Millimeters of mercury (mmHg)

Millimeters of mercury (mmHg) is the unit used to measure blood pressure.^{1International Sports Sciences Association, Strength and Conditioning, p. 94.} In a reading of 120/80, the first number is the systolic pressure. The second is the diastolic pressure.^{1International Sports Sciences Association, Strength and Conditioning, p. 94.} Both are in mmHg. Some books put a space in the unit and write mm Hg.^{2Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 300.}

Trainers record both numbers in mmHg, rounded to the nearest 2 mmHg. So readings are recorded as even numbers.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 215.} The gauge on the cuff may be a mercury column or a dial. Either way, it should sit at eye level.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 215.} The cuff is pumped to 160 mmHg, or to 20 mmHg above the expected systolic pressure. Then air is let out at 2 or 3 mmHg per second.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 215.}

The unit isn’t only for blood pressure. Normal air pressure is 760 mmHg.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 30.} At sea level, the oxygen share of that pressure is about 159 mmHg.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 30.}

Pressure also differs across the blood vessels. In the arteries it runs from about 100 mmHg in the aorta to about 60 mmHg in the arterioles. Arterioles are the smallest arteries.^{5Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 33.} By the time blood gets back to the heart, pressure has fallen to nearly 0 mmHg.^{6Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 134.}