I have a conflicted relationship with the Red Cross. On the one hand, I like donating blood (provided that I never actually see the needle), and believe it’s the right thing to do and indeed that I have a responsibility to do it. On the other hand, I wish they would stop calling me at totally inconvenient times, and that the finger stick didn’t hurt so much, and that I didn’t fail the hemoglobin test so often.
In space, this might get weird for me, because of space flight anemia.
Even in the 60s, they noticed that astronauts who spent time in space often had decreased red blood cell mass afterwards:
FIG. 1. Red blood cell mass changes after spaceflight. Each point represents one individual. Data are expressed as percentage of change from preflight red blood cell mass (mL/kg of body mass).
(from Smith, linked above)
Interestingly, despite invoking the word “anemia,” which to me suggested that the astronauts had iron levels that were too low, Smith says that:
Although the weight of evidence now indicates that iron storage and availability increase during space flight, space food systems provide excessive amounts of dietary iron (about 20 to 22 mg Fe/d).[13 and 26] Expert panels have recommended that iron intake of men and women be reduced to less than 10 mg/d during space flight.  Even with the typically reduced, overall dietary intake, iron consumption exceeds this recommendation ( Fig. 3A). Dietary fiber intake decreases during flight compared with before flight (Fig. 3B), further increasing the risk of elevated iron absorption.
Overdosing on iron can actually be dangerous, in that it can kill you. Could be awkward in space.
Also, it appears to be the case that anemia refers to the amount of red blood cells you have, not your internal iron content, explaining why it’s called “anemia” even if they have extra iron.
So why does it happen? I was struggling through a bunch of blood terminology I didn’t understand until I found this abstract, italics added by me for the important parts:
Upon entering microgravity, the blood volume in the extremities pools centrally and plasma volume decreases, causing plethora and erythropoietin suppression. There ensues neocytolysis, selective hemolysis of the youngest circulating red cells, allowing rapid adaptation to the space environment but becoming maladaptive on re-entry to a gravitational field.
“Hemolysis” means that the cells break open and their contents go floating happily into the bloodstream. Red cell count: decreased. Cue anemia. On the other hand, these people at the Vanderbilt Center for Space Physiology suggest that it’s just decreased production, not hemolysis, that causes space flight anemia.
As pointed out in one of the articles above, it’s hard to study because opportunities to fire people up into space and test their blood aren’t just lying around. Nevertheless, that it happens is pretty clear, so before our future space travelers set out we’ll have to figure out how to deal with it, as will Space Red Cross.