There's a relationship between the diameter or radius of the hose or the vessel and the pressure of the water. So if you constrict a blood vessel or narrow it, you're increasing the blood pressure. The flow is to the power of four of the radius. What that means is basically this: If you put your thumb over the end of that hose and cover half of it, so you have the radius. Let's say you go from one centimeter radius to let's say half a centimeter radius. The change in the flow is actually to thepower of four. And then if you were to dilate or open up that hose, let's say you double it from one cent