Separation of the renal pelvis in either the left or right kidney is often found and reported during a kidney ultrasound, indicating a sign of hydronephrosis. The human urinary system, which includes the kidneys, ureters, bladder and urethra, has the primary function of excretion. The renal pelvis is part of the kidneys and produces urine when blood flows through the kidneys, which first drains to the pelvis, then to the bladder via the ureter and out of the body via the urethra. Therefore, when obstruction occurs in any part of the ureter below the ureter, including the ureteropelvic junction, such as urinary stones, urinary tumors, inflammatory strictures, trauma, and malformations, which are more common in clinical practice, it can cause hydronephrosis. This can be manifested as a separation of the renal pelvis, often greater than 1cm, when a kidney ultrasound is done, and less than 1cm is considered normal.