Two sphincters, one voluntary and one not, guard the exit from the bladder
The human urethra, the tube carrying urine out of the bladder, averages about 4 cm in women but 18 to 20 cm in men, because in males it runs the length of the penis. One study of 109 men found an average of 22.3 cm, with a spread from 15 to 29.
Control comes from two rings of muscle. The external urethral sphincter is striated muscle under conscious command, supplied by the pudendal nerve, while the internal sphincter, formed by smooth muscle at the bladder neck, is run involuntarily by the sympathetic nervous system. When the bladder contracts to empty, the tube's smooth muscle goes slack, then regains its tone once the bladder settles. Its muscle cells are mechanically coupled, so force and electrical signals travel through them as a single unit.
The lining changes along the way. Near the bladder it is transitional epithelium, cells that flatten or round up depending on whether they are stretched; further along come columnar layers, and near the opening the lining becomes flat, layered cells matching the surrounding skin. Small glands in the wall secrete mucus, and the wall itself has three coats: muscular, erectile and mucous. In women the tube is about 6 mm across and opens below the clitoris. In men it also carries semen, and Cowper's fluid from the bulbourethral gland clears away urine beforehand.
The structure begins in the embryo as part of the cloaca, which between the fourth and seventh weeks splits into a urogenital sinus and the start of the anal canal. The sinus then divides in three: its upper part becomes the bladder and its middle part the urethra. After the third month, the male urethral lining multiplies to form the prostate.
Inflammation of the tube, called urethritis, is most often caused by sexually transmitted infections such as gonorrhoea or chlamydia, which frequently occur together. Cancer can also arise in the lining; its most frequent sign is blood in the urine, and it usually begins in transitional cells, though chronic schistosomiasis can alter the lining and lead to squamous cell carcinoma instead.
Source: Urethra
