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Polycythemia is where the hematocrit is above 55%. This can be divided into absolute and relative and absolute can be further divided into primary and secondary.

Absolute polycythemia is where the hematocrit is above 55% because of increased RBC count.

Primary means there is no underlying cause whereas secondary means that something else is causing RBC count to increase. And relative polycythemia is where the hematocrit is increased due to lowered plasma volume, not RBC count.

Now I am wondering, my Kepler Bb people live underground so it gets harder to breathe the deeper down you go. 1 common cause of secondary polycythemia is chronic hypoxia.

So would it be good if everyone had the genetics for an increased RBC count? Then this elevated RBC count would be normal for them and they wouldn't have as much trouble breathing in areas of lower oxygen concentration. On the other hand this requires a higher amount of iron to sustain and if polycythemia was normal than our normal would be anemia.

So unless they had lots and lots of iron rich food nearby, it would be easy to get iron deficiency anemia if the normal is polycythemia.

So I am wondering, should my Kepler Bb people naturally have polycythemia as their baseline level of RBCs to increase oxygen transport when they go deep underground but at a risk of easily getting Iron Deficiency Anemia or have normal RBC count but at a greater risk of getting hypoxia?

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    $\begingroup$ This is one of those questions that needs links to define its terms. $\endgroup$ – SRM Feb 2 '17 at 1:16
  • $\begingroup$ Perhaps you should give more context in the beginning - instead of "Polycythemia is where the hematocrit is above 55%." with links it would be more appropriate to just explain those terms. $\endgroup$ – Zxyrra Feb 2 '17 at 3:45
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    $\begingroup$ Are the tags you created - polycythemia and anemia - beneficial to the site? While there are many circumstances in which tags can be created, these seem very specific. I'm not sure if they will be useful - would disease or medical work as replacements? $\endgroup$ – Zxyrra Feb 2 '17 at 3:59
  • $\begingroup$ Real human adaptation to high altitude doesn’t involve having more oxygen carriers in the blood—that would make it too thick. $\endgroup$ – JDługosz Feb 2 '17 at 5:00
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You're overthinking it.

First off, polycythemia is defined with respect to humans, on Earth. Ending in -cythemia, it's a word describing a "condition involving the cells of the blood." The "normal" for your race will never be a condition (other than perhaps the human condition).

So stepping beyond the word, the real question would be "would people get hypoxic in their normal daily lives, or would they have enough red blood cells to transport oxygen." The answer is obvious: they would have enough red blood cells. Our bodies always adapt to our environment, especially given a long enough period of time for genetics to weigh in.

As for anemia, the answer is also simple. I can rephrase your question as "would people risk not having enough red blood cells if they get iron deficient, or would they not have enough red blood cells all the time?" The answer to that is easy: they'd have enough red blood cells, and they'd deal with any iron deficiency issues as they came up. There's no advantage to intentionally giving yourself a large permanent handicap to avoid a small temporary one.

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    $\begingroup$ I'm not going to answer the question because you've pretty much covered the bases - but if you want evidence for "they would have enough red blood cells" you can link to this handy page about whale blood. $\endgroup$ – Zxyrra Feb 2 '17 at 3:47
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I think you've got your oxygen dynamics backwards. As altitude decreases, hematocrit levels decrease as well because the increased air pressure makes absorbing oxygen easier. Conversely as altitude increases, then hematocrit increases. People in the Andes mountains have high hematocrit levels and athletes train at altitude to naturally increase the hematocrit levels, giving them an advantage when they exert themselves at lower altitude.

It gets harder to breathe down in mines and caves owing not to pressure but to poor air circulation combined with CO, CO2, CH4(methane) and a lot of gases leaching out the rocks. Raising hematocrit might offset lower O2 levels if you postulate poor circulation and exchange with the surface but at the same time, the viscosity of the blood will increase which will cause it to absorb more noxious gasses while at the same time impairing its CO2 transport because the CO2 is solvated in plasma volume which will decrease as hematocrit rises and the higher pressure will tend to keep it solvated. (At altitude, the viscosity and CO2 release problems are lessened by the lower pressure.)

Also, our sense of whether we have our breathe or not is actually measured by the CO2 in the blood as the O2 is bound and inaccessible. That's why breathing to fast, hyperventilating, creates a sense of suffocation. It removes the CO2 to fast. And its why rebreathing into a paper bag fixes the problem, it increase CO2. To much CO2 can suppress the breathing reflex.

If the O2 circulation is poor, then the air volume normally taken by oxygen will be taken by something else, something added to the mix as the nitrogen levels won't change (although people have gotten the bends, like deep sea drivers, when coming out very deep mines to quickly.) Whatever that takes up the volume instead of oxygen won't be inert and won't be healthy.

Polycythemia would make everything worse in high pressure, toxin filled atmosphere. Conversely, if the general air circulation is good i.e. high O2 no toxins, then increased pressure will cause the hematocrit to drop, not rise.

Hypoxia is never simple.

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  • $\begingroup$ Polycythemia would be helpful if O2 was low because these deep people loved big smoky fires. The extra hemoglobin could scavenge CO. $\endgroup$ – Willk Feb 4 '17 at 21:26
  • $\begingroup$ @Will - No, the extra CO (carbon monoxide) will bind to the hemoglobin permanently, rendering it useless for oxygen transport. That is how CO kills. It's insidious because the victim never feels discomfort mere drowsiness and perhaps some disorientation. My father was killed in Aug 1965 while attempting to rescue two of his teammates on an oil storage tank cleaning crew who had unknowingly walked into a storage tank with a freak levels of CO but no other tell tales gases like CO2. It almost got the investigators as well. $\endgroup$ – TechZen Feb 5 '17 at 16:12
  • $\begingroup$ @Will - There are few things more dangerous than a fire burning in an enclosed space. The way that torches are depicted in movies in caves is wildly inaccurate. A foot flame would consume as much oxygen as 4-5 adult males. If you look at pre-electric, flame mining lights/candles, they are tiny flames, about the size of burning match tip. The miners relied on night vision to magnify those tiny lights to useful illumination. Large, "smokey" fires undergo incomplete combustions which means in addition to CO and other toxins, they also give off combustable gasses which go boom, eventually. $\endgroup$ – TechZen Feb 5 '17 at 16:21
  • $\begingroup$ @Will - Remember that blood is not just red cells, blood plasma is a highly complex mixture of proteins, water and carbolic acid (solvated CO2). Red blood cells don't carry CO2. Blood viscosity is very important and complex and polycythemia increases it. Increased pressure also increase solvency of gasses like CO2 & N2. High blood viscosity makes it harder for the plasma to solvate CO2 initially but also makes it harder to release it once it has. Our breathing reflex is triggered by CO2 levels, measured by blood PH, not oxygen. See hypoventilation for the counterintuitive effects. $\endgroup$ – TechZen Feb 5 '17 at 16:33

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