CAUSES OF HIGH AaΔO₂ (continued)

E. "Normal" Ventilation/Perfusion (V'_A/Q) Nonuniformity (mismatch)

Note: Characteristic of upright posture (orthostatic); less important in supine posture

1. higher regions of lungs (apex) get less blood flow than lower (basal) regions

Reason: the reduced intravascular pressure lead to partial (Zone 2) or total (Zone 1) vascular compression, thus increasing vascular resistance

2. higher regions of lungs (apex) get less ventilation than lower (basal) regions

Reason: In the upright posture, gravity reduces intrapleural pressure in the apical regions compared to the base of the lung. This leads to distension (stretching) of apical alveoli at rest, thereby reducing apical compliance

3. Ventilation/Perfusion ratio: The effect of gravity on pulmonary circulation is greater than the effect on alveolar ventilation

Thus, ventilation/perfusion ratio is less at lung base, so blood flowing through basal region is less well oxygenated

4. In a normal person, nonuniformity is not sufficient to cause a large AaΔO₂

Note: the tendency of low P_A-O2 and high P_A-CO2 in underventilated regions is to increase local pulmonary vascular resistance (vasoconstriction) and decrease local airway resistance (bronchodilation) thus increasing V'_A/Q, and vice-versa for the overly ventilated/under perfused regions; this reduces, but does not totally eliminate, the nonuniform V'_A/Q