Adaptation to Hypoxia via Erythropoiesis

Steps:

• At high altitudes, atmospheric pressure drops, reducing oxygen partial pressure.
• This lowers oxygen availability, causing hypoxia in tissues.
• Kidneys detect low oxygen and release erythropoietin (EPO).
• EPO stimulates bone marrow to produce more red blood cells, increasing count to enhance oxygen transport.

Why A is correct:

• Haemoglobin saturation decreases due to lower alveolar PO2 (from Dalton's law of partial pressures), triggering compensatory erythropoiesis to maintain oxygen delivery.

Why the others are wrong:

• B: Partial pressure decrease is the cause of hypoxia but not the direct physiological trigger for RBC increase.
• C: Oxygen percentage remains ~21%; it's the lower total pressure that reduces partial pressure.
• D: CO2 levels do not increase at altitude; Bohr effect relates to CO2's influence on haemoglobin affinity, not RBC production.

Final answer: A