Kidney physiology. The kidneys regulate the fluid and electrolyte balance of the body by continually filtering the blood. This is vital to maintain a constant extracellular fluid volume and composition.To perform this important function they are able to: excrete or conserve salt and water #Health Health, Composition, Extracellular Fluid, Fluid And Electrolytes, Kidney Cancer, Kidney Stones, Physiology, Kidney, Water Health