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( Ureter)
In human anatomy, the ureters are muscular ducts that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually 25-30&_160;cm (10-12 inches) long. In humans, the ureters arise from the renal pelvis on the medial aspect of each kidney before descending towards the bladder on the front of the psoas major muscle. The ureters cross the pelvic brim near the bifurcation of the iliac arteries (which they run over). This "pelviureteric junction" is a common site for the impaction of kidney stones (the other being the ureterovesical valve). The ureters run posteroinferiorly on the lateral walls of the pelvis. They then curve anteriormedially to enter the bladder through the back, at the vesicoureteric junction, running within the wall of the bladder for a few centimeters. The backflow of urine is prevented by valves known as ureterovesical valves, pressure from the filling of the bladder. In the female, the ureters pass through the mesometrium on the way to the urinary bladder. The ureter has a diameter of about 3 millimeters, and the lumen is star-shaped. Like the bladder, it is lined with transitional epithelium, and contains layers of smooth muscle, thereby being under autonomic control. The epithelial cells of the ureter are stratified (in many layers), are normally round in shape but become squamous (flat) when stretched. The lamina propria is thick and elastic (as it is important that it is impermeable).
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