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How Your Body Loses Heat

Sweat originates deep in the secretory coil of the sweat gland and emerges at the skin surface to wet the skin. Sweat secretion over a given region of skin is dependent on both the density of sweat glands number per cm and the amount of sweat secreted per gland. In most people, the back and chest have the greatest sweating rates, while the arms and legs have relatively low sweating rates. Evaporation of sweat occurs when water changes from a liquid to a gas. For this to happen, heat is supplied by skin. As skin loses heat, it is cooled. The necessary thermal energy equals kilocalories abbreviated kcalthe amount of heat needed to raise the temperature of L of waterper liter of water. Therefore, evaporative cooling is most effective when the skin remains very wet, due to copious sweat production. High environmental temperatures and prolonged strenuous exercise may result in thermal sweating rates as high as L/h.

Sweat evaporation also is influenced by the amount of moisture in the air. Hot-dry air receives vaporized sweat readily. In contrast, hot-wet air receives little evaporated sweat because it is heavily laden with moisture. The meteorological measurement known as relative humidity rh provides an index of the amount of water in the air relative to a totally saturated volume of air rh. As the relative humidity of air climbs over -, the effect on heat dissipation becomes obvious as more heat is stored in the body and is sensed as a hot, red skin. In a very humid environment, therefore, the body relies increasingly on nonevaporative dry heat loss via increased skin blood flow. This explains why your skin becomes red and flushed when you exercise in hot-wet conditions.

Figuredepicts the relative contributions of evaporative and nonevaporative heat loss, during rest and exercise, in both hot-wet and hot-dry environments. The evaporative heat loss due to sweat is abbreviated as E, whereas the total nonevaporative dry heat dissipation is abbreviated as R C. This latter term refers to the combined effects of heat loss via radiation and convection. Radiation is the transfer of energy waves that are emitted by one object and absorbed by another. Solar energy from direct sunlight and radiant heat from the ground are examples. Convection is heat exchange that occurs between a solid medium and one that moves i.e. a fluid; this movement is known as a convective current. Air and body fluids are technically considered fluids for this purpose. Conduction is not included in the figure because it accounts for less than of heat loss in most situations. In convection and conduction, heat is transferred from a warm object to a cooler object.

The relative contributions of radiation R, convection C, and evaporation E to total heat dissipation. Rest and strenuous exercise are shown, in hot-dry and hot-wet environments.

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As figureshows, the body regulates its temperature differently in different environments and during different activities. In a hot-dry environment, evaporation accounts for – of all heat dissipation during exercise, as depicted in the lower right quadrant of this figure, and emphasizes the need for wet skin and lightweight, loose-fitting, porous clothing. However, in a hot-wet environment the evaporation of sweat from the skin surface is greatly diminished. The body must, as a result, rely more on radiative and convective heat loss. At rest, the ratio of E to R see top half of figureis similar in hot-wet and hot-dry environments. And, although it is not shown in this figure, R are major avenues of heat loss in a cool-dry environment, accounting for about of such loss.

Several other environmental factors are important in balancing body temperature. These include solar radiation, air speed, barometric pressure, and clothing insulation. However, few people recognize that R may not always act to remove heat from the body. In fact, the body gains heat from the environment when skin temperature is less than air temperature. If you realize that skin temperature usually ranges from to to F, it becomes obvious that R may add to heat storage when air temperature nearsF. In this case, the skin gains heat via R C. Unlike evaporation, R may either remove or add heat to the body, depending on the ambient temperature. This is important when both air temperature is high -C, -F and relative humidity is high – rh because heat loss via R and via evaporation are stifled, and the body stores most of the heat generated during exercise.

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