How your heart works: two pumps, one spark, and what a heart attack really is
Blood pressure: what the two numbers really mean
120 over 80. One heartbeat, two numbers. What does each one measure, and why does the second one never drop to zero?
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In the early 1700s, the English scientist Stephen Hales put a long glass tube into an artery of a horse. The blood shot up the tube, about 2.5 meters high.
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With each heartbeat, the top of the blood rose and fell a few centimeters. That push in the arteries is blood pressure. Today it is given as two numbers, like 120 over 80, and each one measures a different moment.
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