The urethra is a thin-walled muscular tube that drains urine from the bladder and conveys it out of the body. The epithelium of its mucosal lining is mostly pseudostratified columnar epithelium. However, near the bladder it becomes transitional epithelium, and near the external opening it changes to a protective stratified squamous epithelium.
At the bladder-urethra junction a thickening of the detrusor smooth muscle forms the internal urethral sphincter. This involuntary sphincter keeps the urethra closed when urine is not being passed and prevents leaking between voiding. This sphincter is unusual in that contraction opens it and relaxation closes it.
The external urethral sphincter surrounds the urethra as it passes through the urogenital diaphragm. This sphincter is formed of skeletal muscle and is voluntary controlled. The levator ani muscle of the pelvic floor also serves as a voluntary constrictor of the urethra.
The length and function of the urethra differ in the two sexes. In females the urethra is only 3-4 cm (1,5 inches) long and tightly bound to the anterior vaginal wall by fibrous connective tissue. Its external opening , the external urethral orifice, lies anterior to the vaginal opening and posterior to the clitoris.
In males the urethra is approximately 20 cm (8 inches) long and has three regions.The prostatic urethra, about 2,5 cm (1 inch) long, runs within the prostate. The membranous urethra, which runs through the urogenital diaphragm, extends about 2 cm from the prostate to the beginning of the penis. The spongy urethra, about 15 cm long, passes through the penis and opens at its tip via the external urethral orifice. The male urethra has a double function: It carries semen as well as urine out of the body.