Without metabolic information, accurate planning of environmental
systems for long flights is difficult. Importance is also attached to
early evaluation of weightlessness effects on body-fluid equilibria. The
results of Earth orbital flights and of terrestrial water-immersion
experiments suggest the occurrence of undesirable changes, although no
effects leading to operational incapacity have yet arisen.
In both recumbency and immersion, a similar redistribution of body
fluids occurs. It has been suggested that recumbency may affect an
extracellular fluid-volume receptor mechanism which by decreasing
aldosterone secretion by the adrenal gland, would decrease sodium
reabsorption by the renal tubules. Aldosterone excretion decreases
during recumbency and during standing in water, but increases while
standing in air. There is also evidence for cardiac atrial volume
receptor mechanisms which respond to increased filling of the left
atrium with reflex inhibition of release of pituitary antidiuretic
hormone (ADH), resulting in diuresis (Henry-Gauer reflex).
Altered fluid equilibrium in buoyant states is accompanied by shifts in
intracellular and extracellular electrolyte distribution, especially
sodium and potassium. Evidence from recumbency studies indicates a
strong correlation between loss of erect posture or weight bearing and
excretion of calcium stores in bone.
A bone X-ray densitometry method has been developed by Mack, at Texas
Woman’s University, for accurately determining the loss of bone mineral
(±2 percent accuracy) in humans and animals. The heel bone and spine are
X-rayed using a calibrated aluminum wedge as a standard. This technique
will be used for preflight and postflight analysis of the primate being
flown in the 30-day biosatellite. Comparative appraisal of bone mineral
behavior in astronauts participating in the Gemini and Apollo programs
will be invaluable for future flight missions.
Bed rest and immobilization studies by Mack have shown loss of skeletal
mineral and increased calcium in the urine and excreta. Four bed-rest
studies, each extending for 2 weeks, compared different levels of
calcium intake. Four men were used in each study and served as their own
controls during extended ambulatory periods. During 2-week periods, up
to 10 percent of calcium mineral was lost from the heel bone. Calcium
was also determined in the urine and feces. In other studies, isometric
exercises reduced loss of bone mineral during bed rest.
Excretion of calcium in the urine is accompanied by risk of its
deposition as calculi or "kidney stones" in the urinary tract.
Currently, changes in calcium metabolism resulting from weightlessness
over periods up to 2 weeks is not considered a hazard requiring
precautionary measures.
Flights in excess of 2 weeks, however, constitute a problem serious
enough to warrant study on the 11-day orbital flights and the 30-day
biosatellite primate mission. Therapeutic immobilization,
post-poliomyelitis immobility, and experimental restraint in normal
subjects lead to a negative calcium balance, with hypercalciuria.