Effects of mixing patterns in respiratory gas exchange

Consequences of the O2-CO2 dissociation curves for gas exchange in avian and mammalian lung at various altitudes

Gas exchange in avian and mammalian lung was studied with computer simulations using mutually dependent O2-CO2 dissociation curves. The mammalian lung was simulated with an ideal mixing unit. The model of the avian lung consisted of 25 ideal mixing units placed in series with respect to the ventilation but placed in parallel with respect to the perfusion (cross-current arrangement). The consequence of the right shift in the avian oxygen dissociation curve, if compared with the human dissociation curve, was studied by application of both dissociation curves to the avian lung model. The influence of a decreased tension of oxygen in the inspired air was studied under conditions of constant PvCO2 (48 mmHg), PvO2 (30 mmHg), RQ (0.70) and approximately constant levels of PaCO2. At sea-level the avian lung needs 80 per cent of the alveolar ventilation and 64 per cent of the cardiac output to transport the same amount of gas if compared with the mammalian lung. The difference in the need for cardiac output resulted from the right shift in the dissociation curve of avian blood. Changing PIO2 from 115 mmHg (2000 m) to 100 mmHg (3000 m) showed a dramatic difference in the performance of the two lung models. The mammalian lung model needed a more than eight fold increase in cardiac output whereas the avian lung model needed less than a three fold increase to maintain a RQ of 0.7. This difference could be ascribed to the Haldane effect in combination with the cross-current arrangement. The Haldane effect was also responsible for an end expired CO2 tension which exceeded the mixed venous CO2 tension.