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RE: Rearing up on hind legs (was Re: Parrish's neck work ...)



Betty:

On Monday, May 03, 1999 11:11 PM, Betty Cunningham [SMTP:bettyc@flyinggoat.com] 
wrote:
> Whales swim at different depths, so that the pressures of blood and
> every other part of the animal increase with depth.
> Wouldn't this show as an increase in, among other things, what we're
> speaking of as 'blood pressure'?

No.  Blood pressure is the pressure the heart generates by pumping action.  It 
doesn't relate to the pressure on the organism.
>
> Wouldn't increased pressure at depths be similar to the affect of
> changing diameter of the blood vessels? For that wouldn't pressure
> changes change the size of blood vessels all by itself?

No.  Blood is an aqueous solution.  Water is, practically speaking, 
incompressible at physiologically relevant pressures.  The volume of the 
vessels would not change.  There *are* problems with gasses coming out of 
solution at high pressures, but that's a whole different deal.  The problems of 
depth are problems with gas solubility and gas compression.  AFAIK, they are 
unrelated to the fluid dynamics of the sauropod problem.

> Indeed, since whales rise and fall in the ocean depths at speeds
> necessary for feeding (bubble netting krill or teh surmise killing of
> giant squids at depths by sperm whales) and breathing, they could even
> experience this "radical *changes* in blood pressure to avoid either
> passing out from lack of oxygen or suffering an aneurysm from too much
> pressure" that you infer that sauropods suffer from.
>
> Yes? No?

No.  Again, depth, and changes in depth, aren't going to affect the work the 
heart has to do.  Think of it this way: if the heart stops, there is no blood 
pressure -- on the surface or 20,000 leagues under the sea.  Blood pressure is 
not the pressure on the blood, but the pressure generated by the heart, a 
linear force exerted on a fluid in one direction.  Pressure caused by depth is 
the same in every direction.  It does not create a net force in any direction 
in the bloodstream.

(I will omit my usual whining about all the physiologists having been run off 
this List, but I do need to repeat the disclaimer that I'm not one myself.)

  --Toby White