In the normal urinary system, urine is formed by the kidneys then passes into the ureters, which carry it to the bladder for storage. From the bladder, urine passes to the outside through the urethra. One-way valvular structures are located along this passageway to assure that urine only flows in the one direction to the outside. The most important structures are those that prevent urine from flowing backwards from the bladder back into the ureters, and this “one-way” flow system has two important functions.
First, the flow system prevents bacteria (germs) from the outside world from going up into the system to cause infection. The constant one-way flow flushes out any bacteria that may get inside.
Second, the valve-like structures prevent the normal pressure in the lower part of the system from causing urine pressure in the sensitive tissues of the kidney. Such pressure over time can cause damage to the kidney. When urine flows “backward” in the system it is called reflux. Because of worries about infection after birth (there are no bacteria inside the womb to get into the fetal kidney before birth) and pressure damage to the kidney, any evidence of urinary reflux in the developing fetus is closely watched. When there are signs of urinary reflux in the developing fetus, evaluation and prevention of infection after birth are planned. When reflux is a problem after birth, it is often corrected by surgery.