14.3.4 Blood Pressure
The last time you had a medical checkup, the doctor may have told you something like “Your blood pressure is 120 over 80.” What does that mean?
About every 0.8 s, assuming a pulse rate of 75 beats per minute, your heart “beats.” The heart muscles contract and push blood out into your aorta. This contraction, like squeezing a balloon, raises the pressure in your heart. The pressure increase, in accordance with Pascal’s principle, is transmitted through all your arteries.
FIGURE 14.15 is a pressure graph showing how blood pressure changes during one cycle of the heartbeat. The medical condition of high blood pressure usually means that your resting pressure is higher than necessary for blood circulation. The high pressure causes undue stress and strain on your entire circulatory system, often leading to serious medical problems. Low blood pressure can cause you to get dizzy if you stand up quickly because the pressure isn’t adequate to pump the blood up to your brain.
FIGURE 14.15 Blood pressure during one cycle of a heartbeat.
Blood pressure is measured with a cuff that goes around your arm at the height of your heart. Initially, the cuff is pressurized until it squeezes the artery shut and cuts off the blood flow. Then the pressure is slowly reduced. When the cuff pressure drops below the systolic pressure, the pressure pulse during each beat of your heart forces the artery open briefly and a squirt of blood goes through. A doctor or nurse listening through a stethoscope, or a pressure sensor in more modern instruments, records the pressure when the blood first starts to flow. This is your systolic pressure.
This pulsing of the blood through your artery lasts until the cuff pressure reaches the diastolic pressure. Then the artery remains open continuously and the blood flows smoothly. This transition is easily heard in the stethoscope, or sensed by a pressure sensor, and your diastolic pressure is recorded.
Blood pressure is measured in millimeters of mercury. And it is a gauge pressure, the pressure in excess of 1 atm. A fairly typical blood pressure of a healthy young adult is 120/80, meaning that the systolic pressure is of Hg (absolute pressure of Hg) and the diastolic pressure is 80 mm of Hg.