Horan, Michael National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland.
Last reviewed:March 2018
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High blood pressure. Hypertension occurs when there is a state of abnormally elevated blood pressure. Blood pressure is expressed in two numbers (see illustration): the higher number is the systolic blood pressure, which is the pressure exerted by the blood against the walls of the blood vessels while the heart is contracting; the lower number is the diastolic blood pressure, which is the residual pressure that exists between heart contractions, or while the heart is relaxing. Normal blood pressure (typically below 120/80 mmHg) provides sufficient blood flow to the vital organs, including the brain, heart, kidneys, intestine, and skeletal muscles. For diagnostic purposes, blood pressure is considered high when persistently above 140/90 mmHg. The prevalence of this clinical condition in modern society is sufficiently great to warrant its being referred to as a public health problem. See also: Blood; Blood vessels; Circulation; Circulation disorders; Heart (vertebrate); Heart disorders; Public health
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