2021A08 Describe the V/Q ratio at the apex and base of the upright lung.
How does this affect regional gas exchange?

Graphs:

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Ventilation	· Gravity -> o Apical traction -> ↑volume -> ↓compliance -> ↓V o Basal compression -> ↓volume -> ↑compliance -> ↑V
Perfusion	· Gravity -> West’s zones: 1. PA > Pa > Pv: no flow (apex) 2. Pa > PA > Pv: flow in systole (from 3cm above RV to apex) 3. Pa > Pv > PA: flow throughout (from base to 3cm above RV) · Hypoxic pulmonary vasoconstriction: o ↓Ventilation -> ↓pO₂ in individual alveolus -> vasoconstriction o Improves local V/Q matching
Carbon dioxide	· CO₂ flux fairly evenly distributed o Mild gradient in ventilation from base to apex o Effective compensation for ↓V/Q regions § Hyperventilation -> ↓↓CO₂ in well-matched regions § High CO₂ solubility -> PaCO₂/CaCO₂ curve is steep -> V/Q inequality rarely causes hypercapnoea
Oxygen	· O₂ flux almost all near the base o Steep gradient in perfusion from base to apex o Ineffective compensation for ↓V/Q regions § Hyperventilation -> ↔↑O₂ in well-matched regions § Low O₂ solubility + finite Hb binding sites -> OHDC upper part flat -> V/Q inequality often causes hypoxaemia