Diuretic therapy reduces blood volume primarily by increasing what?

• A. Blood viscosity
• B. Urine production
• C. Cardiac output
• D. Hematocrit

Answer: (B)

Diuretic therapy reduces blood volume by increasing urine production. These drugs act on the kidneys to inhibit reabsorption of sodium (and the water that follows), or to otherwise promote water loss in the urine. The resulting diuresis lowers the circulating plasma (extracellular) fluid, which directly reduces overall blood volume. This decrease in volume also lowers venous return to the heart and can contribute to lower blood pressure.

The other factors aren’t the primary mechanism. Blood viscosity is largely determined by hematocrit, which can rise as plasma volume falls, but that’s a secondary consequence rather than how diuretics reduce blood volume. Cardiac output can decrease secondarily due to reduced preload, and hematocrit depends on plasma-volume shifts rather than being the driver of the volume decrease.