Urethra

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 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 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. ==Clinical significance==

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. ==Additional images==

File:Prostatelead.jpg|Position of the urethra in males File:Gray1155 a.png|Transverse section of the penis File:Male urinary meatus.jpg|Male urethral opening on glans penis File:Skenes gland.jpg|Female urethral opening within vulval vestibule File:1116 Muscle of the Female Perineum.png|Muscles of the female perineum File:Slide12BLA.JPG|Urethra. Deep dissection. Serial cross section. File:Penis lateral cross section.jpg|Diagram which depicts the membranous urethra and the spongy urethra of a male File:Female vaginal anatomy.jpg ==See also==