University of Utah, Salt Lake City, Utah
Physiology (Bethesda). 2015 Jul;30(4):260-72. doi: 10.1152/physiol.00056.2014.
Conventional wisdom holds that the avian respiratory system is unique because air flows in the same direction through most of the gas-exchange tubules during both phases of ventilation. However, recent studies showing that unidirectional airflow also exists in crocodilians and lizards raise questions about the true phylogenetic distribution of unidirectional airflow, the selective drivers of the trait, the date of origin, and the functional consequences of this phenomenon. These discoveries suggest unidirectional flow was present in the common diapsid ancestor and are inconsistent with the traditional paradigm that unidirectional flow is an adaptation for supporting high rates of gas exchange. Instead, these discoveries suggest it may serve functions such as decreasing the work of breathing, decreasing evaporative respiratory water loss, reducing rates of heat loss, and facilitating crypsis. The divergence in the design of the respiratory system between unidirectionally ventilated lungs and tidally ventilated lungs, such as those found in mammals, is very old, with a minimum date for the divergence in the Permian Period. From this foundation, the avian and mammalian lineages evolved very different respiratory systems. I suggest the difference in design is due to the same selective pressure, expanded aerobic capacity, acting under different environmental conditions. High levels of atmospheric oxygen of the Permian Period relaxed selection for a thin blood-gas barrier and may have resulted in the homogeneous, broncho-alveolar design, whereas the reduced oxygen of the Mesozoic selected for a heterogeneous lung with an extremely thin blood-gas barrier. These differences in lung design may explain the puzzling pattern of ecomorphological diversification of Mesozoic mammals: all were small animals that did not occupy niches requiring a great aerobic capacity. The broncho-alveolar lung and the hypoxia of the Mesozoic may have restricted these mammals from exploiting niches of large body size, where cursorial locomotion can be advantageous, as well as other niches requiring great aerobic capacities, such as those using flapping flight. Furthermore, hypoxia may have exerted positive selection for a parasagittal posture, the diaphragm, and reduced erythrocyte size, innovations that enabled increased rates of ventilation and more rapid rates of diffusion in the lung.
传统观点认为，鸟类呼吸系统是独特的，因为在通气的两个阶段，空气在大多数气体交换管中都沿着相同的方向流动。然而，最近的研究表明，单向气流也存在于鳄鱼和蜥蜴中，这就引发了关于单向气流真正的系统发生分布、该特征的选择驱动因素、起源日期以及这种现象的功能后果的问题。这些发现表明，单向流存在于共同的合弓类祖先中，与传统观点不一致，传统观点认为单向流是支持高气体交换率的适应。相反，这些发现表明，它可能具有减少呼吸功、减少蒸发呼吸失水、降低热量损失率和促进伪装等功能。在呼吸系统的设计上，单向通气肺和潮汐通气肺之间存在分歧，例如在哺乳动物中发现的那样，这种分歧非常古老，其分歧的最小日期在二叠纪。在此基础上，鸟类和哺乳动物谱系进化出了非常不同的呼吸系统。我认为，这种设计上的差异是由于相同的选择压力，即扩展的需氧能力，在不同的环境条件下作用的结果。二叠纪时期大气中高浓度的氧气放松了对薄的气血屏障的选择，可能导致了同质的、支气管肺泡的设计，而中生代的低氧选择了具有极薄气血屏障的异质肺。这些肺部设计的差异可能解释了中生代哺乳动物生态形态多样化的令人困惑的模式：所有的哺乳动物都是小型动物，它们不占据需要高需氧能力的生态位。支气管肺泡肺和中生代的缺氧可能限制了这些哺乳动物利用大型身体大小的生态位，在这些生态位中，奔跑运动可能是有利的，以及其他需要高需氧能力的生态位，如那些使用拍打飞行的生态位。此外，缺氧可能对矢状位姿势、横膈膜和红细胞体积的减少产生了积极的选择，这些创新使肺通气率增加，扩散速率更快。
