Blood carries nutrients to all the cells of the body where they are broken down further by precisely controlled processes to yield needed energy, ending up as the by-products water, carbon dioxide and heat. The collective term for these processes of converting chemical energy is metabolism, which can be compared to a slow self-regulated combustion. The comparison is quite aptsince metabolism, like combustion, needs a supply of oxygen, which it obtains via the lugs and the bloodstream. These metabolic processes liberate heat as well as mechanical energy, depending on what muscular activity is going on. Figure 6.1 shows these processes diagrammatically.