Respiratory System

Perfusion and Diffusion Limited Transport

Anisha ValliLeave a comment

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 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.

DIFFUSION-LIMITED TRANSPORT:

  • The partial pressure gradient is maintained; that is, equilibrium between alveolar and pulmonary oxygen is not reached before the pulmonary blood leaves the capillaries.

Example 1:

Pulmonary fibrosis, a disease in which the alveolar walls are thickened.

  • As the distance for diffusion increases (through lung fibrosis), the rate of oxygen transport slows, and, consequently, total oxygen transfer is reduced, which leads to hypoxemia (low partial pressure of oxygen in the blood).

Graph illustrates:

  • Partial pressure of oxygen in the alveolar gas remains constant, at 100 mmHg.
  • Partial pressure of oxygen in the mixed venous blood begins at 40 mmHg, but, because diffusion is impaired and its rate decreased, never reaches equilibrium with the alveoli.
  • Shaded area that represents the partial pressure gradient between the alveoli and systemic arterial blood remains wide along the length of the capillary.
  • As we can see, it is the slower rate of diffusion that limits the total transport of oxygen from the alveoli to the pulmonary blood.

Example 2:

High altitude.

  • Drop in atmospheric partial pressure of oxygen leads to a drop in the partial pressure of oxygen in the alveoli.
  • Alveolar-arterial partial pressure gradient narrows, diffusion rate slows
  • To compensate, hemoglobin’s affinity for oxygen is reduced, so that oxygen is more easily released to the body tissues; this shifts the oxyhemoglobin curve to the right (discussed in detail elsewhere) to increase diffusion.

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