A Foley catheter is a flexible tube that is passed through the urethra into the bladder to drain urine. It has two separate lumens running down its length. One lumen is open at both ends and allows urine to drain into a collection bag. The other lumen has a valve on the outside end and connects to a balloon at the tip. The balloon is inflated with sterile water when it lies inside the bladder in order to prevent it from slipping out.