What is blood pressure?
It is the pressure exerted by blood on the walls of arteries, through which it flows. This pressure is not constant even during the duration of a heartbeat, which is 0.6 to 1 sec for most people. The pressure is highest when the heart contracts to squeeze out blood to the arteries. After contraction, the heart relaxes. During this relaxation period, the pressure exerted by the blood on the walls of the arteries is the lowest.
Even though this is a very simplified description of the very complex process of blood circulation, it explains why the doctors use two numbers as a measure of blood pressure. The highest pressure is the systolic blood pressure (SBP), and the lowest one is the diastolic blood pressure (DBP).
While writing your pressure, the doctors use the unit mm. of mercury (Hg).Height of mercury column is the gold standard of pressure measurement. The doctors also make another assumption; they take atmospheric pressure to be zero. So if your pressure is written as 120/80, then your systolic pressure is 120.In actuality, on a normal day when atmospheric pressure is say, 760 mm of Hg your systolic pressure is 760+120=880mm. of mercury and similarly for the DBP. However, this is only a matter of shifting the origin and need not concern us.
What is important is to know what might be the normal blood pressure, whether your blood pressure readings are high, whether you should consider lifestyle modification or begin taking medicine under your doctor’s guidance. These are difficult questions and probably no single answer fits everybody.
We shall take up the issues in next post.