Ureters

Ureters are retroperitoneal tubes positioned on either side of the vertebral column that transport urine from each kidney to the urinary bladder. These thick-walled tubes measure 25–30 cm long and have a diameter of 10 mm at the renal pelvis, narrowing to 1 mm as they enter the posterior bladder wall through the ureteric orifices.

The slit-like shape of the ureteric orifices prevents urine backflow toward the kidneys when the urinary bladder contracts. This specialized anatomical design acts as a one-way valve, ensuring urine moves only from the kidneys toward the bladder during normal urinary function.

Each ureter comprises three layers: the mucosa, muscularis, and adventitia. The innermost mucosa contains transitional epithelium that stretches with urine volume. The middle muscularis layer consists of smooth muscle fibers arranged longitudinally and circularly. The outermost adventitia is connective tissue that anchors and protects the ureter.

Transitional epithelium in the ureter's mucosa layer allows the ureters to stretch and adjust according to the volume of urine flow. This specialized tissue is continuous with the mucosae of the kidney pelvis and the bladder, enabling the ureter to accommodate varying amounts of urine without damage.

The muscularis layer undergoes rhythmic peristalsis, enabling the unidirectional propulsion of urine into the bladder. Its smooth muscle fibers are arranged with longitudinal fibers on the inner layer and circular fibers on the outer layer, creating coordinated contractions that move urine downward.

The adventitia is the outermost layer composed of connective tissue that securely positions and shields the ureters from potential damage. This protective layer anchors the ureters in place within the retroperitoneal space and provides essential structural support for the tubes.

The ureter has a diameter of 10 mm at the renal pelvis, which gradually narrows to 1 mm as it obliquely enters the posterior bladder wall at the ureteric orifices. This tapering design facilitates the controlled flow of urine and contributes to the valve-like function that prevents backflow.