ureter

Each ureter is a muscular tube approximately 25–30 cm long that descends retroperitoneally from the renal pelvis of each kidney. It passes over the psoas major muscle, crosses the pelvic brim, and enters the pelvis to reach the posterolateral wall of the urinary bladder. The ureter enters the bladder at an oblique angle (the ureterovesical junction), creating a valve-like mechanism that prevents urine reflux when the bladder contracts.

The ureter has three natural constrictions where kidney stones are most likely to become lodged: (1) the ureteropelvic junction (UPJ) where the renal pelvis transitions to the ureter, (2) where the ureter crosses over the iliac blood vessels at the pelvic brim, and (3) the ureterovesical junction (UVJ) where the ureter enters the bladder. These narrowings are the sites of approximately 90% of symptomatic ureteral calculi.

Urine movement through the ureter is not passive gravity flow but active peristalsis — coordinated waves of smooth muscle contraction originating in pacemaker cells at the renal pelvis. These peristaltic waves propel discrete boluses of urine from the kidney to the bladder at a rate of 1–5 waves per minute, even when a person is lying horizontally or standing on their head.