What is High Blood Pressure? How is Blood Pressure Measured?

(This is the first of a three part series on high blood pressure. The second part will discuss the signs, symptoms, and complications of high blood pressure, and the final part will describe the treatment of high blood pressure.)

Before you can understand high blood pressure (also referred to as hypertension), you need to know what blood pressure represents. When you have your blood pressure taken, you usually get a result that sounds like 120 over 80 and looks like 120/80 when your doctor writes it down. The 120 is called the systolic blood pressure (SBP). The systolic blood pressure is the amount of pressure in your arteries as the heart pumps blood from its left side to the rest of the body. The 80 is the diastolic blood pressure (DBP). The diastolic blood pressure is the lowest pressure in the arteries just before the heart begins to pump again.

On the left side of the heart, the heart muscle pushes blood from the upper chamber, the left atrium, to the lower chamber, the left ventricle. A valve closes between the atrium and the ventricle so there is no back flow, and the muscle of the ventricle propels the blood out into the body. As the muscle of the ventricle relaxes, a valve closes between the ventricle and the major artery of the body, the aorta, so no blood flows back into the heart.

Blood Pressure Measurement

To measure blood pressure, you pump up a cuff that surrounds the upper arm. There is a tube that connects the cuff to a pressure gauge that reads the pressure in the cuff. You use a stethoscope to hear the sounds below the cuff on your arm. You allow the pressure in the cuff to fall very slowly. You begin to hear a sound when the pressure in the artery becomes slightly greater than the pressure in the cuff. If you glance at the pressure gauge when you hear the sound, you will be seeing the systolic blood pressure. As the pressure declines, the volume of the sound also declines. If you glance at the pressure gauge when the sound disappears, you will be seeing the diastolic blood pressure.

Classification of Blood Pressure for Adults

The levels of blood pressure are classified in the following table:

Category SBP (mm Hg) DBP (mm Hg)
Normal Less than 120 And Less than 80
Prehypertension 120-139 Or 80-89
High Blood Pressure
Stage 1 HBP 140-159 Or 90-99
Stage 2 HBP Equal to or greater than 160 Or Equal to or greater than 100

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