For a better understanding of how diuretics work, it may be helpful to review the basic physiology of body fluids and electrolytes. Electrolytes are minerals made up of electrically charged particles held together by ionic bonds. Ionic bonds are easily broken when dissolved in water, causing the particles to separate into positive and negative ions. For example, sodium chloride (NaCl) breaks into sodium (Na⁺) and chloride (Cl⁻) in body fluid. The major body electrolytes include sodium, potassium, calcium, chloride, magnesium, and phosphorus. Electrolytes are found inside and outside of cells and are acquired through food and water.

Because diuretics cause the excretion of fluid in the form of dilute urine, most diuretics also cause excretion of electrolytes, primarily sodium (Na⁺). Other critical electrolytes, including potassium (K⁺) and calcium (Ca²⁺), are also excreted in the fluid.

Diuretics help to lower blood pressure by increasing the elimination of fluids (and associated electrolytes) from the body: this decreases the blood volume. When there is less blood volume circulating, there is less pressure on the blood vessels and the heart does not pump as forcefully or fast (which reduces cardiac output, the volume of blood pumped by the heart per minute). This concept can be compared to a water balloon. When it is full (has a high volume of water), the pressure of the water pushing on the walls of the balloon is high. When some of the water is removed (lowering of the volume), the pressure is lessened (lower).

For further understanding of the action of diuretics, it is also necessary to briefly review the renal process of excretion. Consult your physiology textbook for additional information on fluids, electrolytes, and renal function.