I&#x27;ll repost the comment I made over there:<p>Have you guys heard about the entire field of study that is aerospace medicine? I’m taking a course on it this semester (I’m a masters student in aerospace engineering)–let me summarize some of the problems that your body goes through during space travel.<p>Humans spend about 70% of their time either standing or sitting. That means the human body is optimized for a hydrostatic pressure gradient like the one illustrated here (<a href="http:&#x2F;&#x2F;wiki.sdstate.edu&#x2F;@api&#x2F;deki&#x2F;files&#x2F;999&#x2F;=1-BP_Change.png" rel="nofollow">http:&#x2F;&#x2F;wiki.sdstate.edu&#x2F;@api&#x2F;deki&#x2F;files&#x2F;999&#x2F;=1-BP_Change.png</a>). Blood pressure is much higher at your feet than at your head. In space, however, there is no gravity to produce this gradient, so the cardiovascular system equalizes its pressure. That’s why astronauts suffer from “puffy face” (higher-than-usual fluid pressures in head) and “chicken legs” (lower-than-usual fluid pressures in legs). In their first couple days in space, astronauts lose about 1L of leg volume from each leg.<p>The CV system relies on internal pressure sensors to figure out how to operate, and the new pressure distribution confuses it. Astronauts lose a lot of blood plasma–it sort of ends up absorbed into the surrounding tissues. This increases the relative concentration of red blood cells, which triggers the body to slow down production of new ones. Also, the heart atrophies because it doesn’t need to pump as hard to move liquid around the body.<p>When you come back from space, your body needs to rapidly readapt from microgravity to one gee. It is not very good at doing that. That’s why 63% of astronauts are unable to stand for ten minutes straight just after their return from short-duration space missions (see video: <a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=TPDST7EePXQ" rel="nofollow">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=TPDST7EePXQ</a>).<p>There are other problems: the cells that are constantly building and destroying your bones fall out of alignment when you’re in microgravity, causing astronauts’ bones to change structure in a way that looks a lot like accelerated aging. Why? Not clear, but possibly related to the lack of repeated loading, as happens when you stand&#x2F;walk in normal gravity. Astronauts end up with huge concentrations of calcium in their blood, which causes kidney stones. Astronauts in space average around 4 hours of sleep per night, and very few of them eat enough calories to maintain their body weight, so they lose muscle mass.<p>Aerospace medicine is about half medical studies of astronauts and half studies of people on earth. It turns out that you can make most of these body changes happen by having people lie on their backs with the bed tilted down six degrees (head downward). Fascinating field. If you want to know more, I recommend the textbook “Space Physiology” by Jay Buckey (amazon link here: <a href="http:&#x2F;&#x2F;smile.amazon.com&#x2F;Space-Physiology-Jay-C-Buckey&#x2F;dp&#x2F;0195137256?tag=s4charity-20" rel="nofollow">http:&#x2F;&#x2F;smile.amazon.com&#x2F;Space-Physiology-Jay-C-Buckey&#x2F;dp&#x2F;019...</a>).