The urethra (: urethras or urethrae) is the tube that transports urine from the bladder to the urethral meatus of the penis or vulva in placental mammals. In males, it also transports 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 in males and females.
Structure
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.
Male
thumb|The human male urethra laid open on its anterior (upper) surface
In the human male, the urethra is on average long and opens at the end of the external urethral meatus. with steady diameter of 6 mm, and travels through the corpus spongiosum. The ducts from the urethral gland (gland of Littré) enter here. The openings of the bulbourethral glands are also found here. Some textbooks will subdivide the spongy urethra into two parts, the bulbous and pendulous urethra. The urethral lumen runs effectively parallel to the penis, except at the narrowest point, the external urethral meatus, where it is vertical. This produces a spiral stream of urine and has the effect of cleaning the external urethral meatus. The lack of an equivalent mechanism in the female urethra partly explains why urinary tract infections occur so much more frequently in females. || Pseudostratified columnar – proximally<p>Stratified squamous – distally</p>
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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.
Female
In the human female, the urethra is about 4 cm long, having 6 mm diameter,
Between the superior and inferior fascia of the urogenital diaphragm, the female urethra is surrounded by the urethral sphincter.
The urethra in female placental mammals is typically shorter than in the male. Further along the urethra there are pseudostratified columnar and stratified columnar epithelia. The urogenital sinus divides into three parts, with the middle part forming the urethra; the upper part is largest and becomes the urinary bladder, and the lower part then changes depending on the biological sex of the embryo.
Ejaculation
The male urethra is the conduit for semen during orgasm.
Clinical significance
thumb|right|[[Micrograph of urethral cancer (urothelial cell carcinoma), a rare problem of the urethra.]]
Infection of the urethra is urethritis, which often causes purulent urethral discharge.
Cancer can also develop in the lining of the urethra. 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. A retrograde urethrogram in which dye is injected into the urethra can reveal the location and structure of the narrowing. Other forms of imaging such as ultrasound, computed tomography and magnetic resonance imaging may also provide further details.)
Foreign bodies in the urethra are uncommon, but there have been medical case reports of self-inflicted injuries, a result of insertion of foreign bodies into the urethra such as an electrical wire.
Other
Hypospadias and epispadias are abnormal developments of the male urethra where the meatus is not at the distal end of the penis (it occurs lower than normal with hypospadias, and higher with epispadias). In a severe chordee, the urethra can develop between the penis and the scrotum.
Catheterisation
A tube called a catheter can be inserted through the urethra to drain urine from the bladder, called an indwelling urinary catheter; or, to bypass the urethra, a catheter may be directly inserted through the abdominal wall into the bladder, called a suprapubic catheter.
Other animals
In all mammals, with the exception of monotremes, and in both sexes, the urethra serves primarily to drain and excrete urine, which in mammals, collects in the urinary bladder and is released from there into the urethra. In addition, the closing mechanisms of the urethra, together with immunoglobulins, largely prevent germs from penetrating the inside of the body. In marsupials, the female's urethra empties into the urogenital sinus.
History
The word "urethra" comes from the Ancient Greek οὐρήθρα – ourḗthrā. The stem "uro" relating to urination, with the structure described as early as the time of Hippocrates. Confusingly however, at the time it was called "ureter". Thereafter, terms "ureter" and "urethra" were variably used to refer to each other thereafter for more than a millennium. The urinary tract as well as its function to drain urine from the kidneys, has been described by Galen in the second century AD. Surgery to the urethra to remove kidney stones has been described since at least the first century AD by Aulus Cornelius Celsus.