Oxygen Pulmonary Gas Exchange
- Experimental studies have demonstrated that in healthy individuals the partial pressure of oxygen in blood largely equilibrates with the partial pressure of oxygen in the alveolar space (Alveolar Oxygen) roughly one-third of the way through its course through the pulmonary capillaries. In other words, the partial pressure gradient of oxygen across the alveolar membrane is eliminated early on as blood moves through the pulmonary capillaries. Given these features, it is clear that oxygen gas exchange is perfusion-limited in a normal, healthy lung. However, a number of physiological and pathological scenarios can modify the character of oxygen gas exchange as described below.
- During heavy exercise, the cardiac output of the heart increases substantially which in turn significantly enhances the blood flow rate through the lung and thus each individual alveolar unit. Empirical studies have shown that heavy exercise results in blood travelling through the pulmonary capillaries nearly three times as fast as during rest. Consequently, during scenarios of intense physical exertion, the partial pressure of oxygen across the alveolar membrane just barely equilibrates by the time blood has reached the end of the pulmonary capillaries.
- As a result, heavy exercise pushes pulmonary gas exchange to the cusp of switching from perfusion-limitation to diffusion-limitation. It is perhaps no coincidence, and likely a product of long evolutionary adaptation, that the body possesses precisely enough of a safety factor in terms of oxygen diffusion that heavy exercise does not result in a diffusion-limited situation. If diffusion-limitation did occur with heavy exercise, then the partial pressure of oxygen in arterial blood, arterial oxygen, would decline with physical exertion, resulting in hypoxemia.
|Alveolar Membrane Thickening|
- Certain pulmonary disease result in pathological thickening of the alveolar membrane which in turn can substantially reduce the diffusing capacity of oxygen and thus its rate of diffusion across the membrane. The rate of oxygen diffusion can be so considerably slowed that the partial pressure gradient of oxygen across the alveolar membrane may not equilibrate by the time blood reaches the ends of the pulmonary capillaries. In such a scenario, oxygen gas is rendered diffusion-limited and results in reductions in the partial pressure of arterial oxygen, thus yielding hypoxemia.
- Reductions in the partial pressures of alveolar oxygen result in reductions in the partial pressure gradient for oxygen across the alveolar membrane. For example, a reduction in alveolar oxygen partial pressures from the normal 100 mm Hg to 60 mm Hg would reduce the partial pressure gradient for oxygen by 40 mm Hg. As explained by Fick's Law, the rate of oxygen diffusion across a membrane is proportional to the partial pressure gradient for the gas across the membrane. Consequently, substantial reductions in the oxygen partial pressure gradient may yield significant drops in the rate of oxygen diffusion across the alveolar membrane.
- In certain scenarios oxygen diffusion can become so slowed that the partial pressure of oxygen does not equilibrate across the alveolar membrane by the time blood reaches the end of the pulmonary capillary. As a result, oxygen exchange can be rendered diffusion-limited. In general, this does not occur in normal individuals at rest but can strongly exacerbate scenarios which place stress of the oxygen diffusion rate. For example, reduced alveolar oxygen partial pressures in individuals with alveolar thickening can severely worsen hypoxemia. Additionally, at high altitude, oxygen partial pressure can be sufficiently reduced such that heavy exercise can yield bona fide diffusion-limitation for pulmonary oxygen exchange.