WHY RENAL CYSTS GROW

The growth of renal cysts leads to morbidity, contributes to mortality , and is a lesson in applied physiology. The fact that chemical, elect rical, osmotic, and hydrostatic driving forces determine cyst volumes is inferred from observations that cyst fluids vary in their concentra tions of osmotically active substances. Most have concentrations of so dium-based salts that are lower and nonsodium-based salts that are hig her than those present in normal plasma. Nonsodium solutes include osm olytes that normally are present in higher concentrations inside of ce lls, including potassium, amino acids, and so-called idiogenic osmoles . The basic process of cyst growth therefore involves 1) the osmotic e quilibration of water across cysts walls that have variable permeabili ty characteristics and 2) the replacement of sodium salts with other o smotically active solutes in cyst fluids. Cyst volume is governed by t he amounts and kinds of osmolytes that enter and become entrapped in l umina. Proliferation and necrosis of mural cells are events that are f undamental to the growth of renal cysts.