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Name: Robert M.
Status: other
Age: 50s
Location: N/A
Country: N/A
Date: 8/26/2004


Question:
Concerning body temperature,thicker blood or thinner does either have anything to do with body temperature regulation.thanks for settling this dispute. Robert


Replies:
Thanks for your question Robert... The viscosity (thickness) of blood, or any liquid for that matter, is controlled by both the chemical structure of its constituent components and temperature. Independent of chemical structure, however, temperature is the primary environmental factor (though ambient pressure may play some role in affecting the viscosity of compressible liquids, as well). Our every day experience with liquids tells us that they become more viscous (thicker) with a decrease in temperature and less viscous(thinner) with an increase in temperature. These temperature induced changes in viscosity are attributable to the way molecules in the liquid react to a gain or loss of thermal energy (heat). When thermal energy from the environment is absorbed by the liquid, the molecules that comprise the liquid begin to move at higher velocities, and thus engage in higher energy collisions with one another and their surroundings. This increased activity increases the volume of the liquid (which is what Boyles Law is all about) and thereby results in an increased average distance between molecules at any given time (you can imagine that the water molecules in steam are much further apart than the same molecules in a sample of liquid water). This increased distance between molecules means that they will have fewer opportunities to effectively slow each other down by virtue of electrostatic interactions, or rather, friction at the molecular level. Thus, heating a liquid reduces its internal molecular friction, it flows more freely, and appears thin. (My apologies if this background info was unnecessary.)

Ive read that the blood viscosity of patients who are suffering from extreme hypothermia can increase to nearly 200% the normal blood viscosity, and would imagine that an opposite viscosity reaction would occur in an individual suffering from hyperthermia (heat exhaustion). Complications, however, arise with the bodys retention or loss of water when exposed to those two extreme conditions. For example, although heat exhaustion would cause a decrease in blood viscosity, it would likely also cause dehydration, resulting in a loss of the liquid component of blood (predominantly water) and a relative increase in the solid component of blood (cells, proteins, etc.) dehydration, therefor, would cause blood viscosity to increase, possibly countering or even exceeding the viscosity change originally brought about by high temperature. In short: body becomes hot, blood gets thin, body loses water, blood gets thick.

As far as how blood viscosity affects temperature regulation (as opposed to how ambient temperature affects blood viscosity), I would imagine that thicker blood would be less effective at distributing heat throughout the body and losing heat to the environment. Viscous liquids resist internal flow; thick blood would resist flow through blood vessels, and would consequently not be as effective at distributing heat to either the areas in the body that are in need of it (for example, the hands or feet), or away from the places that have an excess of it (for example: the head or chest). More important, however, is the issue of how blood vessels respond to ambient temperature and blood viscosity changes.

All things being equal, low ambient temperatures cause blood to become more viscous and blood vessels to constrict (become smaller in diameter). Constricted blood vessels have a decreased surface area than normal, and are less effective at radiating heat away from the blood and out of the body. Conversely, high ambient temperatures cause the blood to become thinner and the blood vessels to dilate (increase in diameter). Dilated vessels have an increased surface area, and can more effectively radiate heat away from the blood and out of the body. That blood vessels act in this way to regulate body temperature is really the beauty of a couple hundred million years of warm-blooded animal evolution.

In the end, Id say that blood viscosity does play a role in regulating body temperature, but really just as a result of the physical laws of nature and not because of any purposeful intent by the body. Changes in viscosity are, for the most part, a hand dealt to the body by its environment, and it has no choice but to do whatever it can to deal with those changes. I hope that this response (unfortunately lengthy as it is... again, my apologies) is of some help.

Scott J. Badham
Department of Geology and Geophysics
University of Wyoming



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