Department of Surgery, Center for Organogenesis and Trauma, University of Texas Southwestern, Dallas, TX 75390, USA.
Louisiana Department of Wildlife and Fisheries, Rockefeller Wildlife Refuge, Grand Chenier, LA 70643, USA.
J Exp Biol. 2024 Sep 15;227(18). doi: 10.1242/jeb.248139. Epub 2024 Sep 25.
Vertebrates utilize various respiratory organs such as gills, lungs and skin in combination with diverse cardiovascular structures, including single-, three- and four-chambered hearts, to enable oxygen delivery and carbon dioxide removal. They also exhibit differences in aerobic and anaerobic metabolism during exertion, but the cardiorespiratory gas transport of all vertebrates is a four-step process governed by Fick's Principle and Fick's Law of Diffusion over the entire range of metabolic rates. Hillman et al. (2013) suggested that previous exercise studies have focused too narrowly on mammals and proposed that the cardiorespiratory system's excess capacity serves an evolutionary role in enhancing CO2 excretion in non-mammalian vertebrates. In contrast, an analysis by Hicks and Wang (2021) concluded that vertebrates maintain effective gas exchange even at peak activity, finding no evidence of arterial hypercapnia at maximal oxygen consumption and thus challenging the proposal of significant limitations to pulmonary or branchial CO2 efflux. In the present study, we investigated the limits for CO2 exchange in exercising American alligators (Alligator mississippiensis) and provide evidence that the cardiorespiratory system is adequately built to sustain CO2 excretion during strenuous exercise and maintain arterial PCO2, with no evidence of diffusion limitation for pulmonary CO2 excretion.
脊椎动物利用各种呼吸器官，如鳃、肺和皮肤，并结合各种心血管结构，包括单、三、四心室心脏，以实现氧气输送和二氧化碳排出。它们在运动时也表现出有氧和无氧代谢的差异，但所有脊椎动物的心肺气体传输都是一个四步过程，受菲克原理和扩散定律控制，适用于整个代谢率范围。Hillman 等人（2013 年）认为，先前的运动研究过于狭隘地关注哺乳动物，并提出心肺系统的过剩能力在增强非哺乳动物脊椎动物的二氧化碳排泄方面具有进化作用。相比之下，Hicks 和 Wang（2021 年）的分析得出的结论是，脊椎动物即使在活动高峰期也能保持有效的气体交换，在最大耗氧量时没有发现动脉高碳酸血症的证据，从而对肺或鳃二氧化碳流出的显著限制提出了挑战。在本研究中，我们研究了运动中的美洲鳄（Alligator mississippiensis）的二氧化碳交换极限，并提供证据表明，心肺系统足以在剧烈运动中维持二氧化碳的排泄，并维持动脉 PCO2，没有证据表明肺内二氧化碳排泄存在扩散限制。
