Maina John N
Department of Zoology, University of Johannesburg, Kingsway, Johannesburg, South Africa.
Anat Rec (Hoboken). 2015 Oct;298(10):1673-88. doi: 10.1002/ar.23162. Epub 2015 Jul 21.
The blood capillaries (BC) and the air capillaries (ACs) are the terminal gas exchange units of the avian lung. The minuscule structures are astonishingly strong. It is only recently that the morphologies and the biomechanical properties of the BCs and the ACs were investigated. Regarding size and shape, the BCs and the ACs differ remarkably. While they were previously claimed to be tubular (cylindrical) in shape, the ACs are rather rotund structures which interconnect across short, narrow passageways. Atypical of those in other tissues, the BCs in the exchange tissue of the avian lung comprise of distinct segments which are about as long as they are wide and which are coupled in three-dimensions. The thin blood-gas barrier (BGB) which separates the ACs from the BCs is peculiarly strong. The causes of the strengths of the ACs and the BCs in general and the BGB in particular are varied and controversial. Here, the recent morphological and physiological findings on the structure, biomechanical properties, and the strengths of the respiratory units of the avian lung and the BGB have been critically examined. Also, in light of the new morphological findings of the ACs and the BCs, the functional model which is currently in use to assess the gas exchange efficiency of the avian lung should be revised and the inappropriateness of the terms 'blood capillary' and 'air capillary' for the gas exchange units of the avian lung is pointed out.
毛细血管（BC）和空气毛细血管（AC）是鸟类肺部的终末气体交换单位。这些微小的结构惊人地坚固。直到最近才对BC和AC的形态及生物力学特性进行了研究。在大小和形状方面，BC和AC有显著差异。虽然它们以前被认为是管状（圆柱形）的，但AC是相当圆润的结构，通过短而窄的通道相互连接。与其他组织中的毛细血管不同，鸟类肺部交换组织中的BC由长宽大致相等的不同节段组成，并在三维空间中相连。将AC与BC分隔开的薄气血屏障（BGB）特别坚固。一般来说，AC和BC以及特别是BGB坚固的原因是多样且有争议的。在此，对鸟类肺部呼吸单位和BGB的结构、生物力学特性及强度方面的最新形态学和生理学发现进行了批判性审视。此外，鉴于AC和BC的新形态学发现，应修订目前用于评估鸟类肺部气体交换效率的功能模型，并指出“毛细血管”和“空气毛细血管”这两个术语用于鸟类肺部气体交换单位的不恰当之处。
