What do millimeters have to do with the measurement of blood pressure?

Answer 1

A common way to measure pressure is in "mm of Hg" or millimeters of Mercury. The reason that this unit of pressure is based on a unit of length is because of how pressure was measured originally. A curved tube full of mercury metal was used by Robert Boyle in the 17th century to measure pressure. Briefly, it works like this. Imagine a tube of glass bent into a U-shape with mercury inside. If both ends of the tube are open to air, the level of the mercury in each side of the "U" will be equal. However, if additional pressure was applied to the air above one side, it would cause the mercury on that side to drop, and the mercury on the other side would rise. The "mm of Hg" refers to the difference in height between the two sides. The bigger the difference in pressure between the two sides, the bigger the height difference. Note, that the measurement is inherently a relative measurement (one side of the tube relative to the other). One atmosphere (atm) is equal to 760 mm Hg. This is because if you have a perfect vacuum on one side of the U-shaped tube and 1 atmosphere or air pressure on the other side, the difference in height will be 760 millimeters (independent of the diameter of the glass tubing). Also, 1 torr is equal to 1 mm of Hg. For more information see: http://www.chm.davidson.edu/ChemistryApplets/GasLaws/Pressure.html