A patient with chronic severe anaemia (haemoglobin 7 g/dL) remains asymptomatic at rest. The principal compensatory mechanism allowing near-normal tissue oxygen delivery is:

• A Leftward shift of the oxyhaemoglobin dissociation curve increasing loading of oxygen in lungs
• B Reduction in capillary density redirecting flow to vital organs
• C Increase in arterial oxygen tension above 100 mmHg
• D Rightward shift of the dissociation curve from elevated 2,3-bisphosphoglycerate, facilitating unloading at tissues ✓

Explanation

Chronic anaemia raises red cell 2,3-bisphosphoglycerate, shifting the oxyhaemoglobin curve to the right, lowering affinity so oxygen unloads more readily at a given tissue PO2. Arterial PO2 and saturation remain essentially normal in anaemia because dissolved oxygen depends on alveolar gas, eliminating option A and C. Increased cardiac output and erythropoiesis contribute, but the curve shift is the canonical textbook answer for preserved extraction.

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

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