High-altitude acclimatization increases red blood cell mass

Steps:

• Low oxygen at high altitude triggers hypoxia-inducible factor, stimulating erythropoietin release.
• Erythropoietin promotes bone marrow production of red blood cells over weeks to months.
• After three months, chronic adaptation results in higher red cell mass to enhance oxygen transport.
• At rest, this polycythemia persists without acute cardiovascular strain.

Why D is correct:

• Hypoxia induces erythropoiesis via erythropoietin, increasing red blood cell mass to improve oxygen-carrying capacity, as defined by the hormone's role in stimulating erythroid progenitors.

Why the others are wrong:

• A: Stroke volume decreases chronically due to reduced plasma volume and right ventricular strain.
• B: Breathing rate increases acutely but normalizes at rest after acclimatization.
• C: Heart rate elevates initially but returns to baseline or lower at rest long-term.

Final answer: D