Gas exchange between lungs and tissues

Gases diffuse from regions where their partial pressure is higher to those where it is lower. This allows CO2 to diffuse out of the lungs into the air, and for O2 to diffuse from the air into the lungs.

O2 also diffuses across the alveolar-capillary membrane due to a difference in partial pressures, though this difference is much smaller between the two. In this case it is also dependent on the lung diffusing capacity DL. DL is the permeability of the barrier between alveoli and capillaries, which is directly proportional to the diffusion coefficient of the gas and surface area, and inversely proportional to thickness. Fick’s Law can be used to find the volumetric flow rate:

V̇x = DL*ΔP

Where V̇x is the volumetric flow rate, ΔP is the pressure difference, and DL is the diffusing capacity.

Partial pressure gradients and surface area also drive gas exchange between the capillaries and tissues, as well as blood perfusion of the tissue.