Pulmonary Gas Exchange - Basic Principles
- Transport of oxygen and carbon dioxide across the alveolar membrane occurs purely by passive diffusion of these gases down their respective partial pressure gradients across the membrane. The fundamental rate at which these gases diffuse is governed in large part by Fick's Law which was originally described by the German physiologist A. E. Fick but is now used as a general principle in thermodynamics. This stands as a rare example of a general physical principle originating from physiological studies.
- In the context of respiratory physiology, the fundamental rate of gas transport across the alveolar membrane is less important than understanding the factors that determine the rate at which gas can be transported away from the lung and into the tissues. For some gases, the factor which limits additional gas transport from the lung is the rate at which the gas diffuses across the alveolar membrane, as determined by Fick's Law (Diffusion-limited Gas Exchange). However, for other gases, the factor which limits additional gas transport from the lung is not the rate of gas diffusion, but rather the rate at which blood is transported through the pulmonary capillaries (Perfusion-limited Gas Exchange).