Ask A Scientist General Science Archive

  
name         Robert M.
status       other
age          50s

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


I’ve 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 body’s 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, I’d 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 it’s 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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