The urethra is the canal that leads from the urinary bladder to allow for urine to be expelled into the environment. In men, the urethra also carries semen, traverses the prostate gland which lies below the bladder and runs through the penis. In women, the urethra is much shorter and terminates just anterior to the vagina.
The urethra is a muscular tube that starts an the bottom of the bladder known as the internal urethral orifice. It is about 20 centimeters long (between 18 to 22 centimeters) in men, whereas in women, the urethra is only 4 centimeters long. The external urethral orifice is where the urethra terminates and in males this is located at the tip of the glans penis. The female urethra terminates in the vestibule between the labia minora, lying between the clitoris and vaginal orifice.
Anatomy of the Urethra in Men and Women
The male uretha can be divided into three parts :
- prostatic urethra
- membranous urethra
- cavernous urethra (spongy urethra)
A portion of the urethra lying between the neck of the bladder before it enters the prostate gland is known as the pre-prostatic urethra and is very short, about 0.5 to 1.5 centimeters in length. The length varies depending on the distension of the bladder. The pre-prostatic urethra is often considered as part of the prostatic urethra.
The prostatic urethra is about 3 centimeters long and runs through the prostate gland. Here the prostatic ducts empty into it through the prostatic sinuses, carrying prostatic fluid to make up the semen. The ejaculatory ducts also empty into prostatic urethra.
The membranous urethra is about 1 to 2 centimeters long and the shortest part of the male urethra. It runs from the base of the prostate gland to the bulb of the urethra, piercing the urogenital diaphragm along its course.
The spongy urethra, also known as the cavernous urethra, is about 15 centimeters long and is completely surrounded by the corpus spongiosus of the penis. It can be further subdivided into the bulbar urethra (proximal) and penile urethra (distal). The penile urethra ends in the external urethral orifice. This is a very narrow canal about 6 centimeters in length.
Arteries, Veins and Nerves
- Arterial supply : prostatic branches of the inferior vesical and middle rectal arteries
- Venous drainage : prostatic venous plexus
- Innervation : prostatic plexus which is an extension of the inferior hypogastric plexus
The blood supply, venous drainage of the distal urethra may be similar to the penis.
The female urethra is shorter and narrower than the male urethra. It runs along the anterior wall of the vagina, passing behind (posterior) and then under (inferior) to the pubic symphysis. The female urethra is studded by several urethral glands, one group of which known as the paraurethral glands being homologous to the male prostate gland. Females do not have an actual internal urethral sphincter muscle because its primary function is to prevent reflux of semen which may occur in males. Along with the vagina, it passes through the pelvic diaphragm to enter the perineum. The lamina propria, like the corpus spongiosus of the male urethra, is rich in nerves and blood vessels.
Arteries, Veins and Nerves
- Arterial supply : internal pudendal and vaginal arteries
- Venous drainage : internal pudendal and vaginal veins
- Innervation : vesical (bladder) plexus, pelvic splanchnic nerves and pudendal nerve