Urinary bladder and urethra

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The urinary bladder and urethra are pelvic urinary organs whose respective functions are to store and expel urine outside of the body in the act of micturition (urination).

The urinary bladder is a distensible muscular sac acting as a reservoir for urine. It contains three openings, two for the ureters (ureteric orifices) and one for the urethra (internal urethral orifice). The triangular region defined by these three openings, the trigone, is relatively smooth and constant in thickness, whereas the rest of the bladder wall is thick and folded when the bladder is empty and thin and smooth when the bladder is distended.

The wall of the urinary bladder is comprised of smooth muscle fibers arranged in multiple directions. These smooth muscle fibers are collectively known as the detrusor muscle. This interwoven arrangement of muscle fibers provides the bladder with the ability to stretch in response to the presence of urine. Contraction of the detrusor muscle compresses the entire bladder and forces the urine into the urethra. Similar to the ureters, the urinary bladder is lined by a mucosal layer of transitional epithelium (urothelium) which helps to accommodate for large volume changes.

The urethra is the excretory canal of the urinary bladder. It conveys urine from the urinary bladder to outside the body. It extends from the internal urethral orifice of the bladder to the external urethral orifice of the external genitalia. The course of urethra is different between males and females.

Like most urinary organs, the urethra is lined with a mucosal layer of transitional epithelium (urothelium). Deep to the urothelium, a dense collagenous lamina propria can be found. Additionally, two layers of smooth muscle (inner longitudinal, outer circular) lie beneath the lamina propria.

The male urethra serves as the terminal duct for both the urinary and genital systems and is divided into four distinct segments (from proximal to distal): the intramural part of male urethra, the prostatic urethra, the membranous part of male urethra and the spongy urethra. Transitional epithelium ends in the membranous urethra, which in turn is lined with stratified or pseudostratified columnar epithelium. The spongy urethra is lined with pseudostratified colum-nar epithelium except at its distal end, where it is lined with stratified squamous epithelium continuous with that of the skin of the penis. Two types of glands empty into the spongy urethra: the  bulbourethral glands (Cowper’s glands) as well as mucus-secreting urethral glands (glands of Littré).

In contrast, the female urethra is solely lined by stratified columnar epithelium, with a few small mucosal glands that secrete mucus, and is bound to the anterior wall of the vagina by an external layer of fibrous connective tissue.

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