Blood requires gravity ????????
Answers
blood requires gravity
Blood flow to all parts of the body is primarily regulated by blood pressure. Arterial blood pressure outweighs gravitational fluid pressure. When I stand up, the pressure difference between my heart (reference level) and my feet in the veins is simply the pressure due to a column of blood (which is only slightly denser than water) about 1000mm tall. That's about the same as 70mmHg. For comparison, my resting blood systolic pressure is about 130mmHg. When I'm the right way up, getting blood up from my heart to my brain needs a systolic of about 50mmHg. In other words, even if I stand on my head, my heart can get blood TO all my extremities.
Getting it back is more fun.
Blood return is partly by pressure (venous pressure is much lower than arterial but is measureable) and partly by gravity from the upper body. The lower body has the opposite problem - gravity is trying to pull venous blood back down, and this pull (while you are standing) is higher than venous pressure. The deep veins of the lower limb have an anatomical feature which allows the muscles of the legs to act as a pump - they have valves. When your leg muscles contract they squeeze the veins. Blood can only flow the the direction the valves allow - towards the heart. When the muscles relax, the veins fill from the capillaries. If you are sat or stood for a long time, e.g. sat on a long-haul flight, this muscular squeezing doesn't occur and you may find your feet swell as fluid passes out of vessels into the interstitial space.
Superficial veins in the arms also have valve but these come into their own not by being squeezed, but when you raise and lower your arm - gravity moves blood out of your arm when you raise it, but it can't flow back in when you lower it.