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The term endothermic has two distinct meanings, depending on the context in which it is used. In chemistry, endothermic means that a chemical reaction or phase transition absorbs heat. (A phase transition is the transformation of matter from a gas, liquid, or solid into a different one of these states.) In physiology, the term endothermic refers to organisms which metabolically generate heat to maintain their body temperature.

Typically, the term endothermic describes a chemical reaction or phase transition that absorbs heat from the environment, in order to bring the reaction back to the temperature at which the reaction began. Heat is a form of energy, like kinetic energy and radiation. The units in which heat is expressed are known as joules or calories. The heat consumed or released in a chemical reaction is known as the enthalpy change, if measured at a constant pressure, or the energy change, if measured at a constant volume. The heat consumed may not be the same under these different conditions, because energy may be transferred to or from the reaction in the form of work, if it occurs at a constant pressure. A positive enthalpy or energy change indicates that a reaction is endothermic. The evaporation of water is a familiar, endothermic phase transition. When water moves from a liquid phase to a gaseous phase, it absorbs heat; that is why drying objects usually feel cool when touched.

Physiologists use the term endothermic when referring to organisms classified as endotherms, such as mammals or birds. Endothermic organisms maintain their body temperature above that of their surroundings by maintaining a high metabolic rate. (Metabolism refers to the material and energy transformations within an organism.) These transformations are highly exothermic of heat releasing, causing the endotherm's body temperature to remain high. The fact that endothermic organisms derive heat from exothermic chemical reactions can make this term confusing when used in physiology.

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