The urine that flows into the bladder is prevented from backing up into the ureters by the:

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Where the two ureters enter the bladder, a small triangular fold of tissue called the trigone keeps urine from flowing back into the ureters after it has emptied into the bladder. From the renal pelvis, the urine flows into the ureters. Blood enters the glomerulus through the afferent arteriole. The glomerulus filters the blood, producing filtrate, the basis of urine. Filtered blood that leaves the glomerulus through the efferent arteriole is returned to the circulation. The internal sphincter, a ring of involuntary muscle located where the bladder and the urethra join, keeps the bladder closed while it fills.

The Urine That Flows into the Bladder Is Prevented from Backing