Major redistribution of body fluids occurs with exposure to weightlessness and all astronauts and cosmonauts have noted a tendency for puffy face and fullness in the upper torso shortly after arriving in space.
These changes are inevitable and represent, simply, the body adjusting and adapting to the new environmental demands. Fluid in the lower torso and legs due to the normal hydrostatic effects of Earth's gravity becomes mobilized into the blood vessels and finds temporary accommodation in our relatively vast pulmonary "reservoir" until the kidneys rid the body of this now unnecessary fluid.
The same process occurs on the surface of the Earth anytime one assumes the horizontal position resulting in the well-known recumbency diuresis as the body readjusts. During zero gravity the end result of all this is a more optimum adjustment of the body fluids to the new demands, and function is preserved.
Replenishing these fluids in the brief interval of time available before the return of Earth's gravity is extremely difficult and considerable judgment is required. If done too early, the bladder inevitably will fill. If done too late, a fainting tendency may complicate recovery.
It is for this reason that crew members must take in fluid and electrolytes prior to de-orbit. Space scientists have even considered IV fluids and fluid and salt retaining hormones if it becomes necessary.
Duane Graveline MD MPH
Former USAF Flight Surgeon
Former NASA Astronaut
Retired Family Doctor