科研之友
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摘要
Maximal aerobic metabolic rates (MMR) in vertebrates are supported by increased conductive and diffusive fluxes of O-2 from the environment to the mitochondria necessitating concomitant increases in CO2 efflux. A question that has received much attention has been which step, respiratory or cardiovascular, provides the principal rate limitation to gas flux at MMR? Limitation analyses have principally focused on O-2 fluxes, though the excess capacity of the lung for O-2 ventilation and diffusion remains unexplained except as a safety factor. Analyses of MMR normally rely upon allometry and temperature to define these factors, but cannot account for much of the variation and often have narrow phylogenetic breadth. The unique aspect of our comparative approach was to use an interclass meta-analysis to examine cardio-respiratory variables during the increase from resting metabolic rate to MMR among vertebrates from fish to mammals, independent of allometry and phylogeny. Common patterns at MMR indicate universal principles governing O-2 and CO2 transport in vertebrate cardiovascular and respiratory systems, despite the varied modes of activities (swimming, running, flying), different cardio-respiratory architecture, and vastly different rates of metabolism (endothermy vs. ectothermy). Our meta-analysis supports previous studies indicating a cardiovascular limit to maximal O-2 transport and also implicates a respiratory system limit to maximal CO2 efflux, especially in ectotherms. Thus, natural selection would operate on the respiratory system to enhance maximal CO2 excretion and the cardiovascular system to enhance maximal O-2 uptake. This provides a possible evolutionary explanation for the conundrum of why the respiratory system appears functionally over-designed from an O-2 perspective, a unique insight from previous work focused solely on O-2 fluxes. The results suggest a common gas transport blueprint, or Bauplan, in the vertebrate clade.
- 出版日期2013-2
