Overview Diffusion-Limited and Perfusion-Limited Gas Exchange Describes gas exchange across alveolar-capillary barrier illustrated by partial pressure / time graph There are two main types Diffusion-Limited Gas Exchange applies to CO Perfusion-Limited Gas Exchange applies to N2O, CO2 Diffusion-Limited Gas Exchange (CO) Gas exchange across alveolar-capillary barrier is limited by diffusion process Net diffusion into pulmonary capillary depends on magnitude of partial pressure gradient Example: CO partial pressure of CO (PACO) in alveolar air is constant along length of capillary partial pressure of CO (PaCO) in capillary blood is zero at beginning of pulmonary capillary largest partial pressure gradient of CO and largest driving force for diffusion of CO from alveolar air into capillary blood at beginning of pulmonary capillary CO diffuses into capillary blood moving along length of pulmonary capillary PaCO rises only slightly along length of pulmonary capillary login to view 2 more bullets CO does not equilibrate by end of capillary regardless of the amount of blood flow partial pressure gradient of CO is maintained along entire length of capillary login to view 1 more bullet Perfusion-Limited Gas Exchange (N2O, CO2) Gas exchange across alveolar-capillary barrier is limited by blood flow through pulmonary capillaries (perfusion) e.g., N2O partial pressure of N2O (PAN2O) in alveolar air is constant along length of capillary partial pressure of N2O (PaN2O) in capillary blood is zero at beginning of pulmonary capillary largest partial pressure gradient of N2O and largest driving force for diffusion of N2O from alveolar air into capillary blood at beginning of pulmonary capillary N2O diffuses into capillary blood moving along length of pulmonary capillary PaN2O rises rapidly along length of pulmonary capillary login to view 1 more bullet N2O equilibrates early along length of capillary partial pressure gradient of N2O is not maintained along length of capillary login to view 1 more bullet only means for increasing net diffusion of N2O is by increasing blood flow through pulmonary capillaries (perfusion)