Function

The urethra transports urine from the bladder to the outside of the body for disposal. The urethra is the only urologic organ that shows any significant anatomic difference between males and females; all other urine transport structures are identical (Pic. 1). 

The brain signals the bladder muscles to contract, which squeeze the urine out of the bladder. Simultaneously, the sphincter muscles of the bladder relax to let urine exit through the urethra. When all the signals happen in this exact order, normal urination occurs.

Anatomical structure

The final passageway for the outflow of urine is the urethra—it is a thin-walled tube that passes urine from the urinary bladder to outside of the body.

The male urethra passes through the prostate gland immediately inferior to the bladder before passing below the pubic symphysis. The length of the male urethra varies between men but averages 20 cm in length. It is divided into four regions: the preprostatic urethra, the prostatic urethra, the membranous urethra, and the spongy or penile urethra.

The preprostatic urethra is very short and incorporated into the bladder wall. The prostatic urethra passes through the prostate gland. During sexual intercourse, it receives sperm via the ejaculatory ducts and secretions from the seminal vesicles. Paired Cowper’s glands (bulbourethral glands) produce and secrete mucus into the urethra to buffer urethral pH during sexual stimulation. The mucus neutralizes the usually acidic environment and lubricates the urethra, decreasing the resistance to ejaculation. The membranous urethra passes through the deep muscles of the perineum, where it is invested by the overlying urethral sphincters. The spongy urethra exits at the tip (external urethral orifice) of the penis after passing through the corpus spongiosum. Mucous glands are found along much of the length of the urethra and protect the urethra from extremes of urine pH. Innervation is the same in both males and females.

Histological structure

The epithelium of the urethra starts off as transitional cells as it exits the bladder. Further along the urethra there are pseudostratified columnar and stratified columnar epithelia (Pic. 2), then stratified squamous cells near the external urethral orifice. There are small mucus-secreting urethral glands that help protect the epithelium from the corrosive urine.  

Development

During the 4th to the 7th weeks of development, the cloaca divides into the urogenital sinus anteriorly and the anal canal posteriorly. The urorectal septum is a layer of mesoderm between the primitive anal canal and the urogenital sinus. The tip of the septum will form the perineal body.

The pelvic part of the urogenital sinus is a narrow canal which in the male gives rise to the prostatic and membranous parts of the urethra.

The epithelium of the urethra in both sexes originates in the endoderm; the surrounding connective and smooth muscle tissue is derived from visceral mesoderm. At the end of the third month, epithelium of the prostatic urethra begins to proliferate and forms a number of outgrowths that penetrate the surrounding mesenchyme. In the male, these buds form the prostate gland.






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Sources

Urethra ―sourced from Boundless licensed under CC BY-SA 4.0
Urethra ―sourced from Wikipedia licensed under CC BY-SA 3.0
Embryonic development of the Bladder and Urethra ―sourced from WikiLectures licensed under CC BY-SA 3.0
The Urinary Excretory System ―sourced from Scribeschool
Ciliated columnar epithelium ―by Straus and Osipov licensed under CC BY-SA 3.0
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