How do water pills (diuretics) work to lower blood pressure?

Removal of fluid: Simple answer is that by removing excess fluid, the blood pressure is lowered. Some diuretics also have effects on the blood vessel walls causing them to dilate. Imagine a baloon with a gallon of water in it. Then remove 1/2 the water. The pressure inside the baloon is less. That is an oversimplification, but not completely.

Diuresis: They act on the tubular part of the microscopic nephron in the kidney which filters out water and prevents reabsorption of that water in various parts of the tubule. Then this return blood deprived of water again circulates through the skin releases a plasma and it is avid for water and sucks it back.