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re: rearing up



Ken Clay followed a valuable cetacean clarification with:

> 3.  In terms of the sauropod rearing question, with their small 
> brains and presumably small-diametered cerebral arteries I have
been
> under the impression they would not require that much of an
increase > in blood pressure to perfuse their brains. 

The increase in pressure would depend almost entirely on the
increased vertical distance between head and heart.  If the animal is
underwater, no problem.  But otherwise, if the head is held 10m above
the heart, you need about 1 extra atmosphere of pressure (100kPa or
15psi).  This requires the heart to be very strong, for a start.

And if we assume a circulation similar to birds and mammals, any part
of the animal level with the heart will be at double atmospheric
pressure, which will probably be uncomfortable.  And lower organs
will be even worse.  The body would be under very high pressure,
which would have to be maintained by a very tight skin, which would
make it difficult to move.  Fortunately, the pulmonary circulation
would be separate, forestalling massive lung haemorrhage.

Accessory hearts in the neck, if they existed, would be able to break
down this pressure into manageable steps.  Peristalsis wouldn't work
- the pressure gradient is much too great.  Valves could have kept
the blood already in the head, so the sauropod could keep going until
the blood around the brain ran out of oxygen.  However long that was.
 Unless it had an accessory lung, which is no more plausible than an
accessory heart.

I wonder if the aorta could have a valve which cut off the blood flow
to everything except the head when the blood pressure rose above a
certain level?  There could be a double systole, with the heart
expelling most of the blood in a low-pressure beat which couldn't
reach the head, then contracting more forcefully to close the valve
and force the remaining blood up the neck.  Utter speculation, but
compared to accessory hearts...

Ronald Orenstein predicted:
 
> This is going to sound like an extraordinarily silly question, but
> here
> goes.  I am not sure I quite understand why rearing would cause
> additional
> blood-pressure problems (barring the increase in activity rate). 
> As the
> heart is in the anterior half of the body, rearing does not
> necessarily
> raise the head higher relative to the heart, because the heart is
> elevated
> too...
> Am I missing something?

If the sauropod reared up, it would tilt its whole body.  You are
describing a sauropod rearing up, but bending its neck back down
again.  Possibly possible, but would it be useful?

In case anyone's wondering how sap reaches the tops of tall trees,
they don't push it much, they pull it.  Yes, really.  And that would
not work in a vertebrate.

Bill
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