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Re: sauropod necks



Ken Clay wrote:

>...Let me give you a few data points
> and then the siphon proponents' answer.  In a giraffe right atrial
> pressure in a standing individual is essentially atmospheric;

And the left atrium should also be about atmospheric.

> left ventricular pressure is 270/150 or so.

This is presumably mm of mercury (above atmospheric pressure).  As
mercury is 13.6 times as dense as water, the systolic pressure is
enough to raise water about 3700mm.  Blood is a little denser, but
even so, this should be enough to reach the brain about 3m up.  (The
diastolic pressure could only raise it 2m, though.)

>  Carotid artery pressure 40cm below the jaw line when the animal is
> lying flat is 280/180, while standing it dropped to 160/110.  

Now that's interesting.  With the giraffe lying the pressures are
similar to those in the ventricle, which is what you'd expect for a
horizontal vessel offering little resistance.

But standing up, the systolic pressure is 160.  If nothing happened
in the neck, it would be 270 - 3000/13.6 = 50.  If the carotid artery
offers any resistance, it will be even lower.  To raise the pressure
to 160 an additional pressure difference of 110mmHg is required.  And
180 is neeeded in the diastole.

> It was estimated that carotid pressure
> at the base of the skull at this time is 125/75.  

Quite a pressure drop in 40cm.  If the estimate is accurate.

> Data on standing jugular pressure show "a nearly atmospheric 
> pressure gradient along the length of the jugular vein." 
> Unfortunately I could not find any data on postural changes in the 
> jugular venous pressure other than " it varied in the same 
> direction as the distal carotid pressure." 

The lower end of the jugular vein would presumably be at about the
same pressure as the right atrium (i.e. atmospheric).  I don't know
what "a nearly atmospheric pressure gradient" means.  A pressure
difference of 1 atmosphere (750mmHg) in either direction would be
implausibly high.  Perhaps it means both ends are about atmospheric
pressure.

> Siphon proponents, using a model of an inverted U tube with a 
> collapsable descending limb (representing the jugular vein), 
> explain venous pressure as being the sum of viscous flow 
> (frictional) pressure and gravitational (hydrostatic) pressure.

> When you add the two pressures together they counterbalance each 
> other and give you a jugular vein with an essentially atmospheric 
> pressure which doesn't collapse. Pressure measurements in model 
> tubes support this reasoning.

That works for the jugular vein.  It's just a question of making the
jugular veins narrow enough, although it would have to dilate when
the giraffe lies down.  But it doesn't explain how blood gets up the
carotid arteries.  

The viscous flow pressure will always be in the opposite direction to
the flow, but in the carotid, there is an extra 110 to 180mmHg in the
same direction.  There would have to be active pumping by the
surrounding muscles.  Quite substantial pumping, comparable to the
pressure difference generated by the heart.  The neck might be
described as an 'accessory heart'.

If the giraffe's neck can generate 80% of the pressure required to
raise its blood, I don't see why sauropod necks couldn't.

I think I understand what's going on, but I can't see why they call
it a siphon.

All the best,

Bill Adlam
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