Diffusion-limited and perfusion-limited gas exchange

The primary role of the lungs is to ensure gas exchange between the external environment, and the blood within the circulatory system.

This happens thanks to a series of branching tubes called airways, which conduct the air down into small thin-walled sacs called alveoli, which are wrapped in an intricate network of tiny blood vessels called pulmonary capillaries.

And the alveolo-capillary membrane, where the layer of alveolar cells lining the alveoli meets the endothelial cells that make up the pulmonary capillary, is where gas exchange occurs.

Now, before we delve into diffusion, perfusion and their limits, remember that gas exchange across the alveolo-capillary membrane happens according to Fick’s law.

Fick’s law states that the net rate of diffusion - V of any particular gas across the alveolar-capillary membrane, is proportional to the pressure gradient across the wall; which is the difference between the partial pressure of the gas in the alveolar sacs, or PA, and the partial pressure of the gas in the blood, or Pa, and also proportional to the surface area of the wall, or A, but inversely proportional to the wall’s thickness - T. And this is all times the diffusion constant - D, which varies from gas to gas. V=(PA-Pa)ADT So, diffusion-limited gas exchange means that a gas like oxygen or carbon dioxide can diffuse across the alveolo-capillary membrane as long as the partial pressure gradient is maintained.

On the other hand, perfusion-limited gas exchange means that if the pressure gradient is not maintained, and the concentration of gases on the two sides of the alveolo-capillary membrane becomes the same, further gas exchange is only possible by increasing blood flow, or perfusion, in the pulmonary capillary.

To understand these concepts, let’s look at a section of an alveolar sac, with a pulmonary capillary running along its surface.

The capillary carries mixed venous blood, which is low in oxygen and high in carbon dioxide. As blood passes along the alveolar capillary wall along the length of the capillary, it exchanges some gas molecules with the interior of the alveolar sac.