Department of Biology, Portland State University, Portland, OR 97207, USA.
J Comp Physiol B. 2013 Feb;183(2):167-79. doi: 10.1007/s00360-012-0688-1. Epub 2012 Jul 10.
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 CO(2) 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 CO(2) 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 CO(2) efflux, especially in ectotherms. Thus, natural selection would operate on the respiratory system to enhance maximal CO(2) 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.
脊椎动物的最大有氧代谢率（MMR）是通过增加环境到线粒体的 O(2) 的传导和扩散通量来支持的，这需要伴随着 CO(2) 排放的增加。一个备受关注的问题是，呼吸还是心血管，为 MMR 时的气体通量提供主要的速率限制？限制分析主要集中在 O(2)通量上，尽管肺对 O(2)通气和扩散的过剩能力除了作为安全因素外，仍然无法解释。MMR 的分析通常依赖于比例和温度来定义这些因素，但不能解释大部分变异，而且往往具有狭窄的系统发育范围。我们比较方法的独特之处在于使用跨类荟萃分析来检查从休息代谢率增加到脊椎动物从鱼类到哺乳动物的 MMR 期间的心肺变量，独立于比例和系统发育。MMR 上的共同模式表明，尽管活动模式（游泳、跑步、飞行）不同，心肺结构不同，代谢率（恒温与变温）差异很大，但控制脊椎动物心血管和呼吸系统中 O(2)和 CO(2)运输的普遍原则。我们的荟萃分析支持先前的研究表明，最大 O(2)运输存在心血管限制，也表明最大 CO(2)排放存在呼吸系统限制，尤其是在变温动物中。因此，自然选择将作用于呼吸系统以增强最大 CO(2)排泄，作用于心血管系统以增强最大 O(2)摄取。这为呼吸系统从 O(2)的角度来看似乎功能上过度设计的难题提供了一个可能的进化解释，这是以前仅专注于 O(2)通量的工作的独特见解。结果表明，在脊椎动物分支中存在共同的气体运输蓝图或 Bauplan。
