West Zone 1 of the lung (apex in an upright person) has a ventilation-perfusion ratio that is greatly elevated. What is the physiological consequence and why does it not normally cause hypoxaemia at sea level?

• A Zone 1 is the most efficiently oxygenated region; high V/Q maximises PAO₂, increasing total O₂ uptake
• B Zone 1 acts as alveolar dead space (high V/Q → little gas exchange per alveolus); it does not cause hypoxaemia because it constitutes a tiny fraction of total lung volume at sea level ✓
• C Zone 1 has intermittent blood flow causing periodic shunting that is compensated by increased mixed venous PO₂
• D Zone 1 wasted ventilation is offset by a compensatory increase in V/Q in Zone 3 that reverses the effect

Explanation

In Zone 1 (Pa > Pa > Pv does not apply; actually Pa < alveolar pressure), pulmonary capillary pressure falls below alveolar pressure at the lung apex, collapsing capillaries and producing alveolar dead space (ventilation without perfusion). The PAO₂ is high but these alveoli contribute little to gas exchange. Under normal conditions Zone 1 is negligible, but in conditions such as haemorrhage (low Pa) or PEEP (high alveolar pressure), Zone 1 expands and dead-space fraction increases. The excess dead-space from Zone 1 does not cause hypoxaemia per se but reduces ventilatory efficiency.

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

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