Blood pressure, force originating in the pumping action of the heart, exerted by the blood against the walls of the blood vessels; the stretching of the vessels in response to this force and their subsequent contraction are important in maintaining blood flow through the vascular system. In humans, blood pressure is usually measured indirectly with a special cuff over the brachial artery (in the arm) or the femoral artery (in the leg). There are two pressures measured: the systolic pressure, which is the force that blood exerts on the artery walls as the heart contracts to pump the blood to the peripheral organs and tissues, and the diastolic pressure, which is residual pressure exerted on the arteries as the heart relaxes between beats.
In apparently healthy individuals, the arbitrary values we consider to be “fair” are:
- Systolic pressure, between 100 and 140 millimeters of mercury (mmHg).
- Diastolic pressure, between 60 and 100 millimeters of mercury (mmHg).
When a doctor says you that your blood pressure is 160 mmHg, he really means 100 mmHg higher than atmospheric pressure. It’s a gauge pressure (is zero-referenced against ambient air pressure), not an absolute pressure. This was done for simplifying the numbers on the reading devices!
Thus, when your doctor says your blood pressure would be about 160 mmHg, me means you have a blood pressure of 160 mmHg + 760 mmHg (air pressure), i.e. 920 mmHg.
Let’s consider a doctor is going to measure your blood pressure in two different days.
- The first day he measures your diastolic pressure and it is 80 mmHg…it means that it is 80 mmHg + 760 mmHg = 840 mmHg
- The second day he measurer again your diastolic pressure and it is 160 mmHg….it means that it is 160 mmHg + 760 mmHg = 920 mmHg
Most of the doctors would assume that the diastolic pressure of the second day is the double of the first day, because they will consider the relative values 80 mmHg and 160 mmHg !!!!
In reality, they should consider the absolute values; thus they have to consider 840 mmGh and 920 mmHg. And they will discover that the difference is just 9% more among the two days!!!