Diffusion-limited Gas Exchange

  • Diffusion-limited Gas Exchange describes the scenario in which the rate at which gas is transported away from functioning alveoli and into tissues is principally limited by the diffusion rate of the gas across the alveolar membrane. The classic example of diffusion-limited gas exchange is transport of Carbon Monoxide (CO) across the alveolar membrane. Hemoglobin within blood can tightly bind enormous amounts of carbon monoxide even at very low CO gas partial pressures. On average, blood traverses the pulmonary capillaries within 3/4 of a second, thus allowing a limited amount of time to exchange whatever gases are present.
  • Because of the enormous CO-binding capacity of hemoglobin within the blood, any CO diffusing into the blood is rapidly bound by hemoglobin and thus cannot contribute to the blood partial pressure of CO. Consequently, by the time blood traverses across the pulmonary capillaries, the blood partial pressure of CO is still very low compared to that in the alveolar space, meaning a large partial pressure gradient remains even in the end-capillary blood. Because the rate of CO diffusion across the membrane is the principal factor limiting further blood transport of carbon monoxide away from the lungs, pulmonary exchange of carbon monoxide is referred to as "Diffusion-limited".

Characteristics of Diffusion- and Perfusion-limited Gas Exchange
Diffusion- and perfusion-limited gas exchange are distinguished by the extent that an alveolar gas’s partial pressure will equilibrate across the alveolar membrane as blood flows through the pulmonary capillaries. Diffusion-limited gas exchange (left) is characterized by incomplete equilibration. In this scenario, the rate of gas diffusion across the alveolar membrane limits its transport away from the lung. In contrast, perfusion-limited gas exchange (right) is characterized by complete equilibration. In this scenario, the rate of gas transport from the lung can only be increased by increasing pulmonary blood flow (i.e. perfusion).