People who have migrated from the planes to an area adjoining...

As a person moves up a hill the $p \mathrm{O}_2$ and total atmospheric pressure decreases. Decrease in $p \mathrm{O}_2$ due to increasing altitude, stimulates the JG-cells of kidney to secrete erythropoietin hormone, increasing the number of RBCs (polycythemia) to compensate the supply of $\mathrm{O}_2$. At higher altitude, haemoglobin has lower binding affinity to $\mathrm{O}_2$ because the primary factor responsible for binding is $p_2$, which decreases at higher altitude.