Urethra is the duct that connects the urinary bladder to the genitals for the removal of fluids out of the body. In males, the urethra travels through the penis and is the route for passage of both urine and semen. In females urethra is shorter and is used only for urination.
Male urethra is about 8 inches long that extends from the neck of the bladder to the external meatus of the glans penis. It provides exit for urine as well as semen during ejaculation. It is divided into three parts, as described below:
- Prostatic urethra:
It is about 1.25 inches long that begins at the neck of the bladder and passes through the prostate from the base to the apex. It is the widest and most dilatable portion of the urethra.
On the posterior wall, there is a longitudinal ridge called the urethral crest, on each side of which is a groove called prostatic sinus. The prostatic ducts open into these sinuses.
On the summit of the urethral crest is a depression known as the prostatic utricle. It is an analog of the uterus and vagina in females. On the edge of the mouth of the utricle are the openings of the two ejaculatory ducts.
- Membranous urethra:
It is the part of urethra that lies within the urogenital diaphragm. It is about 0.5 inches in length and is surrounded by the sphincter urethrae muscle. Membranous urethra is the shortest and the least dilatable part of urethra. The bulbourethral glands are found posterior to this part of urethra but they open into the penile urethra.
Membranous urethra is continuous above with the prostatic urethra and below with the penile urethra.
- Penile urethra:
It is about 6 inches long and is enclosed in the bulb and corpus spongiosum of penis. The terminal part of the penile urethra, which lies within the glans penis, is dilated to form fossa terminalis (navicular fossa).
The ducts of the bulbourethral glands open into the penile urethra below the urogenital diaphragm.
Female urethra is about 1.5 inches in length. It extends from the neck of the bladder to the external meatus, where it opens into the vestibule about 1 inch below the clitoris.
The female urethra traverses the sphincter urethrae muscle and lies in front of the vagina. At the sides of the external urethral meatus, there are small openings of the ducts of the paraurethral glands. Female urethra can be dilated easily as compared to the male urethra.