Rencmal of the gaseous medium in contact with respiratory epithelium of air brcat hing animals in\rolvcs two basic physical considerations : namely (1) mechanisms to induce volume flow of air in and out of the respiratory tract, and (2) rneolianisms to regulate molccixlar diffusion of the gaseous components of air a t the respiratory epithelium. Development of the alveolar lung in vcrteln-ates has been recognized as important not only for the large surface area iiivolved, but also for the effective niechani sm provided to maintain relatively eonstant physical conditions for diffusion in the presence of gross cyclic flow patterns in the breathing exchange. The effectiveness of PO, a s a respirator>-regulator, f o r example, appears
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