"Blood pressure is usually expressed in terms of the systolic (maximum) pressure over diastolic (minimum) pressure and is measured in millimeters of mercury (mm Hg)."(Wikipedia)
It is not division, but two things put together with a stroke "/". In English, the fraction 3/4 can be said "three over four", etc. So blood pressure looks like a fraction, it is spoken "120 over 70".
I guess Russians speaks of numbers with stroke "/" (though not necessarily fractions) using на + acc.?