So the reason that we need a drop in pressure is to make sure that the pressure and the rate at which our fluid is flowing within the capillaries is low so that the capillaries don't actually pop and so that the capillaries can actually exchange those nutrients and waste products effectively and efficiently. Now, one quick way that I can explain why the velocity within the capillaries is low is by using something called the continuity equation. The continuity equation, Q equals AV, comes from fluid dynamics, comes from physics. So within our entire cardiovascular system our Q remains constant. Q is simply the flow of blood within our system. The flow of blood doesn't actually change because we have a closed cardiovascular circuit, a closed cardiovascular system.