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

Blood pressure is measured in $\text{gauge pressure}$. The given values are over (or below) atmospheric pressure, i.e. over and below $\text{760 mm Hg}$.
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 ($\text{1 atmosphere}$ 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.