Effect of forced breathing on RBC size > 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

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.

Source:

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