Royal Institution Christmas Lectures

You as an Engine (1975x1)

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The body can be regarded as a rather special internal combustion engine, insofar as its ability to perform mechanical work is concerned. Some of this work is expended to keep it alive, for instance by pumping the blood round and breathing, whilst the rest appears as external activity. Walking, running and lifting sacks of coal all involve the expenditure of energy which ultimately has to be derived from the fuel, i.e. the food supplied to the body. Just as in a car engine, not all the energy contained in food can be converted into mechanical work; at best only about 20% appears as work, the rest being converted into heat.