Microcytosis, i.e. decrease in the size of RBCs occurs:

– In iron deficiency anaemia,

– During prolonged forced breathing and

– When osmotic pressure of the blood is increased.

I am unable to understand how forced breathing results in microcytosis.

My attempt: I think prolonged forced breathing results in low CO2 concentration in blood which would lead to low H+ ion concentration , but this would lead to hypo-osmic plasma, which would certainly produce macrocytosis.

Source:Textbook of physiology

  • $\begingroup$ I thought H+ concentrations cause pH changes in blood and protein concentrations cause osmotic changes. $\endgroup$ – user 33690 Jul 16 '17 at 9:02
  • $\begingroup$ Co2 in the blood will be in the form of H2CO3, which will dissociate into H+ and HCO3- , if the rate of respiration is increased then, this is lost from the blood, so some amount of osmotically active species is lost from plasma. You are right in that osmotic changes, are mostly due to proteins, and it was just a guess to solve the original problem. $\endgroup$ – JM97 Jul 16 '17 at 9:10
  • $\begingroup$ I am hypothising too here:for maintaining constant blood viscosity->reverse of hamburgers->decreases swelling of RBC->makes the RBCs comparatvly irregular->viscosity rises.feedback occurs decreases the no. of such RBCs thus viscosity is controld.If I findrefrences I would post it as an answr later. $\endgroup$ – user 33690 Jul 16 '17 at 9:35

Forced breathing, a.k.a hyperventilation, causes hypertonic dehydration.

According to Greger and Windhorst, Comprehensive Human Physiology (2014):

Loss of more water than NaCl leads to hypertonic dehydration and hypernatremia. In this condition the intracellular volume is marked decreased, whereas the extracellular volume is less compromised. Possible causes of hypertonic dehydration include excessive loss of hypotonic fluid (e.g., sweat) or hyperventilation.

Therefore, the decrease of the size of red blood cells is due to the dehydration following hyperventilation.


  • Greger, R. and Windhorst, U. (2014). Comprehensive Human Physiology. Berlin: Springer Berlin.

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