What is the primary source of heat in the human body?

• A. Metabolism
• B. Conduction
• C. Convection
• D. Evaporation

Answer: (A)

Heat primarily comes from metabolism—the energy released during cellular respiration when nutrients are converted to usable energy is largely dissipated as heat. Even at rest, the body’s basal metabolic rate generates a steady amount of heat to keep core temperature stable. When the environment is cold, muscles can rapidly contract (shiver), dramatically increasing ATP turnover and producing more heat. There is also some contribution from brown fat, especially in infants, to generate heat without shivering. The other mechanisms—conduction, convection, evaporation (and radiation as a heat-loss mode)—describe how heat leaves the body, not where it originates. So metabolism is the main source of heat production in the body.