Sweat, evaporation

We feel cooler when sweating because the skin loses energy by transferring it to the water on its surface, which then evaporates. This process of water evaporation (sweating) is endothermic because energy passes from the skin to the water, and a body containing less energy has a lower temperature, which is why we feel cooler. 

Figure 4.2 As the perspiration on this athlete evaporates, the athlete will feel cooler because evaporation is an endothermic reaction. Evaporating sweat absorbs heat from her body.

Bakkevig and Nielsen [35] repeated the protocol above, but used low and high work rates with three kinds of underwear (a polypropylene lxl knit, a wool lxl knit, and a fishnet polypropylene) worn under wool fleece covered by polyester/cotton outer garments. Total sweat production and evaporated sweat were the same for all three underwear fabrics, but where the sweat accumulated differed significantly. More sweat accumulated in the wool underwear than either polypropylene at both work rates. At the higher work rate, more sweat moved into the fleece layer from both kinds of polypropylene underwear than for the wool. Most likely for the lxl knits, the thicker wool underwear(1.95 mm) simply holds more water than the polypropylene underwear (1.41 mm) and based on outer layer-to layer wieking results, needs a greater volume of sweat to fill it pores before it starts to donate the excess to the layer above it. 

Sweating begins. Different areas of the body begin sweating earlier than others, but soon the whole body is involved. If sweat evaporation occurs on the skin surface, then the full cooling power of evaporating sweat (670 W-h/kg) is felt. If the sweat is absorbed by clothing, then the full benefit of sweat evaporation is not realized at the skin. If the sweat falls from the skin, no benefit accrues. 

Climate is most stressful in hot weather when there are high ambient temperamres, absence of wind, high relative humidity, and solar radiation. High relative humidity makes it difficult to evaporate sweat from the skin and thus accounts for our main heat-loss mechanism in the heat. Therefore, a jungle climate is generally more stressful than a desert climate. Wind enhances heat loss by convection. Solar radiation is converted to heat in the skin. Maximal values of solar heat are about 1000 W/m around the equator. Minimizing the exposed surface area, for instance, by adopting the posture, can effectively reduce heat strain. 

There are considerable interindividual differences in heat balance. Important is the ability to evaporate sweat. Generally, females sweat less than males, but they compensate for this partly with a more favorable body surface to body mass ratio. 

Heat acclimation is partly based on the enhanced capacity to evaporate sweat and thus is of limited advantage when wearing protective garments. Fencers, for instance, have a great barrier for sweat evaporation. 

In cold conditions, and especially at subzero temperatures, the water vapour produced by the body (or the evaporated sweat) may condense within the clothing. This is due to the fact that air can cmitain less humidity at lower temperatures. The maximum amount of water vapour contained in air is determined by the saturation pressure, which is highly temperature dependent, as shown in the following equation ... 

Evaporating sweat cools the body because evaporation is an endothermic process ... 

The skin receives heat from the core by passive conduction and active skin blood flow (Table 5.3). It transfers this heat to the surroundings by convection, radiation, and evaporative (perspiration and diffusion) mechanisms. All of these mechanisms are unregulated or passive except evaporation from sweating. The sweating process is actively controlled by the humarrs thermoregulatory center where the rate of sweat secretion is proportional to eleva tions in core and skin temperature from respective set point temperatures (Table 5.3). 

Sweating, the other powerful heat loss mechanism actively regulated by the thermoregulatory center, is most developed in humans. With about 2,6 million sweat glands distributed over the skin and neurally controlled, sweat secretion can vary from 0 to 1 I7(h m ). The other, lesser, passive evaporative process of the skin is from the diffusion of water. The primary resistance to this flow is the stratum corneum or outermost 15 pm of the skin. The diffusion resistance of the skin is high in comparison to that of clothing and the boundary layer resistance and as a result makes water loss by diffusion fairly stable at about 500 grams/day. 

The value used in ISO 7933, required sweat rate, SW is based on the heat balance equation (6.1). Assuming the heat storage is equal to 0, the necessary evaporation from the skin, > to ensure a heat balance is calculated as foil... 

P4SR Predicted 4-hour sweat rate. A scale used to predict the evaporation rate from a body under hot conditions. 

Adaptations to Warm Habitats. When water evaporates into the surroundings, the vaporized molecules cany a great deal of heat away with them. One of the best ways to cool an animal s body is to evaporate water from its surface. Adaptations that take advantage of this property include sweating, panting, and licking the body. But water often is a limited resource in warm habitats such as deserts, so many desert animals have adaptations that reduce the amount of water that evaporates from the body. Most... 

This type of cake is often stored for a while after baking because it is to be covered with royal icing. The cake not only needs to cool but the surface needs to equilibrate with the bulk. There is a paradox that if the cake is just wrapped in greaseproof paper and kept in a tin that is not air tight there will be no problem. However, if the cake is wrapped in aluminium foil and kept in an air tight tin there is a possibility of mould formation because with the better moisture barrier any condensation or sweating can not evaporate. 

Producing sweat is one of the body s natural ways of cooling itself, and it operates as follows. Sweat is an aqueous solution of salt and natural oils, and is secreted by glands just below the surface of the skin. The glands generate this mixture whenever the body feels too hot. Every time air moves over a sweaty limb, from a mechanical fan or natural breeze, the skin feels cooler following evaporation of water from the sweat. 

We need the salt in sweat to decrease the water s surface tension in order to speed up the evaporation process (we feel cooler more quickly). The oils in sweat prevents the skin from drying out, which would make it susceptible to sunburn. 

Sometimes we feel hot even when sweating, particularly in a humid environment like a beach by the sea on a hot day. Two processes occur in tandem on the skin evaporation (liquid water - gaseous water) and condensation (gaseous water liquid water). It is quite possible that the same water condenses on our face as evaporated earlier. In effect, then, a cycle of liquid gas -> liquid occurs. The two halves of this cycle operate in opposite senses, since both exo- and endo-thermic processes occur simultaneously. The net change in energy is, therefore, negligible, and we feel no cooler. 

On a hot dag, gour bodg uses this latent heat to cool gou down.Your bodg sweats, and as the sweat evaporates, it absorbs heat from gour bodg. In the reverse, when a gas turns into a liquid or a liquid into a solid, latent heat is released. 

During a marathon, the rate of heat production can be more than tenfold greater than at rest and sufficient to raise the core body temperature by 1°C every eight minutes, if no cooling occurs. The core temperature is normally regulated so precisely that it does not rise more than about 1°C. The main mechanism for cooling is evaporation from the skin. Endurance runners can produce one litre of sweat per hour which removes about 2.4 MJ of heat. The energy used and therefore converted into heat in a marathon is about 12 MJ... 

If these processes produce too much heat, the body attempts to lose heat by vasodilation within the skin (via convection) and sweating (via evaporation of the water in the sweat). Both are well-known characteristics of fever. The patient s experience of alternate shivering and sweating (so well described by Hippocrates) probably represents an impairment of the thermorequlatory centre in the hypothalamus that regulates the balance between heat loss and heat production, resulting in fluctuations in body temperature. 

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