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Alveolar ventilation brings inspired gas with a PO2 of about 150 mm Hg and a PCO2 of about 0.3 mm Hg into the alveoli. At the same time, the right ventricle pumps mixed venous blood with a PO2 of about 40 mm Hg and a PCO2 of about 45 mm Hg into the pulmonary capillaries. Oxygen diffuses from the alveoli into the pulmonary capillaries at the same time that carbon dioxide diffuses from the pulmonary capillaries. The PO2 and PCO2 of an alveolar-capillary unit are determined by the relative ventilation and perfusion of the unit. Increasing the ventilation relative to the perfusion increases the PO2 and decreases the PCO2 of the alveolus. Increasing the perfusion relative to the ventilation decreases the PO2 and increases the PCO2 of that alveolus.


Alveolar ventilation V˙A is normally about 4 to 6 l of air per minute, and pulmonary capillary blood flow Q˙c has a similar range, so the ratio of ventilation to perfusion for the whole lung is about 0.8 to 1.2. However, ventilation and perfusion must be matched on the alveolar-capillary level. The V˙A/Q˙c for the whole lung is really of interest only as an approximation of the situation in all the alveolar-capillary units of the lung.

Clearly, alveoli that are ventilated but not perfused constitute alveolar dead space and contribute nothing to gas exchange in the lung (Figure 29-1). Similarly, alveoli that are perfused but not ventilated constitute an intrapulmonary shunt and return mixed venous blood to the systemic circulation.

Figure 29-1

The effect of changes in the ventilation-perfusion ratio on the alveolar PO2 and PCO2. (A) Normal V˙A/Q˙c. (B) V˙A/Q˙c= 0. (C) V˙A/Q˙c is infinite. (Reproduced with permission from West JB: Ventilation/Blood Flow and Gas Exchange, 5th ed. Oxford, UK: Blackwell Publishing; 1990.)


Alveolar-capillary unit A in Figure 29-1 has a normal ventilation-perfusion ratio. Inspired air entering the alveolus has a PO2 of about 150 mm Hg and a PCO2 of nearly 0 mm Hg. Mixed venous blood entering the pulmonary capillary has a PO2 of about 40 mm Hg and a PCO2 of about 45 mm Hg. This results in an alveolar PO2 of about 100 mm Hg and an alveolar PCO2 of 40 mm Hg. The partial pressure gradient for oxygen diffusion is thus about 100 – 40, or 60 ...

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