how is urine prevented from flowing back to the ureters?

The ureters join the bladder at an angle so a flap is created. This flap prevents urine flowing back from the bladder into the ureters and the kidneys (vesico-ureteral reflux or VUR). This valve is also important as it stops the high pressure created at urination affecting the kidneys. The valve also lets you get rid of the urine in the bladder in one go, as the urine cannot travel anywhere else.