How is blood pressure measured? There are two measurements for blood pressure. The first number is the pressure of the blood when it is being pumped by the heart, called the systolic pressure. The second number is the pressure of the blood between heartbeats, called the diastolic pressure.
The amount of blood we have in our bodies depends on our size. Generally, about 10% of our body weight is blood, so an 80 kg person will probably have about 7.5 liters of blood. Unlike water, one kilogram of blood is slightly less than a liter. We all have a different sized heart, but generally two people of the same weight and size will have a similar sized heart. An adult’s heart weighs approximately between 200 and 400 g. There are obviously extremes, but it is a fairly small organ. In order of size, the heart is the fourth largest organ in the body, behind the liver, brain, and lungs. The heart uses about 7% of the body’s energy to function. It beats approximately once a second, which is about 2.5 billion beats in an 80-year life. The heart is a very strong muscle and can pump blood over 10 meters. The heart pushes about 70 ml of blood with every beat and it circulates all of the blood in the body about once a minute.
Why is blood pressure important? Pressure is a measurement of how much force is being exerted on an area. If you let a small amount of air into a bottle, there are not many particles in the bottle and there is a lot of space between the particles. The particles are not pressing on the sides of the bottle and so there is low-pressure in the bottle. The particles are free to move wherever they please. If you keep forcing more and more air into that bottle, the number of particles in the space goes up and the particles start to be squeezed against the sides of the bottle. The force these particles are exerting on the sides of the bottle is pressure and because there are so many particles, there is high-pressure in the bottle. If the power of the pump pushing air into the bottle is greater than the structural limit of the bottle, it will explode because of the pressure.
When the heart pushes the 70 ml of blood forward, that 70 ml has to push against all of the blood that is in front of it. If there were a lot of space in our arteries, the 70 ml of blood the heart pumped could spread out and the molecules in the blood wouldn’t press on the sides of the arteries. The pressure would be low. There is not a lot of space in the arteries, and the new particles in the 70 ml of blood mix have nowhere to go, so they press against the sides of the arteries, creating higher pressure. The heart is strong enough to overcome this pressure and push the weight of blood all around our circulation. However, if your arteries become narrower, there is less space for the blood to go and it presses on the sides of the arteries with even greater pressure. The heart needs to work harder to overcome this extra pressure to push blood around the body, which can lead to heart problems. High blood pressure can also make arteries explode in the same way the air made the bottle explode. If the heart keeps pushing the blood in, but it cannot get out, the artery might explode and cause a stroke.
So, how is blood pressure measured? It is usually measured with a sphygmomanometer. (Sphygmos is Greek for pulse and manometer is French for “pressure meter”.) It has an inflatable cuff that goes around the upper arm and something to measure the pulse. This could be a stethoscope if the test is being done manually or a sensor if the test is being done by a machine. The cuff is usually placed at the same height as the heart. The cuff is inflated until it cuts off the brachial artery, which is the artery carrying blood down the arm. The pressure inside the fully inflated cough is indicated with a dial. Once the blood flow has stopped, the air is gradually let out of the inflated cuff and the amount of pressure in the cuff, as indicated on the dial, starts to drop. At the point where the pressure in the cuff has dropped enough for the blood in the artery to start flowing again, the doctor will hear a whooshing sound in the stethoscope and will mark down the reading on the dial. The pressure of the cuff at the point where the blood started flowing again is the systolic pressure. The doctor will keep listening until the whooshing sound dies away and the blood flow returns to normal. This is the diastolic blood pressure. And this is what I learned today.
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