An individual who lives at a high altitude may normally have an increased RBC count because

• A. high altitudes cause vascular fluid loss, leading to hemoconcentration.
• B. hypoxia caused by decreased atmospheric oxygen stimulates erythropoiesis.
• C. the function of the spleen in removing old RBCs is impaired at high altitudes.
• D. impaired production of leukocytes and platelets leads to proportionally higher red cell counts.

Answer: (B)

High-altitude hypoxia stimulates erythropoietin production, which drives erythropoiesis. The lower oxygen tension sensed by the kidneys prompts them to release more erythropoietin, signaling the bone marrow to make more red blood cells. Over weeks to months, this increases the red cell count and hematocrit, improving the blood’s capacity to carry oxygen in a low-oxygen environment. Temporary hemoconcentration from dehydration can contribute a bit, but the lasting rise in red cells is due to erythropoietin-driven production. Spleen function or changes in leukocytes and platelets don’t explain this adaptation.