Which option best describes the effect of diuretics on preload?

• A. Increases afterload
• B. Decreases preload
• C. Decreases afterload
• D. Increases preload

Answer: (B)

Preload is the filling pressure of the heart before it contracts, largely set by venous return to the ventricles. Diuretics cause the kidneys to excrete more salt and water, which lowers circulating blood volume. With less volume returning to the heart, venous return falls and the left ventricular end-diastolic pressure decreases, so preload decreases. This is the primary way diuretics change the heart’s loading conditions. They don’t directly reduce the resistance the heart has to pump against (afterload), so the description that best fits their effect is a decrease in preload.