Evaporation, of sweat

Heat dissipation can also be achieved by increased production of sweat, because evaporation of sweat on the skin surface consumes heat (evaporative heat loss). Shivering is a mechanism to generate heat. Autonomic neural regulation of cutaneous blood flow and sweat production permit homeostatic control of body temperature (A). 

Endothermic processes are common in our everyday experience. Examples include the melting of ice cubes in a glass of water, or the evaporation of sweat from our skin. In these cases, the greater entropy of the products favors the utilization of energy to allow these processes to occur (Case III). A dramatic example is a chemical cooling pack that contains a salt (usually ammonium nitrate,... 

In a sedentary state body temperature can be regulated by varying the amount of clothing worn or by adjusting the surrounding temperature. However, these mechanisms are inadequate when strenuous work is undertaken and temperature regulation must then be achieved by the evaporation of sweat from the surface of the skin. 

As the effects of exercise on the body are examined, clearly the primary effects are on body fluids and fuels. Movement of the body requires additional fuels and the process of conversion of these fuels into energy produces heat which must be dissipated, in large part, by evaporation of sweat from the skin. 

At low cirabient temperatures a greater portion of the metabolic heat production (depending upon exercise intensity and clothing) is dissipated by convection and radiation and a minor portion by evaporation of sweat and respiratory water. As ambient temperature rises, the portion of heat dissipated by convection and radiation decreases progressively in concert with a proportional increase in the rate of sweating and evaporative heat loss. The coordination of the rate of heat loss between conduction, radiation, and evaporation is so precise that, for ambient dry-bulb temperatures between 5 C and 29 C, the equilibrium level of core (rectal) temperature is related directly to the intensity of the exercise load and is independent of environmental temperature (25). 

Evaporative or latent heat loss from the skin is proportional to the difference between the water vapor pressure at the skin and the ambient air, and the skin wettedness, which is a measure of the amount of moisture on the skin. It is duo to Iho combined effects of the evaporation of sweat and the diffusion of water through the skin, and can be expressed a.s... 

The high specific heat of water enables it to act as a heat buffer, thus minimizing the effects on the cells of fluctuations in environmental temperature. The high latent heat of vaporization permits humans to use the evaporation of sweat as a cooling mechanism. 

When work is performed in heavy clothing, evaporation of sweat from the skin to the environment is limited by layers of wet clothing and air. The magnitude of decrement in evaporative cooling is a function of the clothing s resistance to permeation of water vapour. 

Under normal physiological conditions, the following heat transfer terms should be incorporated in a lumped or distributed parameter model metabolic heat generation conduction and convection within the body heat exchange with the environment by radiation, conduction and convection heat loss from the skin by evaporation of sweat and water diffused across the skin and respiratory heat loss. Expressions for each of these terms, along with the parameter values, are given elsewhere (Cooney, 1976). 

The evaporation of sweat from skin can be used to cool the body. 

In hot, humid conditions, the vapor pressures in air and at the skin surface are nearly the same. Such conditions limit cooling through evaporation of sweat. In hot, dry environments the difference in vapor pressures is large and evaporation is rapid. In hot, dry conditions the limiting factor may be the maximum rate of sweat production. 

Not only do the 3D woven fabrics provide improved balUstic performance in their soft form, but also the 3D honeycomb weave can form a type of device to be used for body armour ventilation and evaporation of sweat and therefore improve comfort. 

K - heat lost or gained by conduction that is, skin contact with a hot or cold surface S - the heat stored in the body if any, due to inability to lose sufficient heat through radiation, convection or evaporation E - heat lost from the body due to evaporation of sweat. 

Undesirable heat strain results from a combination of factors - clothing, activity levels, age, health and fitness status. The body can pick up or lose heat from conduction (very little) convection (movement of air over the body) or radiation. It can also lose heat by evaporation of sweat however, it is not desirable for even a normal fit heaWy young man to sweat more than 1 litre/hour over 8 hours (women s tolerances have not been as well studied). Also, in conditions where heat retention is at the rate of 73 W for an hour, the body core temperature will rise approximately 1.2°C. A rise above this can lead to heat syncope (fainting) heat exhaustion anhidrosis (sweating stops) and heat stroke. Figures 8.5 and 8.6 show the full range of effects. 

In hot conditiotts, evaporation of sweat is a rrrajor factor. This relies on what is called latent heat . This is the heat energy needed to break down the attraction of water molecules in liquid water to form a vapoirr with almost no attraction between molecrrles. This heat energy does not raise the temperature of the water. In fact, as airflow over the sweat strips off water molecules from the sirrface, this heat energy (latent heat of evaporation) can only be obtained by drawing heat from the skin. This cools the body. 

Secretion is effected in the deep parts called glomeruli. The secretory cells are cuboidal and are surrounded by a layer of myoepithelial cells which have contractile properties. The majority of the sweat glands are of the eccrine type. The sweat they produce is slightly acidic and is mostly composed of water. The evaporation of sweat from the skin allows a decrease in body heat. 

Heat stress and illness is a major concern when personnel are working in chemical protective clothing. The body s principal means of cooling is through the evaporation of sweat When personnel are working in chemical protective clothing, sweat is trapped inside the clothing and can not evaporate. This will raise the body s core temperature and can result in heat related illness. 

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