.

The urethra is divided into four parts in men, named after the location:

There is inadequate data for the typical length of the male urethra; however, a study of 109 men showed an average length of 22.3 cm (SD = 2.4 cm), ranging from 15 cm to 29 cm.

The urethra in male placental mammals is typically longer than in females.

In the human female, the urethra is about 4 cm long, having 6 mm diameter, and exits the body between the clitoris and the vaginal opening, extending from the internal to the external urethral orifice. The meatus is located below the clitoris. It is placed behind the symphysis pubis, embedded in the anterior wall of the vagina, and its direction is obliquely downward and forward; it is slightly curved with the concavity directed forward. The proximal two-thirds of the urethra is lined by transitional epithelial cells, while the distal third is lined by stratified squamous epithelial cells.