Perfusion-limited O2 transport. In the lungs of a normal person at rest, O2 transfer from alveolar air into pulmonary capillary ­lood is perfusionlimited (although not to the extreme that N2O is perfusion-limited) (see Fig. 5-19A). PAO2 is constant at 10

In the context of O2 transport in the lungs, "perfusion-limited" refers to the mechanism by which the transfer of oxygen from the alveolar air into the pulmonary capillary blood is primarily dependent on the blood flow through the pulmonary capillaries. At rest in a normal individual, the partial pressure of oxygen in the alveoli (PAO2) is approximately 100 mm Hg. The mixed venous blood entering the pulmonary capillaries has a partial pressure of oxygen (PaO2) around 40 mm Hg. This creates a substantial partial pressure gradient, allowing for the diffusion of oxygen from the alveoli into the blood. The diffusion of O2 is facilitated by the fact that oxygen binds to hemoglobin in red blood cells, which keeps the free O2 concentration low in the plasma and helps maintain the partial pressure gradient throughout the initial portion of the capillary. Equilibration of O2 occurs roughly one-third of the way through the capillary, at which point the partial pressure of oxygen in the blood (PaO2) becomes equal to the partial pressure of oxygen in the alveolar air (PAO2). Once this equilibrium is reached, if there is no increase in blood flow, no additional net diffusion of oxygen will take place. Thus, O2 transport in this scenario is classified as perfusion-limited because the amount of O2 that can be transferred into the blood is contingent upon the flow of blood through the pulmonary capillaries. If the pulmonary blood flow increases, such as during exercise, the total amount of O2 that can be transported also increases. Conversely, if the blood flow decreases, the total amount of O2 transported decreases as well. In summary, the exchange of oxygen in the lungs under normal resting conditions is perfusion-limited, meaning that the rate and amount of oxygen transfer relies heavily on the rate of pulmonary blood flow.