Putting It All Together—Manned Space Flight

There are 3 main sources of pressure in physiology: atmospheric, hydrostatic, and mechanical. Unless a feasible method to generate ‘artificial gravity’ is developed, the astronauts on board will experience about 888 days of weightlessness during a Mars mission. What does this mean physiologically? The mechanical pressures generated by the heart, blood vessels, and the muscles of respiration will remain unchanged, except for whatever atrophy occurs during the mission. One interesting and apparently intractable problem of reduced gravity is muscle wasting. The skeletal muscles of respiration will not atrophy because they still will be constantly used and exercised. What about hydrostatic pressures? Without gravity, there is no possibility of a hydrostatic pressure gradient. What is the practical effect of losing the hydrostatic pressure gradient? Apparently, it has very little effect because astronauts survive and their brains seem to remain perfused during exposure to weightless environments. There is another physiologic challenge in space, a decrease in total blood volume, which results in orthostatic intolerance upon returning to Earth.