Proteins are also used as a source of energy, but their components, the amino acids,
are often too valuable to squander in this way, and are used to construct other
proteins instead. When proteins are oxidized (to urea, CO(NH2)2), the equivalent
enthalpy density is comparable to that of carbohydrates.
The heat released by the oxidation of foods needs to be discarded in order to maintain
body temperature within its typical range of 35.6–37.8°C. A variety of mechanisms
contribute to this aspect of homeostasis, the ability of an organism to counteract
environmental changes with physiological responses. The general uniformity of
temperature throughout the body is maintained largely by the flow of blood. When
heat needs to be dissipated rapidly, warm blood is allowed to flow through the
capillaries of the skin, so producing flushing. Radiation is one means of discarding
heat; another is evaporation and the energy demands of the enthalpy of vaporization
of water. Evaporation removes about 2.4 kJ per gram of water perspired. When vigorous
exercise promotes sweating (through the influence of heat selectors on the
hypothalamus), 1–2 dm3 of perspired water can be produced per hour, corresponding
to a heat loss of 2.4–5.0 MJ h−1.