By Michael A. Grippi, MD - 24th Mar 2020 - Course previews

Simplifying Fick's law and lung gas exchange

Fick's law describes the process whereby gas movement across the alveolar-capillary membrane occurs through the process of diffusion. In this video, from our Blood Gas Analysis Essentials course, you'll learn about the impact of diffusing capacity and other factors that affect gas transfer across membranes, as well as how this relates to respiratory pathologies such as emphysema and pulmonary fibrosis.

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Video transcript

The alveolar surface area of the lung is quite expansive, and contains an extensive network of pulmonary capillaries, making it ideal for gas exchange. Gas movement across the alveolar capillary membrane occurs by diffusion. And this process is described by Ficks law, flux of a gas equals the diffusing capacity of the membrane times the pressure gradient across the membrane.

According to Ficks law, the rate of gas transfer across a tissue playing or membrane is directly proportional to the difference in partial pressures of the gas on the two sides of the membrane. And the membranes diffusing capacity. The diffusing capacity of a membrane is dependent on several components, tissue plane or membrane area, tissue thickness, solubility of the gas and the molecular weight of the gas.

And now if we substitute the equation for diffusing capacity back into Ficks law, we get this equation. So to reiterate, there are a number of factors that can influence the movement or flux of gas molecules across the membrane, the surface area of the membrane, the thickness of the membrane, the solubility of the gas, the molecular weight of the gas, and the driving pressure gradient across the membrane.

From the looks of the formula, it may seem a little complex, but the changes are fairly intuitive. Let's consider each component individually. Diffusion is enhanced if there's more membrane surface area available for gas transfer. Diffusion is enhanced if membrane thickness is reduced, constituting a shorter path length for gas transfer.

Similarly, diffusion is enhanced if the gas has greater solubility in the membrane. And diffusion is enhanced with a lighter gas that is a gas of lower molecular weight. Finally, diffusion is enhanced if there is a greater driving pressure across the membrane. And we can apply this to the movement of gas, let's say oxygen across the alveolar capillary membrane.

So, loss of alveolar surface area, as seen in emphysema will decrease diffusion of oxygen into the lung. Similarly, an increase in the thickness of the alveolar wall, as seen in pulmonary fibrosis will limit diffusion of oxygen. And a decrease in the partial pressure of oxygen in inspired gas will reduce the pressure gradient for oxygen diffusion across the lung.

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