If you’ve ever been to the doctor
to get your blood pressure tested, you may have wondered what those numbers
that show up stand for. In this section you will discover what they mean,
and you will no longer be a victim to blood pressure ignorance.
In order to understand blood pressure,
you must first be familiar with the term blood flow. Blood flow is the volume
of blood that flows past a certain point in the cardiovascular system during
a specified amount of time. An example of this can be demonstrated at a gas
station. Imagine you’re using the gas hose to fill up a car. The car has a
20 gallon gas tank. Let’s say that you fill it up in five minutes. You have
measured the amount of gas that moved through the gas hose during a period
of time (5 minutes). This means that the gas pump has a gasoline flow of 20
gallons per 5 minutes (or 4 gallons / 1 minute). This also means that any
point in the hose is going to have the same gasoline flow as the nozzle, 4
gallons / 1 minute.
The cardiovascular system works
in much the same way. If we measured the blood flow from the heart into the
aorta, we can assume that it is the same amount as what we would find if we
measured the total blood flow through the lungs, left heart, right heart, and
the systemic circulation.
The driving force behind blood
flow is blood pressure. Blood pressure is a measurement of the horsepower that
drives the blood throughout our bodies. In order to explain this more clearly,
I will use a car’s tire as an example. When you puncture a tire, the air rushes
outward because the pressure is greater inside the tire. The same goes for the
differences in blood pressure within our circulatory system. The blood will
flow out of the higher pressure zone. It doesn’t flow due to a given pressure,
but instead due to the pressure gradient, or pressure difference between two
points. Therefore, the blood flow between two points is directly proportional
to the difference in blood pressure between the two points (P1 - P2).
Don't understand? Check out this demonstration:
Click on the red arrow to continue the demonstration.
If you do not see the demonstration you don't have the flash plug-in. You can
get it here.
So, why do we require a heart as
pressure to maintain blood flow if the blood pressure remains constant? Because
friction exists between blood cells and the vessel wall. Thus, resistance is
created in the blood flow. The more resistance that exists, the lower the blood
flow. Therefore, we must add one more variable to our blood flow (F), blood
pressure (P1 - P2) equation, resistance (R). The end result is:
F =(P1 - P2) / R
Blood pressure is usually measured
in one of two units: millimeters of mercury (mm Hg) or pounds per square inch
(psi). The average aortic blood pressure is 90 mm Hg, meaning that the pressure
within the aorta equals that of the downward pressure applied by a column of
mercury 90 mm tall.One psi is equivalent
to 50 mm Hg.
We hope this puts blood pressure
in prospective for you, and next time you go to the doctor, you’ll know what
the blood pressure numbers stand for.
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