Almost everyone knows what it is like to have his blood pressure taken. But few really know what the doctor is doing, and fewer still comprehend the meaning of the figures that register blood pressure. Actually, taking blood pressure and interpreting the results is a simple process.

The device used to measure blood pressure is called a sphygmomanometer (from the Greek, meaning pulse measurement); it measures the air pressure needed to raise a column of mercury. To use it, the doctor pumps air into a cloth cuff wound tightly round the patient's arm. As the cuff expands, the column of mercury rises...

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