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Perfusion-limited Gas Exchange

  • Perfusion-limited Gas Exchange describes the scenario in which the rate at which gas is transported away from functioning alveoli and into tissues is principally limited by the rate of blood flow through the pulmonary capillaries and thus across the alveolar membrane. The classic example of perfusion-limited gas exchange is transport of nitrous oxide N2O across the alveolar membrane. Nitrous Oxide displays a rapid diffusion rate across the alveolar membrane and is not bound by any proteins within blood. Consequently, the gas partial pressure of nitrous oxide within blood rises rapidly following the transport of only minimal amounts of the gas. As a result, the partial pressure of nitrous oxide equilibrates rapidly as blood traverses the pulmonary capillaries, thus eliminating the partial pressure gradient across the alveolar membrane long before blood reaches the end of the capillaries.
  • Given the rapid equilibration of nitrous oxide, enhancing the diffusion rate of the gas across the alveolar membrane would not increase the blood transport rate of nitrous oxide away from the lungs. Consequently, the principal factor limiting blood transport of N2O away from the lungs is merely the rate of blood flow through the pulmonary capillaries, a scenario termed "Perfusion-limitation".

Characteristics of Diffusion- and Perfusion-limited Gas Exchange
Diffusion- and perfusion-limited gas exchange are distinguished by the extent that an alveolar gass partial pressure will equilibrate across the alveolar membrane as blood flows through the pulmonary capillaries. Diffusion-limited gas exchange (left) is characterized by incomplete equilibration. In this scenario, the rate of gas diffusion across the alveolar membrane limits its transport away from the lung. In contrast, perfusion-limited gas exchange (right) is characterized by complete equilibration. In this scenario, the rate of gas transport from the lung can only be increased by increasing pulmonary blood flow (i.e. perfusion).