The ventilation-perfusion (V/Q) ratio varies between apex and base of an upright lung. Which of the following correctly describes the difference?
- A V/Q ratio is highest at the apex (~3.3) because perfusion falls more steeply than ventilation from base to apex due to gravity; this makes the apex functionally similar to dead space ✓
- B V/Q ratio is highest at the base (~3.3) because blood flow and ventilation are both maximal at the base
- C V/Q ratio is uniform throughout the lung at ~0.8, with gravity having no effect in healthy individuals
- D V/Q ratio is lowest at the apex because hypoxic vasoconstriction causes redirection of blood flow to the apex in upright position
Explanation
In an upright lung, both ventilation and perfusion increase from apex to base due to gravity, but perfusion (Q) increases much more steeply (approximately 5-fold from apex to base) than ventilation (approximately 3-fold). This means the V/Q ratio is highest at the apex (~3.3, i.e., over-ventilated relative to perfusion — physiological dead space) and lowest at the base (~0.6, i.e., under-ventilated relative to perfusion — physiological shunt). The average overall V/Q ratio is ~0.8. At the apex, high PaO2 and low PaCO2 (relative to base) prevail due to the high V/Q. At the base, higher blood flow contributes more to the mixed alveolar output. Hypoxic vasoconstriction (option D) would redirect flow away from hypoxic areas, not to the apex.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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