I am not a medical professional but if I recall correctly blood pressure is the measurement of resistance of flow. Blood pressure that is too high has a very high resistance to flow which would indicate restrictions, like pinching off a garden hose. If your body is swollen it squishes the veins so the heart has to work harder to push the blood through the veins. On the other side low blood pressure has very little restriction of flow which could indicate a heart pumping problem meaning the heart isn't pushing enough blood, like just barely opening the faucet with a garden hose attached to it. I suppose you could also have a major blood leak and have low blood pressure too but it would probably be obvious unless you were bleeding internally. The way I have always understood it your heart is responsible to inflate your veins and keep the blood moving. When they use the cuff they measure the pressure while listening to the flow of blood. They see what pressure the flow stops at, which is how hard your heart can push, then they measure the pressure at which the blood starts to flow again which indicates how hard your heart pushes normally. If you think of your heart as a pump, it can only push so hard before it can't push anymore. If you have high blood pressure at rest your heart is already pushing that much closer to its high limit, when stressed your heart might not be able to keep up and then bad things happen.