Pressure is one of the most context-dependent measurements in science and everyday life. The same physical quantity — force per unit area — gets measured in pounds per square inch when you're checking tire pressure, in millimeters of mercury when discussing blood pressure, in atmospheres in chemistry, in pascals in physics, and in bars on European instruments. Understanding how these units relate prevents costly mistakes and safety hazards.
Blood Pressure: Millimeters of Mercury
Blood pressure uses a unit inherited from early medical measurement: millimeters of mercury (mmHg). A sphygmomanometer (blood pressure cuff) measures how high the pressure in your arteries can raise a column of mercury. Normal blood pressure is around 120/80 mmHg. The 120 is systolic (peak pressure when the heart contracts); 80 is diastolic (pressure when the heart is at rest between beats).
In SI units, normal blood pressure of 120/80 mmHg converts to approximately 16.0/10.7 kPa. Medical contexts outside the US sometimes use kPa instead of mmHg — a patient with 16/10.7 kPa blood pressure has 120/80 mmHg. For most patients, this conversion is irrelevant because their providers use the same unit system. But those reading international medical literature or managing health records across systems may encounter kPa.
Atmospheric pressure changes with altitude and weather, which is why blood pressure medications and some medical conditions produce different effects at altitude. At sea level: 101.325 kPa = 760 mmHg = 1 atm. At Denver's elevation (5,280 feet / 1,610 m): about 84.3 kPa = 632 mmHg = 0.83 atm. The 17% reduction in atmospheric pressure is enough to require acclimatization and affects conditions like altitude sickness, certain medications, and aerobic performance.