West John B
Department of Medicine, University of California San Diego, La Jolla, California 92093-0623, USA.
Am J Physiol Regul Integr Comp Physiol. 2009 Dec;297(6):R1625-34. doi: 10.1152/ajpregu.00459.2009. Epub 2009 Sep 30.
Two opposing selective pressures have shaped the evolution of the structure of the blood-gas barrier in air breathing vertebrates. The first pressure, which has been recognized for 100 years, is to facilitate diffusive gas exchange. This requires the barrier to be extremely thin and have a large area. The second pressure, which has only recently been appreciated, is to maintain the mechanical integrity of the barrier in the face of its extreme thinness. The most important tensile stress comes from the pressure within the pulmonary capillaries, which results in a hoop stress. The strength of the barrier can be attributed to the type IV collagen in the extracellular matrix. In addition, the stress is minimized in mammals and birds by complete separation of the pulmonary and systemic circulations. Remarkably, the avian barrier is about 2.5 times thinner than that in mammals and also is much more uniform in thickness. These advantages for gas exchange come about because the avian pulmonary capillaries are unique among air breathers in being mechanically supported externally in addition to the strength that comes from the structure of their walls. This external support comes from epithelial plates that are part of the air capillaries, and the support is available because the terminal air spaces in the avian lung are extremely small due to the flow-through nature of ventilation in contrast to the reciprocating pattern in mammals.
两种相反的选择压力塑造了空气呼吸脊椎动物血气屏障结构的进化。第一种压力已被认识了100年，即促进扩散性气体交换。这要求屏障极其薄且面积大。第二种压力直到最近才被认识到，即在面对屏障的极端薄度时维持其机械完整性。最重要的拉伸应力来自肺毛细血管内的压力，这会导致环向应力。屏障的强度可归因于细胞外基质中的IV型胶原蛋白。此外，通过肺循环和体循环的完全分离，哺乳动物和鸟类中的应力得以最小化。值得注意的是，鸟类的屏障比哺乳动物的薄约2.5倍，而且厚度也更加均匀。气体交换的这些优势之所以出现，是因为鸟类的肺毛细血管在空气呼吸动物中独一无二，除了来自其管壁结构的强度外，还在外部得到机械支撑。这种外部支撑来自作为空气毛细血管一部分的上皮板，并且由于鸟类肺部的终末气腔由于通气的流通性质而极小，与哺乳动物的往复模式形成对比，所以这种支撑是可行的。
