Localized or diffuse collapse of alveoli in the lungs may, if the
condition persists, lead to arterial hypoxia which may be extremely
undesirable under the stresses of space flight. The alveoli are probably
unstable when pure oxygen is breathed; they tend to collapse if there is
blockage of the airways, especially at low pressures. This collapse
occurs because each of the gases present in the alveoli (oxygen, water
vapor, and carbon dioxide) is subject to prompt and complete absorption
from the alveoli by the blood.
The alveoli are normally stabilized against collapse by the presence of
inert and relatively insoluble gas (nitrogen) and an internal coating of
lipoprotein substances with low surface tension.
Theoretical and experimental results strongly suggest the desirability
of using oxygen-inert gas atmospheres for long missions to avoid
atelectasis and other gas absorption phenomena, such as retraction of
the eardrum. However, further experimental evidence is required both to
confirm this point and to establish its upper limit of suitability of
pure oxygen atmospheres.
At Ohio State University in 1962, scientists studied the effect on young
rats exposed for 27 days to 100 percent oxygen (with no nitrogen), at a
reduced barometric pressure equivalent to 33 000 feet altitude. The rats
showed no difference in growth rate, oxygen consumption, food and water
intake, or behavior from control rats in air at 1 atm.