Histology of the urinary system

Kidney

The kidneys are paired retroperitoneal organs of the urinary system. Their function is to filter blood and produce urine. Each kidney consists of a cortex, medulla and pelvis.

The nephron is the main functional unit of the kidney, responsible for removing metabolic waste and excess water from the blood. Depending on their distribution and morphology, there are two main types of nephrons in the kidney; cortical and juxtamedullary. 

The renal corpuscle is the filtration apparatus of the nephron. Each corpuscle consists of two main elements; the glomerulus and glomerular (Bowman's) capsule.

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Ureters

The ureters are bilateral, muscular, tubular structures, responsible for taking urine from one kidney to the urinary bladder for storage, prior to excretion. The lumen of each ureter is lined by a mucosal layer of urothelium (transitional epithelium) and a thick fibroelastic lamina propria lies underneath the epithelium. The mucosa of the ureters executes a protective function in general.

The remainder of the ureteral wall is composed of multiple layers of smooth muscle and connective tissue. There are two muscular layers in the wall of the ureter: an inner longitudinal and a middle circular layer. In the lower third segment of the ureters, another outer longitudinal layer can be found proximal to the bladder. As the ureter is usually embedded within the retroperitoneal adipose tissue, this also forms the adventitia (outer layer) of the ureter.

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Urinary bladder and urethra

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 contains three openings, two for the ureters (ureteric orifices) and one for the urethra (internal urethral orifice), with the triangular region defined by these three openings known as the trigone. The wall of the urinary bladder is comprised of smooth muscle fibers collectively known as the detrusor muscle. 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 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.

Solidify your knowledge of the histology of the urinary bladder and urethra with this study unit:

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• Kidney histology
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