29.20 : Ureters

The ureters are retroperitoneal tubes located on either side of the vertebral column. They are responsible for transporting urine from each kidney to the urinary bladder. These tubes have thick walls and are approximately 25-30 cm long. Their diameter is around 10 mm at the renal pelvis, gradually narrowing to 1 mm as the ureter obliquely enters the posterior bladder wall through the ureteric orifices. The shape of these orifices is slit-like, which helps to prevent urine backflow toward the kidneys when the urinary bladder contracts.

Regarding their histology, each ureter comprises three layers: the mucosa, muscularis, and adventitia. The innermost layer is the mucosa, composed of transitional epithelium, continuous with the mucosae of the kidney pelvis and the bladder. This layer allows the ureters to stretch and adjust according to the volume of urine flow.

Next, the middle layer is the muscularis, which comprises smooth muscle. The inner layer has longitudinal fibers, while the outer fibers are circularly arranged. This layer undergoes rhythmic peristalsis, which enables the unidirectional propulsion of urine into the bladder.

Finally, the outermost layer is the adventitia, which comprises connective tissue that securely positions and shields the ureters from potential damage.