The ureters are muscular tubes—about 16 inches (40 centimeters) long—that attach at their upper end to the kidneys and at their lower end to the bladder. (See also Overview of the Urinary Tract.)
Urine formed in the kidneys flows through the ureters into the bladder, but it does not flow passively. The ureters push each small amount of urine along in waves of contraction, at low pressure. At the bladder, each ureter passes through an opening in the bladder wall, which closes when the bladder contracts to prevent urine from flowing back into the ureter (reflux).
