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Sauropod necks and blood pressure

        Here's a silly question I thought I'd throw out to stir up some 
more talk about the necks of sauropods.  Perhaps someone out there who's 
familiar with giraffe anatomy can answer this one.  Supposing for a 
moment that a sauropod's heart is carried rather close to the vertebral 
column, wouldn't there be at least some "siphoning" effect in the blood 
column of the neck?  After all, if the blood on the return trip to the 
body mass enters any large organ where it's allowed to pool, and the 
blood leaving the heart for the brain leaves the heart _above_ the level 
of the "pool," wouldn't the weight of the blood in the return column at 
least partially offset the pressure required to pump the blood up in the 
first place?  Not to say that high systemic blood pressures wouldn't 
still exist in the animal's neck, but would the heart have to do all the 
work?  Or is gravity helping to some degree?  I saw some speculation a 
few years ago that sauropods would need up to eight hearts to pump blood 
UP the neck, so I'm wondering if blood perfusion to the head effectively 
nullifies the siphoning effect of the blood in the return column.
        Of course, I don't know that the heart in sauropods is situated 
above the level of any organ which could pool the blood (perhaps the heart 
itself?), but I thought I'd ask.