Which hematologic response increases within two hours of ascent?

• A. Erythropoietin increases
• B. Platelet count increases
• C. White blood cell count decreases
• D. Hemoglobin decreases

Answer: (A)

When you ascend to a high-altitude environment, the body’s tissues experience less oxygen, which triggers a rapid hormonal response from the kidneys. The kidneys sense the hypoxia and quickly increase erythropoietin production. Erythropoietin then acts on the bone marrow to stimulate the production of red blood cell precursors, starting this cascade within hours. This makes erythropoietin the earliest hematologic response to altitude change, occurring well within two hours. The other options don’t reflect this rapid hormonal adjustment: platelet counts, white blood cell counts, and hemoglobin levels do not rise as an immediate response to acute hypoxia.