This section is from the book "A Manual Of Pathology", by Guthrie McConnell. Also available from Amazon: A Manual Of Pathology.
Hydronephrosis is a cystic dilatation of the kidney resulting from an obstruction of the ureter. Is more likely to occur if the obstruction is of an intermittent character. When the ureter is obstructed, as by a calculus, by inflammatory changes, by twists or kinks, or by pressure from new growths either within or similar conditions on the outside, it and the pelvis of the kidney will begin to dilate. The urine, being unable to escape, collects and gradually causes the tissues to stretch. The pelvis becomes larger, the calices are flattened out, and the renal tissue becomes atrophic until, as a result of the combined pressure and atrophy, there is a mere shell of kidney substance remaining. The fluid contained within the sac is at first practically normal urine. As long as there is any secretory tissue left, urine is excreted, but when that ceases, the salts in the fluid are either precipitated or absorbed and the remaining liquid is watery. Such cysts may become infected or hemorrhages may take place within them. When filled with pus, the condition is known as pyonephrosis.