Why do you think diurectis are often prescribed as a treatment for high blood pressure?

Water creates pressure when it is present in the body. Lowering the amount of water in the blood through diuretics lowers blood pressure. Think of a garden hose with water running through it. If you shut off the end where the spout is, the pressure will increase in the hose, if you open it an let the water run out, the pressure inside the hose will drop. The same principle works in your body. If you have less water in it, by urinating more, your blood pressure would drop.