The circulatory system consists of a pump, the heart, and the attached plumbing of blood vessels. The dimensions of both pump and plumbing must be scaled to the demands, and it seems evident that in vertebrates the transport of gases is the most important consideration. If this demand is met, it appears that most or all other functional requirements on the circulatory system will be satisfied. This applies to the transport of nutrients, metabolic intermediates, excretory products, hormones, heat, and so on.

The circulatory system serves also in the transmission of force; blood is used as a hydraulic fluid to achieve, for example, ultrafiltration (in the kidney and in fluid exchange in the capillaries) and volume changes (e.g., erection of the penis). The needed force is supplied by the heart, as reflected in the blood pressure; we shall see that blood pressure appears to be a scale-independent physiological parameter.

The mammalian heart

The rate of oxygen consumption in mammals, relative to body size, decreases with increasing body size. It is therefore somewhat surprising to find that the relative size of the heart of small and large mammals is similar, that both mouse and elephant have hearts that are around 0.6% of their body mass. There is a great deal of information available on the size of the mammalian heart (e.g., Clark, 1927; Crile and Quiring, 1940; Grande and Taylor, 1965; Holt et al., 1968).