Within the first few days of arrival at 4000 m, before any significant rise in red cell mass, hemoglobin concentration measurably increases. The immediate mechanism responsible is:

• A Hypoxia-induced diuresis reducing plasma volume and hemoconcentrating the blood ✓
• B Erythropoietin-driven reticulocyte release expanding red cell volume
• C Splenic contraction releasing stored erythrocytes into circulation
• D Fluid shift from the intravascular compartment into exercising muscle

Explanation

Early altitude exposure triggers a diuresis with reduced plasma volume, so hemoglobin and hematocrit rise within days through hemoconcentration, well before erythropoiesis contributes appreciable new cells. Erythropoietin peaks within hours but the marrow needs about a week or more to raise red cell mass detectably. Humans lack a significant splenic erythrocyte reservoir, unlike diving mammals, ruling out option C.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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