Figure 1 is a diagrammatic representation of the nephron. It is important for readers to have some knowledge of the fluid and electrolyte balance of the body which is maintained in a constant state by the automatic electronic filtration system provided by the kidney. Each human kidney contains approximately one million nephrons. Each nephron is a mini filter that includes a glomerulus that contains a group of capillary blood vessels. The primary duty of the glomerulus is excretion of water and solutes. A considerable amount of sodium and water passes from the glomerulus into the renal tubule and then a substantial amount of sodium and water and other electrolytes must be reabsorbed into the circulation in correct proportions to maintain the constancy of the body fluids. The primary duty of the renal tubules are the retention and conserva¬tion of water and essential electrolytes such as sodium and potassium. The tubules have a secretory function that removes a vast quantity of water from the tubules and returns the fluid, sodium, potassium, and other electrolytes in correct proportions to the circulating blood. This normal physiologic activity is inhibited by diuretics.
The nephrons filter more than 180 liters daily (125 ml per minute). This volume of water passes into the tubules and approximately 179 liters are reabsorbed daily with only about 1–2 liters eliminated as urine. Without the intervention of the tubules between the glomeruli and the renal pelvis, which joins the ureter and the bladder, the whole water and soluble constituents of the body would be lost in less than 24 h. This situation occurs in conditions that cause obstruction of the tubules or renal tubular necrosis.