What prevents the backflow of urine from the urinary bladder to the kidneys?

1. peristaltic action of muscles of the ureters
2. peristaltic action of muscles of the kidneys
3. pulling action of the force of gravity

The urinary system is made up of the kidneys, bladder and ureters (tubes that connect the kidneys to the bladder). The kidneys act as filters removing extra fluid and waste from your blood to make urine. Urine passes from the kidneys into the ureters, which drain into the bladder. When the bladder is full, urine flows out of the body through a tube called the urethra. The ureters normally enter the bladder at a diagonal angle and have a special one-way valve system that prevents urine from flowing back up the ureters in the direction of the kidneys. If this system doesn’t work, urine can flow back towards the kidneys. This is called vesicoureteric reflux (also known as urinary reflux). Sometimes vesicoureteric reflux (VUR) can be associated with other 'plumbing' problems in the urinary tract and with small, damaged or malformed kidneys.

So I believe the answer is 1.