Why write another small book on respiratory physiology? I have a dozen or so texts on my bookshelf that could already be used interchangeably to teach the subject. For profit, I might as well buy lottery tickets. Not that my publisher is ungenerous, you understand, it's just that the market is not that big and there are many contenders for a share. No, I write from the idealistic standpoint that I think I have something different to say, some­ thing that is importantly different about how gas exchange works and with an approach that is different from other authors. With few changes, basically the same text or chapters on respiratory physiology have been written, by different authors, for decades. One could almost interchange the tables of contents of most of them. Most seem to have copied the figures and concepts used by the others. Few have done more than accept and perpetu­ ate the conventional wisdom. In this text, I have attempted to start from fundamental principles of biology, chemistry, and physics and ask at each step, "Does it make sense?" The mechanisms and structures of gas exchange exist because, scientifically and logically, they "can't not be" as they are. The nature of our environment and the capabilities ofliving tissue are such that only certain opportunities have been available to the evolution of gas exchange.