The metabolic rate of an animal is maintained through the steady consumption of fuel and oxygen. From the viewpoint of scaling, much attention has been given to the supply of oxygen, and relatively little to the supply of fuel. In this and the next two chapters we shall focus on the gas transport system, especially as it pertains to oxygen.

First we shall consider the effects of scale on gas-exchange organs: lungs and gills. These organs must have a size and diffusion capacity adequately scaled to the need for oxygen. In the next chapter, the role of blood in gas transport will be considered, and because information is available mostly for vertebrates, in particular for mammals, the discussion will be limited to these. Following that we shall examine the circulatory system, which consists of a pump (the heart) and conduits through which blood is pumped.

When we analyze the function of the respiratory system from the viewpoint of scale, we should keep in mind that most of the available information has been obtained for animals at rest. The normal animal, if not asleep, spends much of its time being active and moving about, and the supply system for oxygen must be scaled to meet these extra demands (except during brief periods of non-steady-state conditions, such as a burst of maximal activity).