Stratification of inspired air in the elongated lungs of the carpet python, Morelia spilotes variegata

Using lung gas tensions via a triple lumen catheter to monitor ventilation distribution (VA) and radioactive techniques to study blood flow distribution (Q), the distribution of ventilation to perfusion ration (VA/Q) was studied in the elongated alveolar lung of the Carpet Python, Morelia spilotes variegata. In the resting, sleeping and agitated states both alveolar oxygen (PAO2) and carbon dioxide tensions (PACO2) were 'stratified' (unevenly distributed) within the alveolar lungs at end inspiration, during breath holding for up to 6 minutes and, when VA was low, at end expiration. The blood flow was also stratified. The degree of stratification of VA was influenced by the rate and depth of breathing and the length of the breath hold which preceeded the gas sampling. Similar results were obtained with a glass lung model. In both resting and sleeping states VA/Q ratios were similar over the proximal 75% of the alveolar lungs whereas VA nearly always exceeded Q over the distal 25%. The anatomic features of the lung are proposed as a possible mechanism for maintaining a uniform VA/Q distribution. Since the anatomical arrangement places the heart at the apical regions of the lungs, absence of cardiac mixing, combined with low respiratory rates, enables stratification to continue for very long periods within the aveolar lungs of the snake.