Maina J N
Department of Anatomical Sciences, The University of the Witwatersrand, 7 York Road, Park Town 2193, Johannesburg, South Africa.
Comp Biochem Physiol A Mol Integr Physiol. 2000 Aug;126(4):491-515. doi: 10.1016/s1095-6433(00)00218-x.
The sheet-flow design is ubiquitous in the respiratory microvascular systems of the modern gas exchangers. The blood percolates through a maze of narrow microvascular channels spreading out into a thin film, a "sheet". The design has been convergently conceived through remarkably different evolutionary strategies. Endothelial cells, e.g. connect parallel epithelial cells in the fish gills and reptilian lungs; epithelial cells divide the gill filaments in the crustacean gills, the amphibian lungs, and vascular channels on the lung of pneumonate gastropods; connective tissue elements weave between the blood capillaries of the mammalian lungs; and in birds, the blood capillaries attach directly and in some areas connect by short extensions of the epithelial cells. In the gills, skin, and most lungs, the blood in the capillary meshwork geometrically lies parallel to the respiratory surface. In the avian lung, where the blood capillaries anastomose intensely and interdigitate closely with the air capillaries, the blood occasions a 'volume' rather than a 'sheet.' The sheet-flow design and the intrinsic fractal properties of the respiratory microvascular systems have produced a highly tractable low-pressure low-resistance region that facilitates optimal perfusion. In complex animals, the sheet-flow design is a prescriptive evolutionary construction for efficient gas exchange by diffusion. The design facilitates the internal and external respiratory media to be exposed to each other over an extensive surface area across a thin tissue barrier. This comprehensive design is a classic paradigm of evolutionary convergence motivated by common enterprise to develop corresponding functionally efficient structures. With appropriate corrections for any relevant intertaxa differences, use of similar morphofunctional models in determining the diffusing capacities of various gas exchangers is warranted.
片流设计在现代气体交换器的呼吸微血管系统中无处不在。血液通过错综复杂的狭窄微血管通道渗透，扩散成一层薄膜，即“片层”。这种设计是通过截然不同的进化策略趋同形成的。例如，内皮细胞连接鱼类鳃和爬行动物肺中的平行上皮细胞；上皮细胞分隔甲壳类动物鳃、两栖动物肺以及肺螺类动物肺上的血管通道中的鳃丝；结缔组织成分交织在哺乳动物肺的毛细血管之间；在鸟类中，毛细血管直接附着，在某些区域通过上皮细胞的短延伸相连。在鳃、皮肤和大多数肺中，毛细血管网中的血液在几何形状上与呼吸表面平行。在鸟类肺中，毛细血管密集吻合并与气毛细血管紧密交错，血液形成的是“体积”而非“片层”。呼吸微血管系统的片流设计和内在分形特性产生了一个高度易处理的低压低阻力区域，有利于实现最佳灌注。在复杂动物中，片流设计是通过扩散进行高效气体交换的一种规定性进化结构。这种设计便于内部和外部呼吸介质通过薄组织屏障在广阔的表面积上相互接触。这种全面的设计是由共同目标驱动以发展相应功能高效结构的进化趋同的经典范例。在对任何相关类群间差异进行适当校正后，使用相似的形态功能模型来确定各种气体交换器的扩散能力是合理的。
