The urethra (: 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 Placentalia. In males, it carries semen through the penis during Ejaculation.
The external urethral sphincter is a striated muscle that allows voluntary control over urination.
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
In the human male, the urethra is on average long and opens at the end of the external urethral meatus.
The urethra is divided into four parts in men, named after the location:
|
| Transitional
|
Crosses through the [[prostate gland]]. There are several openings at the posterior wall:
-
the ejaculatory duct (2 lateral) receives spermatozoon from the vas deferens and ejaculate fluid from the seminal vesicle
-
the prostatic sinus which has openings for several prostatic ducts where fluid from the prostate enters and contributes to the ejaculate
-
the prostatic utricle, which is merely an indentation.
These openings are collectively called the verumontanum (colliculus seminalis).
The prostatic urethra is a common site of obstruction to outflow of urine in BPH patients | Transitional
|
| | Pseudostratified columnar |
| | Pseudostratified columnar – proximally Stratified squamous – distally
|
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,
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.
[Manual of Obstetrics. (3rd ed.). Elsevier. pp. 1-16. .]
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.
Microanatomy
The cells lining the urethra (the
epithelium) start off as transitional cells as it exits the bladder, which are variable layers of flat to cuboidal cells that change shape depending on whether they are compressed by the contents of the urethra.
Further along the urethra there are pseudostratified columnar and stratified columnar epithelia.
The lining becomes multiple layers of flat cells near the end of the urethra, which is the same as the external skin around it.
There are small mucus-secreting urethral glands, as well as bulbo-urethral glands of Cowper, that secrete mucous acting to lubricate the urethra.
The urethra consists of three coats: muscular, erectile, and mucous, the muscular layer being a continuation of that of the bladder.
Blood and nerve supply and lymphatics
Somatic (conscious) innervation of the external urethral sphincter is supplied by the
pudendal nerve.
Development
In the developing
embryo, at the hind end lies a cloaca. 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.
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.
The cells lining the urethra (the epithelium) come from
endoderm, whereas the connective tissue and smooth
muscle parts are derived from
mesoderm.
After the third month, urethra also contributes to the development of associated structures depending on the biological sex of the embryo. In the male, the epithelium multiples to form the prostate. In the female, the upper part of the urethra forms the urethra and paraurethral glands.
Function
Urination
The urethra is the vessel through which urine exits the bladder. During urination, the urethra's smooth muscle relaxes as the bladder contracts to expel urine in a pressurized stream. Following this, the urethra re-establishes muscle tone by contracting the smooth muscle layer, and the bladder returns to a relaxed, quiescent state. Urethral smooth muscle cells are mechanically coupled to each other to coordinate mechanical force and electrical signaling in an organized, unitary fashion.
Ejaculation
The male urethra is the conduit for
semen during
orgasm.
Urine is removed before ejaculation by
pre-ejaculate fluid – called Cowper's fluid – from the bulbourethral gland.
Clinical significance
Infection of the urethra is
urethritis, which often causes purulent urethral discharge.
It is most often due to a sexually transmitted infection such as
gonorrhoea or
chlamydia, and less commonly due to other bacteria such as
ureaplasma or
mycoplasma; trichomonas vaginalis; or the viruses herpes simplex virus and
adenovirus.
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.
Treatment usually involves
that treat both gonorrhoea and chlamydia, as these often occur together.
A person being treated for urethritis should not have sex until the infection is treated, so that they do not spread the infection to others.
Because of this spread, which may occur during an incubation period before a person gets symptoms, there is often
contact tracing so that sexual partners of an affected person can be found and treatment offered.
Cancer can also develop in the lining of the urethra.
Injury
Passage of
through the urethra can be painful. Damage to the urethra by kidney stones, chronic infection, cancer, or catheterisation can cause narrowing, called a urethral stricture.
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.
Injuries to the urethra (e.g., from a pelvic fracture
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
urinary 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.
[ This may be to relieve or bypass an obstruction, to monitor urine output, or because a person has difficulty urinating, for example due to a neurological cause such as multiple sclerosis.][ Complications that are associated with catheter insertion can include catheter-associated infections, injury to the urethra or nearby structures, or pain.]
Other animals
In all , with the exception of , 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 antibody, largely prevent germs from penetrating the inside of the body.
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.
Kidney stones have been identified and recorded about as long as written historical records exist.
==Additional images==
See also
External links
