BIOLOGY Volume 2 - A Guide to General Biology - 2004

19. HOMEOSTASIS

19.3. Thermoregulation

19.3.6. Heat Loss

Although the primary mechanisms of heat gain differ between ectothermic and endothermic animals, the pathways of heat loss are identical in both cases: radiation, convection, conduction, and evaporation. In practice, the first three mechanisms can either warm or cool the body depending on the Temperature gradient (flowing from warmer to cooler), whereas evaporation results strictly in heat loss.

RADIATION. Radiant heat is dissipated in the form of electromagnetic waves within the long-wave infrared spectrum, which lies beyond the visible range. However, these waves can be captured on special infrared-sensitive film using dedicated equipment (Fig. 19.7). The rate of heat loss or gain is proportional to the temperature difference between the body and the environment. In humans, radiation accounts for approximately 50% of total heat loss, and it serves as the primary regulated pathway of heat dissipation in animals.

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Fig. 19.7. Infrared photograph of a woman with cold hands. The lighter the region on the image, the higher its temperature.

CONVECTION. In this process, heat is transferred between the Organism and the surrounding environment via convective (bulk) currents of air or Water, driven by pressure gradients. For instance, air or water in contact with The surface of an ectothermic animal becomes warmed and rises. The rate of heat loss through convection is directly proportional to the velocity of the surrounding fluid medium. This mode of heat dissipation can be reduced by insulative body coverings such as feathers, fur, or clothing.

CONDUCTION. In this case, heat is transferred through direct physical contact between bodies, such as between an organism and the ground or the air. The molecules of any object are in constant motion due to their kinetic energy, moving faster at higher temperatures; this energy is then propagated down its gradient. For most terrestrial species, conduction accounts for a minor fraction of total heat exchange, but for aquatic and subterranean animals, it can be quite significant. While conductive cooling through contact with solid objects on the ground is negligible in humans, direct heating of the surrounding air accounts for about 15% of our heat loss.

EVAPORATION. Heat is lost from the body surface during The conversion of water into vapor. The vaporization of 1 ml of water requires 2.45 kJ of energy. Evaporation is one of the key limiting factors governing the Geographical Distribution of both animals and plants. In humans, cutaneous heat loss via evaporation occurs continuously, even in the absence of overt sweating. Evaporation also takes place across the respiratory surfaces of the Lungs, with water vapor being expelled during exhalation. Although these losses cannot be entirely eliminated, they can be regulated to a certain extent by controlling perspiration and pulmonary ventilation.