The ureters are muscular tubes—about 16 inches (40 centimeters) long—that attach at their upper end to the kidneys Kidneys The kidneys are bean-shaped organs that figure prominently in the urinary tract. Each is about 4 to 5 inches (12 centimeters) long and weighs about one third of a pound (150 grams). One lies... read more and at their lower end to the bladder Bladder The bladder is an expandable, muscular sac. Urine accumulates in the bladder as it arrives from the ureters. The bladder gradually increases in size to accommodate an increasing volume of urine... read more . (See also Overview of the Urinary Tract Overview of the Urinary Tract Normally, a person has two kidneys. The rest of the urinary tract consists of the following: Two ureters (the tubes connecting each kidney to the bladder) The bladder (an expandable muscular... read more .)
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).