You meant "systolic & diastolic"?

A try here, but may not be a riddle you want: one day, you are gonna meet the Stolic couple. From far away, you SEE Stolic (sy-stolic) because it's taller (Systolic is the point of greatest pressure, ring a bell? ). However, you cannot figure out the other one. Why? Because it's being dyed i.e. Di-a-Stolic.

Hope it helps.