KEY POINTS

1. The composition of a gas mixture can be described by the fractional composition or partial pressure.

2. Alveolar gas composition is determined by alveolar ventilation and the exchange of oxygen and carbon dioxide.

3. Exchange of oxygen and carbon dioxide between the alveolus and pulmonary capillary blood occurs by diffusion.

4. The exchange of gases between the tissues and blood also occurs by diffusion.

5. The amount of alveolar ventilation in relation to pulmonary capillary blood flow—the V/Qratio—determines the adequacy of pulmonary gas exchange.

6. The composition of the systemic arterial blood is determined by the composition of the capillary blood that drains each alveolus.

7. Right-to-left vascular shunts allow blood to bypass ventilated lung.

8. Part of each breath ventilates dead-space and does not participate in gas exchange.

9. Arterial oxygen (Pao₂) and carbon dioxide (Paco₂) tensions are measured to evaluate gas exchange.