What is blood pressure?
While visiting
your local grocery store, you see one of those automatic blood pressure
machines. You sit down, have your blood pressure measured, and out spits
a couple of numbers
Systolic = 120
Diastolic = 80
Or, you visit your doctor, and she measures your blood pressure to be 120 over 80.
What do these numbers mean?
Blood pressure is the pressure of the blood exerted on the walls of vessels, from the inside pushing out.
When the heart is relaxed - that is, between beats, blood is moving through the vessels at a relative slow rate …So it is not pushing very hard on the blood vessels. This is when blood pressure is at its lowest. "Diastolic pressure" is the term used for this blood pressure, and is the lower of the two numbers.
When the heart contracts, the blood picks up a lot of speed moving through these vessels. This causes the pressure exerted on the vessel walls to be greater … this is the "Systolic pressure" or the higher of the two numbers.
A simple
equation relates these two:
Blood pressure = speed of blood flow x resistance
If the speed
of blood flow increases, as when the heart beats, the blood pressure decreases.
What is that other thing, "resistance". A good way to think
of resistance is to imagine washing your car with a garden hose. If you
want to have the hose spray harder (equivalent to increasing blood pressure),
you stick your thumb over part of the end of the hose. This is increasing
the resistance ot water flow through that hose, increasing the pressure
of that water in the hose. So, resistance is just that - resistance to
fluid flow through a vessel. Quick thought question here for you - can
you think of an equivalent in the human body of putting your thumb over
the end of the hose?
So, what does the "120" stand for … it is the same pressure required to move a column of mercury up 120 millimeters.
The chemical symbol for mercury is "Hg" (do you recall those days from chemistry in high school?) (show pic of sphymomanometer reading 120)
Blood pressure is measured in "mmHg"
An individual is thought to have "high blood pressure" when the systolic pressure at rest is greater than 140 mmHg and / or the diastolic pressure at rest is greater than 90 mmHg
How do we measure blood pressure?
More information
on blood pressure at National
Insitutes of Health