Human beings, along with other mammals, birds, fishes, and a few other species, differ from most simpler life forms by being able to maintain their body temperature above that of their environment. This requires energy. Energy is the ability to do work. For human beings, this means both physical and chemical work. We do physical work when we move our bodies, but even when we are quiet, we are still expending energy for processes such as pumping blood through our arteries, making urine in the kidneys, operating our brains, digesting and absorbing food, and maintaining the important differences between the inside and outside of cells.

The processes of energy metabolism in humans and other animals are sometimes referred to as thermogenesis. The process of thermogenesis typically ...

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