Of course we take gravity for granted – it’s reliable and has not changed in the billions of years that life has evolved. As a result of this consistent experience, we have evolved systems to get the blood back up to the heart. We have valves in our veins, muscle pumps and the negative thoracic pressure of respiration to pull the blood uphill. But the venous blood in our heads can just fall back down to the heart, so our jugular veins are generally valveless and when we are upright they are empty with zero pressure. These aspects of human anatomy and physiology have interesting implications for our eyes, for glaucoma, or for intracranial hypertension. Patients often ask, “Is it OK for me to do downward-facing dog?”
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