Perfusion-limited transport of oxygen
- When the partial pressure gradient across the alveolar-capillary wall drops to zero, net diffusion ceases.
- 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 illustrates:
- 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.