Adaptation to low oxygen partial pressure at high altitude

Steps:

• High altitude reduces atmospheric pressure, lowering partial pressure of oxygen (PO2) while oxygen percentage remains 21%.
• Low PO2 causes tissue hypoxia, triggering kidney release of erythropoietin hormone.
• Erythropoietin stimulates bone marrow to produce more red blood cells (RBCs).
• Increased RBCs provide more hemoglobin to bind and transport limited available oxygen.

Why B is correct:

• Lower PO2 at altitude reduces oxygen saturation of hemoglobin (per oxygen-hemoglobin dissociation curve), so more hemoglobin via extra RBCs compensates by increasing oxygen uptake capacity.

Why the others are wrong:

• A: RBC increase targets oxygen transport, not CO2 excretion or pH regulation, which is handled by hyperventilation.
• C: Oxygen percentage in air is unchanged at 21%; issue is lower PO2, not percentage affecting lung uptake.
• D: Hemoglobin affinity for oxygen slightly decreases (right shift via 2,3-BPG), but RBC increase primarily adds more carriers for transport, not affinity change.

Final answer: B