Why does a blood pressure monitor go tight on the arm while measuring?
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First it is important to understand that pressure is force applied to a specific area. Imagine that instead of blood we had very tiny animals walking (flowing) inside of our arteries. Suddenly something (the inflatable cuff)starts pushing in from the outside and these animals would soon be crushed. N an attempt to save themselves these animals (blood) start pushing the walls out. Whenever they are able to stop the walls from continuing shrinking that menas their collective force has equaled the force that is pushing into them, so if we know how much pressure the cuff is inflated at that time we also know how much pressure these animals are using (blood pressure). The question now would be; how do we know that the two pressures have reached an equilibrium? Well, instead of trying to do incredibly precise and delicate changes we came up with an easier way that doesn’t require this equilibrium. Let’s add some more details to the scenario. When this “animals” (blood) travel through our arteries they do it very silently, unless they crash against an uneven surface (which are not common in our bascular system). When the walls start closing down on them, the animals closer to the walls will start crashing into the walls and will make sounds, these sounds would get louder as the walls keep closing, finally when the walls collapse completely things will go quiet again, since there is no movement. At this moment we know the pressure in the cuff is greater that the pressure the animals are capable of producing so we start releasing the pressure slowly until we hear a pulsating sound. At this point we know the pressure the animals (blood) are applying to the wall is slightly larger that the pressure of the cuff, but since only some animals would be able to pass and most will just pass but crashing against the narrow opening we will hear that, and we could say at this time even though technically the pressure created by the animals is greater than the cuff they are very close together so we call that the diastolic, or maximum pressure (when the heart and the arteries contract). From that point on as we keep lowering the cuffs pressure, we will keep hearing sounds with each heart and arterial contraction (pulse) until the animals are able to walk (flow) freely (meaning there is no sound since they don’t crash against anything) and that would be the diastolic pressure, or the lowest pressure; when the heart and the arteries are relaxed.
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