Question

When a physician quotes a blood pressure, to what does this relate?

Answer 1

Blood pressure is measured in `"gauge pressure"`. The given values are over (or below) atmospheric pressure, i.e. over and below `"760 mm Hg"`.

Explanation:

Historically, columns of mercury (or even water) were used for pressure measurement. The difference in heights of the columns represented the difference in pressure. A dense liquid such as mercury gives measurements convenient to the experimentalist (`"1 atmosphere"` will support a column of water approx. 10 m high, which would be a rather inconvenient measurement to make). You are possibly too young to remember the old blood pressure monitors, but these were filled with (short) columns of mercury.

(https://tse1.mm.bing.net/th?id=OIP.M77c8bedd91f4f5149d1440554c80f99fo0&pid=15.1&P=0&w=200&h=150)