|MadSci Network: Medicine|
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  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);  how the mean blood pressure in Hungarian adolescents is found to be somewhat higher than that found in US American adolescents: and  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  http://hyper.a hajournals.org/cgi/content/abstract/27/1/108  http://www.ncbi.nlm.nih.gov/entrez/q uery.fcgi?cmd=Retrieve&db=PubMed&list_uids=12544434& dopt=Abstract  http://www.kup.at/kup/pdf/4204.pdf
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