Kidneys make urine that flows down a tube called the ureter to the bladder. Ureters enter the bladder obliquely through a tunnel in the bladder muscular tissue.
This orientation acts like a flap valve that prevents urine from flowing in reverse up the ureter as the pressure inside the bladder increases during bladder filling, and even more so when the bladder contracts to expel urine.
If the ureters don’t enter the bladder properly, or if there is an obstruction downstream that increases bladder pressure, urine can reflux from the bladder into the ureter and kidney – a condition that is called vesicoureteral reflux (VUR). Girls are five times more likely to have VUR than boys. VUR is usually a congenital defect. The incidence of VUR is increased if a sibling or parent has a history of VUR.