Ureter

The ureters are slender tubes that convey urine from the kidneys to the bladder. Each ureter begins at the level of L2  as a continuation of the renal pelvis. From there, it descends behind the peritoneum and runs obliquely through the posterior bladder wall. This arrangement prevents backflow of urine during bladder filling because any increase in bladder pressure compresses and closes the distal ends of the ureters.

Histologically, the ureter wall has three layers. The transitional epithelium of its lining mucosa is continuous with that of the kidney pelvis superiorly and the bladder medially. Its middle muscularis is composed chiefly of two smooth muscle sheets: the internal longitudinal layer and the external circular layer. An additional smooth muscle layer, the external longitudinal layer, appears in the lower third of the ureter. The adventitia covering the ureter's external surface is typical fibrous connective tissue.

The ureter plays an active role in transporting urine.  Incoming urine distends the ureter and stimulates its mucularis to contract, propelling urine into the bladder. (Urine does not reach the bladder through gravity alone) The strength and frequency of the peristaltic waves are adjusted to the rate of urine formation. Each ureter is innervated by both sympathetic and parasympathetic fibers, but neural control of peristalsis appears to be insignificant compared to the way ureteral smooth muscle responds to stretch.