Hydronephrosis is a condition where one or both kidneys become stretched and swollen as a result of a build-up of urine inside the kidneys. Unilateral hydronephrosis is swelling of one kidney due to a backup of urine. Hydronephrosis occurs as the result of a disease. Hydronephrosis can result from anatomic or functional processes interrupting the flow of urine. This interruption can occur anywhere along the urinary tract from the kidneys to the urethral meatus. The rise in ureteral pressure leads to marked changes in glomerular filtration, tubular function, and renal blood flow.