Optimization of the mammalian respiratory system: symmorphosis versus single species adaptation.

The study delves into the adaptation and optimization of the mammalian respiratory system, specifically focusing on the principle of symmorphosis and its possible application to the Thoroughbred racehorse. It uses a mathematical model to examine the extent to which different components of the equine oxygen transport system influence oxygen flux.

The Principle of Symmorphosis

• Symmorphosis is a principle coined by Taylor and Weibel, suggesting that mammalian respiratory systems are optimized with no additional structure relative to the maximum oxygen flow (VO2max).
• Despite not being a universal design principle, the researchers propose that it might apply to highly adapted aerobic animals, such as Thoroughbred racehorses.

The Equine Oxygen Transport System

• The investigators use a mathematical model derived from empirical data of the equine oxygen transport system to unveil the contribution of each respiratory transport step to the total limitation to oxygen flux at normoxic VO2max.
• These steps were assessed either as the fractional change (F) in VO2max for a 1% change in each component or the fraction of total oxygen pressure drop (R(int)) across each component at VO2max.

Findings of the Study

• The study found that, when calculated as F, alveolar ventilation (VA) and pulmonary diffusing capacity (DLO2), are significant limiting factors.
• Circulatory convection (Q) was nearly as limiting, while the peripheral tissue diffusing capacity (DTO2) was only one-third as important.
• Conversely, calculated as R(int), DLO2 was the major factor, VA and DTO2 contributed significantly, and Q had the smallest impact.
• The results were compared to similar studies in humans where Q was the single major limiting factor, pointing out iter-species differences in limiting factors of the respiratory system.

Implications on Aerobic Power in Horses

• The findings suggest that the strong selection for aerobic power in horses has caused an optimization of their respiratory systems’ malleable components until the least malleable structure, the lungs, has become a major limitation to oxygen flux.
• However, symmorphosis cannot definitively determine if such a design is optimized, because each system falls within a continuous distribution of relative optimization among species.
• Nevertheless, the symmorphosis concept is useful for establishing a comparison framework for a single species against a quantitatively defined hypothesis of optimal animal design and comparing it with other species based on the same criteria.