Ureters

Ureters

Once formed in the nephrons, urine passes from the collecting ducts to enter the calyces of the kidney, and then through the renal pelvis and one of the ureters into the bladder. The urethra passes urine to outside the body via the ureteral openings, which are slit-like in appearance. Each thin ureter is about 30 cm long, descending behind the parietal peritoneum to run parallel to the vertebral column. The ureters begin at the spinal L2 level, continuing from the renal pelvis. They descend behind the peritoneum and join the uri-nary bladder by running obliquely through its poste-rior wall from underneath.

The wall of each ureter has three layers: the mucous coat (transitional epithelium), muscular coat, and fibrous coat. Urine is propelled by the muscular walls of the ureters. A flap-like fold of mucous mem-brane covers the opening through which urine flows from each ureter into the bladder. These folds keep urine from backing up and flowing back into ureters from the bladder. Backflow of urine is also stopped by any increase in bladder pressure, which compresses and closes the distal ureter ends.

The ureters are distended by incoming urine, which stimulates their musculari to contract. This propels the urine into the bladder, assisted by grav-ity. Peristaltic waves vary in strength and frequency according to the urine formation rate. Neural control of the peristaltic waves is believed to be insignificant in comparison with how the smooth muscles of the ureters stretch. Each ureter is innervated by both sym-pathetic and parasympathetic fibers.