

Re: rearing up

Of course, arteries are not rigid tubes.  They are elastic and have a lot of
muscles in their walls.  They can absorb and return pressure to the blood
column.  A column of blood the length of a sauropod neck in the upright
position would still need to be raised by the heart, which would take a
pretty high systolic pressure regardless of how rigid or relaxed, patent or
constricted the lumen.  Vasospasm by the arterial walls might have prevented
dangerous surges when they lowered their heads, as might capacitance retes.
I have read that dinosaurs probably didn't fill their braincases with brains
as much as birds and mammals, although that may be an outdated belief.  If
there was some space in what was already a fairly small cavity, there may
have been room for pressure absorbing blood-filled sinuses.  Do you have any
clearer idea how gravitational effects on the venous side would allow less
work on the arterial side?  I just don't see what that means.  By the way,
do giraffes have valves in their neck veins?   If they do, in which
direction are they oriented to prevent backflow?  If this is too mammalian
to discuss here, then please ignore this portion of my post.

"k. clay" wrote:

> I appreciate all the dialog on this interesting subject.  These types of
> discussions allow me to review my anatomy and physiology, and are quite
> enjoyable.  I have a few more thoughts and then will return to lurking.
> I also agree that mammals and birds may not be good models for sauropods
> because of brain size differences.  Within a smooth rigid tube, flow (F)
> varies with the pressure drop over a unit of length (P/L) , the radius
> of the tube (r), and the viscosity of the fluid (n), giving us the
> always dreaded Hagen-Poiseuille equation:  F=(P)(pi)(r4)/8(L)(n)--note
> that flow varies with the fourth power of the radius.  I argue that flow
> requirements for rearing sauropod brains to function are small, that
> cerebral artery radiuses (radii?) are correspondingly small, and hence
> even with a long neck length the pressure changes would not have to be
> great.  This can be tested if someone could give a good estimate of the
> size of the carotid arteries.  I haven't taken any physics classes for
> some time (and I never really understood them) so someone please let me
> know if this reasoning is flawed.  Lastly, are giraffes good model for
> rearing sauropods? There is an article in Comp Biochem Physiol A Physiol
> 1997 Nov; 118 (3) 573-6 (I have only read the abstract) that suggests
> that the gravitational effects on giraffe neck arteries are
> counterbalanced by the gravitational effects on the neck veins and "that
> the heart does not spend extra energy to raise the blood to the head."
> As usual there are also studies disagreeing with this conclusion.--Ken
> Clay, M.D.