Dilation of the pelvis of the kidney, caused by a blockage in the flow of urine. In each kidney, the urine passes through the renal calix into the renal pelvis, and from there into the bladder via the ureter. The bladder opens to the exterior through the the urethra. A blockage anywhere in these ducts can result in hydronephrosis because urine backs up above the obstruction. If the emptying of the bladder is impeded, it is known as urine retention. An accumulation of urine in the bladder makes it difficult to discharge the urine from the kidneys through the ureters to the bladder, and the result is that urine builds up in the two renal pelvises. A ureter can be blocked by a stone, inflammation or a tumour. A kink in a ureter, such as may occur with a floating kidney or a congenital predisposition in one or both ureters, can also cause hydronephrosis.

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