This might seem like a petty point to make , but it is intimately linked to myth # 3 , so we´ll do it anyway.
Rapid ascents start physiological responses , and , after the higher respiratory and heart rate, the increased red blood cell production is the most commonly mentioned.
It seems straight forward : the body senses the lowered oxygen saturation as you ascend (more on that later) , orders some more hemoglobin , which is delivered.
One of the stumbling points here is delivery time : the unspoken assumption here is this a more or less Fed Ex-like process : sign the order and start looking for the mail man . This would be more like Indian mail , though : a blood cell is a complex product , and will take more than a week to produce.
As with other myths , there is a valid observation at work , with the wrong explanation .
Red blood cell concentration does go up , but it´s the same old blood cells still going around : concentration goes up since the blood volume decreases after increased urine production. After descent the blood quickly goes back to normal values : don´t count on having a higher hemoglobin level a week after descending below 2000 meters.
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