ABSTRACT
All living things require energy to sustain life. If a plant, that energy comes from the sun. If an animal, the energy must be provided by the food that is consumed. Warm-blooded animals are kept warm by their metabolisms. All the reactions that comprise intermediary metabolism release heat as a byproduct. Some reactions are exogonic; that is, they release more heat than they consume. Others are endogonic; they consume more energy than they release. None of the reactions in a living system are 100% efficient. The reaction produces a product(s) plus heat. This is the heat that sustains the body temperature and yet also escapes from the body via radiation or evaporation (insensible water loss and sweating) from the body surface. If a body is neither gaining nor losing weight, the energy released as heat is equal to that needed by the body to sustain its metabolism. Thus, the heat that is produced is equal to the total food energy (corrected for digestive loss and internal energy conversion to mechanical, chemical, and electrical energy) that must be provided on a daily basis to sustain the body. Altogether, the living system is a heatgenerating system. That heat can be measured directly using a calorimeter or indirectly by measuring the oxygen consumed and the carbon dioxide produced. Using equations that relate heat production to the gas exchange (CO
and O
), the energy used by the body can be calculated.
