This ability is measured as fractional sodium excretion. Fractional sodium excretion is the percentage of filtered sodium which is excreted in the urine. The more potent the diuretic, the greater the ability to interfere with the reabsorption of sodium from the reanl tubules resulting in a larger amount of sodium remaining in the excreted urine. The greater the amount of sodium in the urine, the greater the volume of urine. Diuretics are classified by their mechanism of action .
Diuretics work at different locations within the nephron.
Osmotic diuretics cause expansion of the extracellular fluid volume by relocating intracellular fluid to the extracellular space, specifically to the plasma. The expanded blood volume leads to increased renal blood flow which results in a larger amount of blood being filtered into glomerular filtrate. Osmotic diuretics also prevent reabsorption of sodium and water from the renal tubules which results in a larger volume of urine being produced. Therefore mannitol will draw water out of neuronal cells and is used to treat brain edema. Carbonic acid spontaneously breaks down to hydrogen ion and bicarbonate.