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Comment by ceejayoz | Hacker News Reader

> Animal bodies are full of blood, it's not like hearts pump blood up there, they just pump it into movement.

If that were the case, G forces wouldn't make people black out. https://en.wikipedia.org/wiki/G-LOC

Does high G actually cause a higher or lower blood pressure at heart level? (search engines fail to give an answer) I think the problem there is blood packing to the lower body, and a sauropod would have enough blood and strong enough veins to be able to hold its head upright.

It's not really obvious to me why the sauropod heart->head->heart loop shouldn't be considered as a "siphon". For all the blood that's going up there should be an equal amount of blood going down.

> It's not really obvious to me why the sauropod heart->head->heart loop shouldn't be considered as a "siphon".

Because blood vessels are flexible. Siphoning would be disastrous for the body - capillaries would snap shut under the suction and we'd die.

What would be the force actually shutting the capillaries, if the veins themselves aren't collapsing? The upper part of a siphon still has internal pressure, just less than at its base.

For a siphon to work at all requires negative pressure at the top.

This is answered above.

If the condition is brought on slowly, i.e. because the animal grew to a large size, I'd think that it would have enough blood to fill their vessels at the increased pressure required to have positive blood pressure everywhere.

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