Evidence for minimal oxygen heterogeneity in the healthy human pulmonary acinus

Abstract

It has been suggested that the human pulmonary acinus operates at submaximal efficiency at rest due to substantial spatial heterogeneity in the oxygen partial pressure (Po₂) in alveolar air within the acinus. Indirect measurements of alveolar air Po₂ could theoretically mask significant heterogeneity if intra-acinar perfusion is well matched to Po₂. To investigate the extent of intra-acinar heterogeneity, we developed a computational model with anatomically based structure and biophysically based equations for gas exchange. This model yields a quantitative prediction of the intra-acinar O₂ distribution that cannot be measured directly. Temporal and spatial variations in Po₂ in the intra-acinar air and blood are predicted with the model. The model, representative of a single average acinus, has an asymmetric multibranching respiratory airways geometry coupled to a symmetric branching conducting airways geometry. Advective and diffusive O₂ transport through the airways and gas exchange into the capillary blood are incorporated. The gas exchange component of the model includes diffusion across the alveolar air-blood membrane and O₂-hemoglobin binding. Contrary to previous modeling studies, simulations show that the acinus functions extremely effectively at rest, with only a small degree of intra-acinar Po₂ heterogeneity. All regions of the model acinus, including the peripheral generations, maintain a Po₂ >100 mmHg. Heterogeneity increases slightly when the acinus is stressed by exercise. However, even during exercise the acinus retains a reasonably homogeneous gas phase.