Question

How does the human body react to different gravities? What is the maximum gravity the human body can withstand?

According to Ames Research Center's expert on humans in space and gravitational biosignatures, a person has, in fact, survived 2 G (twice Earth's gravity) for 24 hours--the record for long duration exposures. Theoretically, a human could adapt to a gravity environment that was between 2 and 3 times that of Earth. At 4 G, however, people can not sustain an adequate bloodflow to the brain, let alone stand upright.

In regards to a reduced gravity scenario, there does not appear to be any critical problem for surviving either in lunar (.16G), or Martian (.38G) gravity. While walking is a bit awkward, reduced gravity on such planetary bodies should not produce any serious physiological problems. However, in microgravity environments (= .000001G), such as inside the shuttle, Skylab, Mir, or the Space Station, long term effects may prove more critical.

When the force of gravity is this low, there seem to be several adverse effects on different physiological systems. For example, since weight-bearing muscles are no longer needed in microgravity, these muscles quickly atrophy. Likewise, bones deteriorate and become brittle (losing mass at a rate of 1-2% per month!). Similarly, since the heart no longer has to pump blood “uphill,” its strength deteriorates as well. Other physiological changes include confusion of the vestibular system (balance organs are not stimulated normally, resulting in motion sickness), changing blood composition, and dehydration due to blood pooling (fluids tend to shift to the upper part of the body, causing it to rid itself of what appears to be too much fluid). While these physiological problems may seem dire indeed, there is actually a large amount of research in this area. For example, the NSBRI (National Space Biomedical Research Institute) is just one of hundreds of institutions in collaboration with NASA investigating the physiological problems of humans in space. See http://www.lifesci.arc.nasa.gov for more information.
October 29, 2001