Key Points

• O2 moves from the atmosphere to tissues by a series of convective (bulk flow) and diffusive transport steps, which decrease Po2 at each step.

• Alveolar Pco2 is inversely related to alveolar ventilation at any given metabolic rate. Physiologic dead space is the functional difference between total ventilation and effective ventilation of the alveoli.

• O2 diffusion from alveolar gas to pulmonary capillary blood is effective at equilibrating alveolar and arterial Po2 in normal individuals under most conditions.

• O2 delivery by the cardiovascular system depends on cardiac output and the arterial-venous O2 difference. The shape of the blood-O2 equilibrium curve is advantageous for O2 transport to tissues.

• The pathway for O2 diffusion is longer in tissues than in the lung, so Po2 differences between blood and tissue are much larger than the blood-gas Po2 difference in the lung.

• There are four main causes of arterial hypoxemia. Hypoventilation differs from other causes of hypoxemia because it does not increase the alveolar-arterial Po2 difference, and it can occur in patients with normal lungs.

• Diffusion limitation can result from thickening of the blood-gas barrier in lung disease, but oxygen breathing rapidly alleviates this form of arterial hypoxemia.

Essential Medical Physiology, Third Edition

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