When a person moves to higher altitudes, the PO2 and total atmospheric pressure decrease. Hypoxia stimulates the JG-cells of the kidney to release erythropoietin hormone, which stimulates erythropoesis in bone marrow causing polycythemia. Hypoxia will also increase breathing rate. Initially the size of RBCs will also increase but with increase in number of RBCs, the size of RBCs becomes normal.