NORMAL RESTING of blood to service the rising energy requirements in children as they grow. This must be matched by similar improvements in cardiac output. Basic metabolic rate (BMR), related to the increasing body mass and relative decrease of surface area, becomes increasingly inversely related to body size. Resting BMR must be matched to cardiac output for healthy growth. The heart develops from two tubes lying side by side and it is the complex changes in this muscle pump as it develops that gives rise to the eventual double circulation in the heart: receiving blood from the body systems and re-oxygenating it in the lungs before delivering it back to the body tissues (see Figure 4.1).