Externally Heated Animals, Internally Heated Animals
An organism maintains homeostasis, a steady internal state, only if its body temperature stays within prescribed limits. Cellular activities require an optimum amount of heat. They depend on enzyme action, and enzymes function within a narrow range of temperature. For this reason, living things can only tolerate a limited rise or drop in temperature from the optimum. Mechanisms exist that regulate body temperature to stay within its survival limits.
Cells require a source of heat for their activities. The heat source can be external or internal. The Sun is an external source of heat for living things. Cellular metabolism is an internal heat source. During cellular respiration, the chemical energy in food is converted to high-energy phosphate groups in adenosine triphosphate, ATP. In the process, cells lose about 60% of the released energy as heat. Plants and most animals give up this heat to the environment. Birds, mammals, and some fish, however, make use of some of the heat that metabolism yields.
Organisms exchange heat with the environment by four major physical processes. First, there is conduction, the direct transfer of heat from a warmer to a cooler object. We cool off during a swim because the heat of the body is conducted to the cool water. During another process called convection, a breeze passes over a surface and brings about heat loss. This is why a fan cools us on a hot day. Radiation, the emission of electromagnetic waves, transfers heat between objects that are not in direct contact. Animals absorb heat that the sun radiates. Finally, evaporation, the physical change of liquid to gas removes heat from a surface. Sweating and panting are cooling processes. Perspiration evaporates from the skin, and panting increases evaporation from the respiratory system. Both procedures take heat away from the body.
- Temperature Regulation - Externally Heated Animals
- Temperature Regulation - Internally Heated Animals
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