MadSci Network: Medicine

Re: When rested, why is blood pressure so consistently 120 over 80?

Date: Tue Jan 25 03:44:50 2005
Posted By: Jens Peter Bork, M.D., Internal Medicine, Erlangen University Hospital
Area of science: Medicine
ID: 1105423001.Me

Hi Tom, 

many thanks for your question regarding normal blood pressure.

The resting blood pressure is  **not**  consistently 120/80, and there is no
magic at all to these numbers. Blood pressure is distributed along a more
or less bell-shaped curve in the population. It is true that a lot of
studies find that the maximum of the curve lies at about 120/80 mmHg - but
it is no way fixed there. Not only can it indeed differ between people of
different sizes, it can even differ between entire populations. In the
references, you find [1] an example of how mean blood pressure in healthy
people, again distributed along a Gaussian bell curve, differs between two
West African tribes (Pygmies and Bantus); [2] how the mean blood
pressure in Hungarian adolescents is found to be somewhat higher than that
found in US American adolescents: and [3] you find a nice picture of what
these distribution curves look like (the article is in German, but never
mind; I am referring to figure - "Abbildung" - 3).

Regarding the part of your question dealing with standing under water or on
a mountain top BP is determined by the body's regulatory actions far more
than by changes in external pressure. As long as the external change is
moderate (standing in 5 ft. deep water; standing on Mauna Kea) and not too
extreme (300 ft. apnea dive; doing Mt. Everest without oxygen), your blood
pressure will be determined by what you do far more than by what medium you
are doing it in. The main regulators of blood pressure are heart rate and
the resistance of peripheral arteries - they will take care of blood
pressure and act accordingly. This is not because 120/80 is "magic" but
because 120/80 (or 100/70, or 135/85; but not 250/160 or 55/20) is adequate
to our physiological needs: A brain more than 1 ft. above the heart needing
a lot of blood all the time, ankles more than 3 ft. below the heart whose
arteries should not be ripped apart or become sclerotic with the combined
arterial and hydrostatic pressure produced by the beating heart and the
vertical distance; kidneys and guts that need a more or less constant blood
flow; etc. If external circumstances change in the swimming pool or on
Mauna Kea, the regulatory system of the body will make sure that the BP
remains adequate and compensate for any external changes (which, in the
case of the swimming pool and Mauna Kea are not too big anyay).

There has been a lot of discussion about what these smoothly distributed
curves tell us regarding our definition of high blood pressure. Just as the
distribution of resting BP values  in a population is continuous and
smooth, so is the risk of cardiovascular complications in people whose
blood pressure lies towards the upper end of the curve. This smoothness is
not adequately mirrored in our hypertension definitions and treatment
recommendations; take the old "140/90" threshold, for example: While in the
real world someone with a resting BP of 139/89 has only minimally less
cardiovascular risk than someone with a resting BP of 141/91 (assuming the
values are representative), one of them would be classified as normotensive
and would not be treated, while the other one would be seen as hypertensive
and thus receive treatment. This shows us that recommendations like these
are more than a little arbitrary; it is a constant point of debate how to
refine the treatment guidelines so that they reflect the smoothness of the
blood pressure (and risk) distribution curves better. You can see the
smooth association between resting BP (diastolic) and two major
cardiovascular complication in figure (Abbildung) 1 of the German article I
am citing; Schlaganfall means stroke, Herzinfarkt means myocardial infarction.

I hope this helps. Apart form the three references, any good physiology or
Internal Medicine/Cardiology book will help you with this subject. I cannot
name a specific one since I am not familiar with the American books on the
market, but your University library should be well equipped.

Have fun exploring!
Yours truly, 
Jens Peter Bork

[1] http://hyper.a

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