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Jeffrey Cutler, senior scientific adviser for the National Heart, Lung, and Blood Institute at the National Institutes of Health, explains.
Blood pressure readings include two numbers: systolic blood pressure and diastolic blood pressure. The top number is the systolic pressure, which is the pressure within the arteries while the heart is pumping, whereas the bottom number is the diastolic pressure, a measure of pressure within the arteries while the heart is resting and refilling with blood. The exact origin of the designation of 120/80 as the threshold for "normal blood pressure" is unknown. It may have come from the large amount of data available early in the 20th century from life insurance exams and what happened to these people as they aged, or it could have arisen from the beliefs of medical practitioners, as so much traditional knowledge has. But modern epidemiologic studies have confirmed with a great deal of certainty that risk of a heart attack or stroke begins to increase in adults when the usual systolic blood pressure is 115 or higher and/or the usual diastolic blood pressure is 75 or higher. The risk steadily increases with higher and higher readings, so the traditional 120/80 level remains reasonable as a threshold for getting a doctor's attention.
The inquirer is correct in thinking that blood pressure (as conventionally measured in the arm in a sitting position) should change with a person's height. It does in fact increase with height, which ensures that the brain, located at the highest point of the circulatory system for most of the day, gets sufficient blood flow and oxygen, despite the pull of gravity and other forces. But the effect is fairly small, which is why the 120/80 figure is not adjusted for taller people. Furthermore, there are many other personal characteristics that affect blood pressure--for example, age, gender, race/ethnicity, weight/obesity--and we do not adjust the normal level based on these factors either, which is done to keep things simple to some extent. The one thing that the best medical evidence continues to tell us is that regardless of personal characteristics, it is healthier to have blood pressure as close to the normal range as possible.