[5] The internal sphincter, formed by the involuntary smooth muscles lining the bladder neck and urethra, receives its nerve supply by the sympathetic division of the autonomic nervous system.

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.

[10] The urethra is divided into four parts in men, named after the location:[10] These openings are collectively called the verumontanum (colliculus seminalis).

[15] In the human female, the urethra is about 4 cm long,[10][12] having 6 mm diameter,[12] and exits the body between the clitoris and the vaginal opening, extending from the internal to the external urethral orifice.

This, over the fourth to the seventh week, divides into a urogenital sinus and the beginnings of the anal canal, with a wall forming between these two inpouchings called the urorectal septum.

[18] The cells lining the urethra (the epithelium) come from endoderm, whereas the connective tissue and smooth muscle parts are derived from mesoderm.

[22] Investigations such as a gram stain of the discharge might reveal the cause; nucleic acid testing based on the first urine sample passed in a day, or a swab of the urethra sent for bacterial culture and sensitivity may also be used.

[23] When cancer is present, the most common symptom in an affected person is blood in the urine; a physical medical examination may be otherwise normal, except in late disease.

Damage to the urethra, such as by kidney stones, chronic infection, cancer, or from catheterisation, can lead to narrowing, called a urethral stricture.

[24] The location and structure of the narrowing can be investigated with a medical imaging scan in which dye is injected through the urinary meatus into the urethra, called a retrograde urethrogram.

[28] Complications that are associated with catheter insertion can include catheter-associated infections, injury to the urethra or nearby structures, or pain.

In addition, the closing mechanisms of the urethra, together with immunoglobulins, largely prevent germs from penetrating the inside of the body.

[31] It was only in the 1550s that anatomists such as Bartolomeo Eustacchio and Jacques Dubois began to use the terms to specifically and consistently refer to what is in modern English called the ureter and the urethra.

[33] Surgery to the urethra to remove kidney stones has been described since at least the first century AD by Aulus Cornelius Celsus.