Anatomy and Physiology: Oh, the Pressure!
Oh, the Pressure!
Blood pressure is an amazing thing. It is simply the pressure exerted by the blood on the vessel walls. It doesn't sound like much, but without it, we would never be able to pump oxygenated blood against gravity to the brain. You have probably experienced a miniature form of this if you stand up too quickly. Simple inertia—a body in motion stays in motion, and a body at rest stays at rest, unless acted upon by a force—will keep the blood in place, despite the sudden elevation of your head. The subsequent loss of blood pressure is what makes you feel dizzy.
Systolic and Diastolic
For some reason, some people are terrified of having their blood pressure taken. This fear response (from the sympathetic nervous system; see The Central and Peripheral Nervous Systems) raises the patient's blood pressure. This situational high blood pressure has been affectionately called white-coat hypertension.
First of all, even though every vessel experiences pressure against its walls, when you have your blood pressure taken, the measurement is of your arterial blood pressure. Even though the arteries are deeper than the veins, the pressure is still strong enough to stretch the arterial walls with each heartbeat, and you can feel that as a pulse. Remember that even though the heartbeat is a double beat (the sound based on the slamming shut of the valves; see The Heart), the blood flow out of the heart is a single pulse out of both ventricles. That is why your pulse is a single beat.
The high and low pressures make perfect sense when you think about the heart (see The Heart). The left ventricle pumps the blood out to the systemic circuit. The higher number, or the systolic pressure, is a measurement of the pressure when the left ventricle contracts, or is in systole. (Makes sense, no?) When the left ventricle relaxes, or is in diastole, there is still a pressure being exerted on the vessel walls; this is the smaller number, or the diastolic pressure.
Normal blood pressure varies from childhood to adulthood. High blood pressure, or hypertension, is when one's systolic pressure exceeds 139, and diastolic exceeds 89. The risks of hypertension include heart attack (see The Heart) and stroke. The risks (which are less severe, in general, than those for hypertension) of hypotension, or low blood pressure, include dizziness or syncope (fainting).
Measuring off the Cuff
It turns out that measuring blood pressure is an extremely simple concept. By compressing an artery with a cuff attached to a meter, the blood pressure also exerts pressure against the cuff. If the pressure of the cuff is higher than the systolic pressure, all blood flow stops. This is the same principle behind using a tourniquet to stop blood flow, whether it be to stop bleeding, or to take a blood sample. On the other hand, if the pressure of the cuff is lower than the diastolic pressure, the blood flows freely through the vessel. Despite the simplicity of the concept, taking a blood pressure reading is a definite skill; some may even call it an art.
The second sound is so quiet that it cannot be heard if there is too much ambient noise. At the scene of a car accident, for example, a paramedic can palpate, or feel the pulse return at the systolic pressure. This type of reading is called a “BP palp.”
The artery being compressed when taking blood pressure is the brachial artery, and the true name of a blood pressure meter is a sphygmomanometer. First, raising the pressure of the cuff must stop the blood flow. As the pressure on the cuff is released, a sound can be detected with a stethoscope as the pressure of the blood first breaks through the cuff; this sound, a bit of a thudding with each pulse (if there is a needle on the meter, you will see it jump with each pulse), is the systolic pressure.
In the same way, the sound will slowly get quieter, becoming a swishing sound, until it finally stops when the pressure of the cuff is lower than the diastolic pressure. These sounds are called sounds of Korotkoff; the name is a Russian one, and due to differences between the English alphabet and the Cyrillic alphabet, the name is sometimes spelled Korotkov, or Korotkow.
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.