The Reason Your Blood Flows Differently In Space
When you see images of astronauts floating around the International Space Station, you can tell something looks different about them. Sure, their hair might be standing on edge, but there's something else about their appearance that you might not be able to put your finger on exactly. If you look closely at their face, though, you might see veins bulging on their necks or puffiness on their faces. Well, you can thank gravity for that. Or rather ... the lack thereof.
Our bodies have developed with Earth's gravity, so the ordinary acts of walking around the grocery store or playing fetch with our dogs keep the internal operations of our bodies humming like a well-oiled machine. Put the body in microgravity, and things get wonky. According to NASA, our bodies have no external force to work against without the usual level of gravity, so the cardiovascular system gets "lazy."
Here on Earth, gravity pulls blood and other body fluids down to the abdomen and legs. It's why standing for a long time causes legs and ankles to swell. But without that gravitational force in space — artificial or otherwise — blood instead gets pushed up into the chest and head. The fluid shift actually causes the heart and blood vessels to get less than normal blood flow, which in turn causes puffiness and bulging veins in astronauts (via NASA).
In space no one likes a puffy face
This overall lack of gravity dramatically impacts the body and circulatory system. NASA and other space agencies worldwide have spent a lot of time and effort studying the effects of zero gravity to help combat the impact that living in space has on the body.
According to NASA, blood in the chest and head can have adverse health effects, such as hearing loss, increased brain pressure that leads to brain swelling, and eye deformation known as Spaceflight Associated Neuro-ocular Syndrome (aka SANS). The lack of gravity causes muscles to deteriorate from atrophy, and the heart changes shape from the typical oval shape seen on Earth to more of a rounded ball.
When astronauts return to the gravity-laden Earth, blood and fluid are pulled back down into the abdomen and legs. That fluid shift and the other changes to the body make it harder for it to control a drop in blood pressure. It can cause some astronauts to become dizzy and sometimes faint, a condition called orthostatic intolerance.
Bodies adapt to whatever environment they find themselves in, and space, unfortunately, is not a very friendly one.