The body masses of extant mammals span over seven orders of magnitude. Within that size range there is extraordinary diversity of function, phylogenetic diversity that understandably presents fertile ground for uncovering biological insights. Remarkably transcending that diversity, are patterns that reveal body size-dependent constraints of “form and function”, patterns that become visible only through comparison. Thus, “Comparative Physiology” provides an additional tool for discovery of additional biological insights that may be otherwise hidden. Among these are the linear (isometric) scaling of volumes and the disproportionate (allometric) scaling of biological times and rates. When the diffusion of oxygen through the lungs and tissues is re-examined through this lens it is apparent that body size alone has profound impacts. The smallest mammals have no apparent “structural reserve;” oxygen diffusion in both the lungs and tissues is apparently functioning at full capacity. Because small body size is the ancestral state, it may be an evolutionary consequence of increased body size that large mammals seem to have “excess capacity” for oxygen diffusion in both the lungs and tissues. There is scant evolutionary evidence that physiological variables pivot around “normal” values of humans.

现存哺乳动物的体重跨越七个数量级。在此大小范围内，功能异常丰富，系统发育的多样性可以理解为揭示生物学见解提供了沃土。显着超越多样性的是那些揭示了身体大小相关的“形式和功能”约束的模式，这些模式只有通过比较才能看到。因此，“比较生理学”为发现可能被隐藏的其他生物学见解提供了额外的工具。其中包括体积的线性（等距）标度和生物学时间和速率的不成比例（异轴）标度。当通过该镜片重新检查氧在肺和组织中的扩散时，很明显仅身体尺寸会产生深远的影响。最小的哺乳动物没有明显的“结构储备”；氧在肺和组织中的扩散显然以最大容量起作用。因为小体型是祖先状态，所以可能是大体型哺乳动物的进化结果，即大型哺乳动物似乎对肺和组织中的氧气扩散具有“过量的能力”。很少有进化证据表明生理变量围绕人类的“正常”价值而变化。