As a person moves up a hill the pO2 and total atmospheric pressure decreases. Decrease in pO2 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 O2. At higher altitude, haemoglobin has lower binding affinity to O2 because the primary factor responsible for binding is p2, which decreases at higher altitude.