Perfusion-Limited Transport
Gas transport across the alveolar and pulmonary capillary walls is either perfusion limited or diffusion limited.
The
partial pressure gradient of oxygen between the alveoli and the pulmonary blood flow drives diffusion; this is true whether transport is perfusion or diffusion limited.
Oxygen transport is perfusion-limited in healthy lungs.
Diffusion-limited in fibrosis (a pathology) and high altitude (an environmental change).
Perfusion-Limited Oxygen Transport
When the partial pressure gradient across the alveolar-capillary wall drops to zero, net diffusion ceases.
The only way to increase transport is to increase in capillary blood perfusion increases transport, which is another way of saying that transport is limited by perfusion.
Graph Details:
Partial pressure of oxygen of inspired alveolar gas is constant, at 100 mmHg.
Mixed venous blood arrives at the pulmonary capillary with a partial pressure of 40 mmHg.
As it passes through the capillary, its partial pressure of oxygen rapidly increases.
At approximately 1/3rd of the way through the capillary it equilibrates with that of the alveolar gas.
No further net diffusion occurs.
The area between the alveolar and arterial oxygen partial pressures represents the decreasing gradient between the two.
