Although your knowledge of blood pressure (BP) is limited, you likely know that it is unhealthy for the pressure to be excessively high. BP is a measure of how hard the heart must work to deliver oxygen-rich arterial blood to all the cells of the body. It is likely that you also know that there is a top number, the systolic pressure, and a bottom number, the diastolic pressure. the systolic pressure is the force exerted by the heart when the heart muscle contracts, pushing blood out to all parts of the body. The pressure is measured in the arteries, The diastolic pressure is the pressure in the arteries when the muscle is relaxing.
Unlike other muscles in the body, the heart never has an opportunity to rest. Imagine pushing a box of rocks across the floor. You push the box a few feet, and then you stop. When you stop, your muscles relax and you rest. Now imagine pushing that same box of rocks up an incline. You push the box a few feet, and then you stop. When you stop actively pushing, you do not completely relax. You must exert some effort to prevent the box from sliding backward. This is what the heart experiences. Even when it is resting, the heart must put forth some effort.
To understand blood pressure measurement, a garden hose provides a useful analogy. When holding a garden hose that has water flowing out of it, you can bend the hose tightly enough to block the flow completely. If you slowly begin to allow the hose to unbend, water will begin to trickle through. As this trickle begins, you will hear a noise that is made as the water is pushing through a small opening that you have allowed to form.
When BP is measured, an inflatable bladder, or cuff, is placed around their arm over their brachial artery. The cuff is inflated to a level greater than the systolic pressure. This is like the garden hose when the flow of water is stopped completely. The person listening with the stethoscope will no hear anything. As the cuff is slowly allowed to deflate, the individual will begin to hear a noise each time the heart beats. This noise appears when the pressure in the cuff drops just below the pressure exerted by the heart. Like the garden hose, the water pressure is greater than the pressure blocking the hose, but because the opening is narrow, turbulence is created. The turbulence is the sound heard through the stethoscope.
As the cuff continues to deflate, the pressure compressing the brachial artery decreases until it is less than the pressure in the arteries during the heart’s resting phase. At this point, there is no more turbulence and the sound disappears. Referring back to our garden hose, the hose is not blocked at all and water flows freely and smoothly.