Scientists in the fields of hematology and immunology study the red and white blood cells of the body, respectively, both which experience change during space flight.
Scientists have consistently observed decreases in both red blood cell mass and plasma volume after space flight. Losses of 2% to 21% of red blood cell mass, losses of 12% to 33% of hemoglobin mass, and losses of 4% to 16% of plasma volume occur regularly in microgravity. Investigators view this as part of the physiologic adaptation to weightlessness. Microgravity itself is most likely the major influence on this loss, though factors such as bone remodeling, muscle atrophy, and nutritional and metabolic disturbances may also have an effect. Regardless of the duration of the space flight, restoration of the lost blood cell mass can require 4 to 6 weeks following return to Earth. Although this space flight-induced anemia does not appear to compromise the health and performance of astronauts during or after flight, in-flight illnesses or injuries could alter cardiovascular and respiratory requirements to the extent that the reduction in blood cell mass could cause a problem.
White blood cells, which help the immune system identify and destroy external or internal pathogens, also appear to experience a transient reduction in number in microgravity. However, less is known about the changes in immune function during microgravity exposure than is known about changes in red blood cell parameters. It is important to understand the full effect of space flight-induced immune suppression, so its impact on crew health and safety can be determined and appropriate therapeutic measures developed.