Metabolic Rate

Metabolic Rate

The metabolism of the body simply means all the chemical reactions in all the cells of the body, and the meta-bolic rate is normally expressed in terms of the rate ofheat liberation during chemical reactions.

Heat Is the End Product of Almost All the Energy Released in the Body. In discussing many of the metabolic reactions in the preceding, we noted that not all the energy in foods is transferred to ATP; instead, a large portion of this energy becomes heat. On average, 35 per cent of the energy in foods becomes heat during ATP forma-tion. Then, still more energy becomes heat as it is trans-ferred from ATP to the functional systems of the cells, so that even under optimal conditions, no more than 27 per cent of all the energy from food is finally used by the functional systems.

Even when 27 per cent of the energy reaches the func-tional systems of the cells, most of this eventually becomes heat. For example, when proteins are syn-thesized, large portions of ATP are used to form the peptide linkages, and this stores energy in these link-ages. But there is also continuous turnover of proteins— some being degraded while others are being formed. When proteins are degraded, the energy stored in the peptide linkages is released in the form of heat into the body.

Another example is the energy used for muscle activ-ity. Much of this energy simply overcomes the viscosity of the muscles themselves or of the tissues so that the limbs can move. This viscous movement causes friction within the tissues, which generates heat.

Consider also the energy expended by the heart in pumping blood. The blood distends the arterial system, and this distention itself represents a reservoir of poten-tial energy. As the blood flows through the peripheral vessels, the friction of the different layers of blood flowing over one another and the friction of the blood against the walls of the vessels turn all this energy into heat.

Essentially all the energy expended by the body is eventually converted into heat. The only significant exception occurs when the muscles are used to perform some form of work outside the body. For instance, when the muscles elevate an object to a height or propel the body up steps, a type of potential energy is created by raising a mass against gravity. But when external expen-diture of energy is not taking place, all the energy released by the metabolic processes eventually becomes body heat.

The Calorie. To discuss the metabolic rate of the bodyand related subjects intelligently, it is necessary to use some unit for expressing the quantity of energy released from the different foods or expended by the different functional processes of the body. Most often, the Calorie is the unit used for this purpose. It will be recalled that 1 calorie—spelled with a small “c” and often called a gram calorie—is the quantity of heat required to raisethe temperature of 1 gram of water 1°C. The calorie is much too small a unit when referring to energy in the body. Consequently, the Calorie—sometimes spelled with a capital “C” and often called a kilocalorie, which is equivalent to 1000 calories—is the unit ordinarily used in discussing energy metabolism.