ABSTRACT
The pelviureteric junction (PUJ) is the most common site of obstruction in the pediatric upper urinary tract. The PUJ is formed during the fifth week of embryogenesis. Urine flows across it from the kidney and down the length of the ureter by the twelfth week. Flow occurs when renal pelvic pressure exceeds upper ureteral pressure. The pressure gradient is created partly by the hydrostatic force of the filtered urine, but principally by the peristaltic contractions originating in the region of the renal pelvis called the pacemaker and progressing across the PUJ and down the ureter. Hydronephrosis develops either when the PUJ is obstructed or when the normal peristaltic waves are impeded.
