Male urethra

Preprostatic urethra

The preprostatic urethra is the first part of the male urethra, which passes through the neck of the bladder, just below the internal urethral orifice. This part is also referred to as the intramural part of the urethra.

The preprostatic urethra is about 0.5-1.5 cm in length but varies depending on whether the bladder is filled with urine or empty.

This part of the urethra is surrounded by the smooth muscle of the internal urethral sphincter which prevents retrograde flow of semen into the bladder during ejaculation.

Prostatic urethra

The prostatic urethra is the part of the urethra that passes through the prostate. It is about 3-4 cm long and extends between the preprostatic and the membranous part of the urethra. The posterior part of the prostatic urethra is marked by an elevation known as the urethral crest.he prostatic sinuses are two depressions on each side of the crest where the prostatic ducts empty prostatic fluid into the urethral lumen. The urethral crest enlarges to form a rounded elevation which is known as the seminal colliculus. In the center of the seminal colliculus is a small indentation, the prostatic utricle, an embryological remnant that is thought to be the homologue of the uterus. The ejaculatory ducts also open up in the prostatic urethra, on each side of the prostatic utricle. These ducts carry sperm from the testes and fluid from the seminal vesicles into the urethra. Thus the urinary and reproductive tracts merge at this point.

The prostatic urethra is a common site of obstruction to the outflow of urine in patients with benign hypertrophy of the prostate.

Membranous urethra

The membranous part is also known as the intermediate part of the urethra. This is the second shortest part of the urethra and extends between the prostatic urethra to the penile urethra. It is also the narrowest and least dilatable portion of the urethra when compared to its other parts. 

This part passes through the deep perineal pouch. The membranous urethra is about 1-1.5 cm long and is surrounded by the external urethral sphincter.

The external urethral sphincter plays an important role in voluntary control of urinary flow.

Spongy urethra

The spongy (or penile) urethra is the largest and final part of the urethra. It is approximately 15 cm long and is divisible into two main parts: the bulbar urethra and the pendulous urethra. The bulbar urethra is located in the bulb of the penis, while the pendulous urethra runs along the length of the penis. 

The spongy urethra is surrounded by the erectile tissue of the penis, the corpus spongiosum. This part of the urethra is widened in two places: first in the bulb of the penis and distally at the end of the penis (as the navicular fossa). It opens externally through the urethral meatus (external urethral orifice), which is a vertical slit-like opening slightly behind the tip of the penis. The bulbourethral glands and the urethral glands of Littre open into the spongy urethra.

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Vasculature and lymphatics

Innervation

The urethra is innervated by the prostatic plexus. The prostatic plexus includes nerves with a mixture of fibers derived from the sympathetic and parasympathetic trunks, as well as from visceral afferent fibers. This plexus lies on the posterolateral aspect of the prostate and seminal vesicles on each side, and is derived from the pelvic plexus. The striated muscle of the external urethral sphincter is innervated by perineal branches of the pudendal nerve.

More information about the anatomy of the male urinary bladder and urethra is provided below:

Functions

In males the urethra allows for the passage of urine from the bladder to the outside of the body (urination) and is also responsible for the expulsion of sperm during ejaculation. In combination with the above, it also functions in preventing the retrograde flow of either of these fluids and protects the urinary tract from bacteria and other microorganisms.