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whatever necks?



Ken Clay wrote:

> Okay, I know you were all hoping this thread was dead and buried...

I was hoping it would revive, because I thought siphoning was
impossible, but was too busy at the time to explain why.

The theory goes:

> 2) a
> siphon principle is functioning in giraffe necks; that although
> gravitational forces lower the blood pressure of the neck arteries,
> they
> are also lowering the pressure of the neck veins and thus perfusion
> pressure (arterial pressure minus venous pressure) is unchanged. 
> Along
> with this it is noted that the cerebral veins and arteries are
> surrounded by cerebrospinal fluid in a rigid cranium and that these
> vessels do not collapse from postural changes...

For the siphon to work, the top must be below atmospheric pressure. 
At least, that's how I understand the word 'siphon'.  I believe a
structure can only be below ambient pressure if it's surrounded by
rigid elements.

This applies to vertebrate lungs inside a ribcage, and to brains
inside a skull, but it does not apply to jaw muscles.  If the giraffe
or sauropod is going to eat, rather than just take a quick look
around, it needs to get plenty of blood to its jaws and tongue.

I thought a blood pressure below atmospheric pressure would make the
arteries and veins collapse.  (I don't know how capillaries would
behave.)  It would be no good stiffening them like plastic hoses,
because muscles have to change shape.

I wonder if some kind of circular or helical reinforcement would
work.  As in the tetrapod trachea, or the insect tracheal system. 
Maybe blood vessels with that kind of structure could stretch and
bend enough without buckling.  Anything like that in giraffes' heads?

A possible alternative would be to have large amounts of myoglobin in
the head muscles.  That would give them a bit more endurance without
blood.

All the best,

Bill Adlam
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