Why are animals like spiders, geckos, … not affected by blood running into their head when staying head down for a long period of time?
Spiders and insects just don’t have a large enough volume of blood in them, and considering they have open blood circulation (blood isn’t just enclosed in the vessels), this doesn’t cause problems.
Some animals (like geckos) just adapted to this, just like woodpeckers adapted to their head being constantly smashed at a deadly rate.
I think it could be due to a few reasons-
– open circulatory system so blood flow is different
– they weigh very less which might help in upside down position or various different positions
– they are very adept at holding on to various surfaces and not “fall over”
– their balancing (vestibular system) may not be as “ground-centric” in the sense that a human’s is meaning that for us the ground is below and other things are in reference to the surface we stand on
which makes sense as we humans are larger (relatively) and gravity affects us more and might have had more impact in the evolution of our balancing system and correction. But this might not be the case for them, so even if they are upside down, according to them they are in a normal position because there is no up or down for them, any surface is any surface
I’d like to add that the animals mentioned have their heads pretty much horizontal to their hearts, while ours are vertical in most situations. Our hearts work much harder to pump blood against gravity upwards into our head, so when we are upside down the blood rushes into our heads.
Animals that have their heads explode from blood pressure problems have fewer babies
(Also read the one about humans being adapted to stand upright and our hearts pump blood UP)