Heat dissipation during physical activities, like exercise, is a critical part of thermoregulation. Humans have a basal (at rest)metabolic rate of about 0.63 W/kg, or roughly 50 W for the average person, where a watt (W) is one Joule per second. But exercising just certain groups of muscles may metabolize 0.090 g (0.5 mmol) of glucose per minute, producing 24 W of mechanical energy. Since muscle energy conversion is only 22-26% efficient, about 76 W of heat energy is produced at the same time.[1].  Figure \(\PageIndex{1}\): Perspiration on a Runner's Face[2] If this metabolic waste heat energy can't be dissipated, heat stroke or other consequences may result. Humans use perspiration from the skin as one means of thermoregulation, because it has a cooling effect due to the latent heat of evaporation of water. Animals with few sweat glands, like dogs, pant to achieve evaporative cooling through the tongue and mouth. Few other animals depend on sweating to cool down. High humidity can reduce the rate at which the body can lose heat by evaporation. Light clothing that wicks sweat away but reflects radiant heat from the sun aids thermoregulation. plastic raincoats can actually contribute to heat stress. [3] The volume of water lost in perspiration depends on exercise level, and ranges from 100 to 8,000 mL/day. [4]. Mild dehydration (at about 1% body mass loss) may impair cognition[5]. It's not hard to lose that much; a body mass loss of 3 to 5% can result from cycling without hydration (water intake) for 2 hours in Summer(35 °C). Other effects include loss of blood volume by 3 to 6%, increasing body temperature, higher heart rates, and poor blood circulation. These effects are easily eliminated by replacing 50 to 80% of the fluid lost in sweat[6]
When perspiration is evaporated, a large amount of heat is absorbed even though the temperature remains constant at about skin temperature. The heat that is absorbed when perspiration evaporates (by breaking bonds between water molecules, thereby increasing its potential energy) is called the latent heat of vaporization. The kinetic energy of the water molecules (rotation, vibration, and limited translation) remains constant during phase changes, because the temperature does not change. The heat of vaporization is usually recorded at the boiling point, but the heat absorbed by evaporating perspiration is similar, as suggested by the values in the table below. When heat is supplied at a steady rate to a liquid at atmospheric pressure, the temperature rises until the boiling point is attained. After this the temperature remains constant until the enthalpy of vaporization has been supplied. Once all the liquid has been converted to vapor, the temperature again rises. In the case of water the molar enthalpy of vaporization is 40.67 kJ mol–1. In other words \[\ce{H2O (l) -> H2O (g)} \nonumber\]
Example \(\PageIndex{1}\): Heat Dissipated in Exercise How much perspiration must be evaporated if it is the only means of dissipating the 75 W of heat energy mentioned in the introduction, over a period of 30 minutes? Assume the heat of vaporization is about 43 kJ/mol at body temperature, and that no other heat loss (radiation and convection from the skin, etc.) is involved. \[\text{30 min} \times \text{60} \dfrac{s}{min} \times \text{75} \dfrac{J}{s} = \text{135 kJ}\nonumber\] This seems like a lot of energy! But how much perspiration would be evaporated? \[\dfrac{\text{135 kJ}}{\text{43 kJ/mol}} = \text{3.1 mol or 57 g of water, about 2 oz}\nonumber\]
Protective gear worn by fire fighters, welders, and especially bomb disposal technicians may prevent evaporation of perspiration. The bomb suits worn to protect against Improvised Explosive Devices (IEDs) use an ice-based chiller unit and recirculating water to aid thermoregulation.[8] Only a small amount of ice is necessary, because it takes a large heat of fusion to melt ice, even though the temperature remains at 0 °C while the heat is absorbed. Figure \(\PageIndex{2}\): Bomb Suit[7] The heat energy which a solid absorbs when it melts is called the enthalpy of fusion or heat of fusion and is usually quoted on a molar basis. (The word fusion means the same thing as “melting.”) When 1 mol of ice, for example, is melted, we find from experiment that 6.01 kJ are needed. The molar enthalpy of fusion of ice is thus +6.01 kJ mol–1, and we can write \[\ce{H2O (s) -> H2O (l)} \nonumber\] (0°C) ΔHm = 6.01 kJ mol–1 To remove the same amount of heat (135 kJ produced in 30 minutes) as the evaporation of 2 oz of perspiration, how much ice would be required? \[\dfrac{\text{135 kJ}}{\text{6.01 kJ/mol}} \times \text{18 g/mol} = \text{404 g}\nonumber\] (less than a pound) of ice. Selected molar enthalpies of fusion are tabulated below. Solids like ice which have strong intermolecular forces have much higher values than those like CH4 with weak ones.
*http://www1.lsbu.ac.uk/water/data.html Heat energy is absorbed when a liquid boils because molecules which are held together by mutual attraction in the liquid are jostled free of each other as the gas is formed. Such a separation requires energy. In general the energy needed differs from one liquid to another depending on the magnitude of the intermolecular forces. We can thus expect liquids with strong intermolecular forces to have larger enthalpies of vaporization. The list of enthalpies of vaporization given in the table bears this out. Two other features of the table deserve mention. One is the fact that the enthalpy of vaporization of a substance is always higher than its enthalpy of fusion. When a solid melts, the molecules are not separated from each other to nearly the same extent as when a liquid boils. Second, there is a close correlation between the enthalpy of vaporization and the boiling point measured on the thermodynamic scale of temperature. Periodic trends in boiling point closely follow periodic trends in heat of vaporiation. If we divide the one by the other, we find that the result is often in the range of 75 to 90 J K–1 mol–1. To a first approximation therefore the enthalpy of vaporization of a liquid is proportional to the thermodynamic temperature at which the liquid boils. This interesting result is called Trouton’s rule. An equivalent rule does not hold for fusion. The energy required to melt a solid and the temperature at which this occurs depend on the structure of the crystal as well as on the magnitude of the intermolecular forces. From ChemPRIME: 10.9: Enthalpy of Fusion and Enthalpy of Vaporization Contributors and Attributions
Perspiration, also known as sweating, is the production of fluids secreted by the sweat glands in the skin of mammals.[1] Heat HyperthermiaPreventionDrinking waterTreatmentAntiperspirant Two types of sweat glands can be found in humans: eccrine glands and apocrine glands.[2] The eccrine sweat glands are distributed over much of the body and are responsible for secreting the watery, brackish sweat most often triggered by excessive body temperature. The apocrine sweat glands are restricted to the armpits and a few other areas of the body and produce an odorless, oily, opaque secretion which then gains its characteristic odor from bacterial decomposition. In humans, sweating is primarily a means of thermoregulation, which is achieved by the water-rich secretion of the eccrine glands. Maximum sweat rates of an adult can be up to 2–4 liters per hour or 10–14 liters per day (10–15 g/min·m2), but is less in children prior to puberty.[3][4][5] Evaporation of sweat from the skin surface has a cooling effect due to evaporative cooling. Hence, in hot weather, or when the individual's muscles heat up due to exertion, more sweat is produced. Animals with few sweat glands, such as dogs, accomplish similar temperature regulation results by panting, which evaporates water from the moist lining of the oral cavity and pharynx. Although sweating is found in a wide variety of mammals,[6][7] relatively few (exceptions include humans and horses) produce large amounts of sweat in order to cool down.[8][9] 1610s, "a breathing through," a sense now obsolete, from French perspiration (1560s), noun of action from perspirer "perspire," from Latin perspirare "blow or breathe constantly," from per "through" (from PIE root *per- (1) "forward," hence "through") + spirare "to breathe, blow" (see spirit (n.)). Applied by 1620s to "excretion of invisible moistures through the skin," hence its later use as a euphemism for "sweat" (1725).[citation needed]
Sweat contributes to body odor when it is metabolized by bacteria on the skin. Medications that are used for other treatments and diet also affect odor. Some medical conditions, such as kidney failure and diabetic ketoacidosis, can also affect sweat odor. Areas that produce excessive sweat usually appear pink or white, but, in severe cases, may appear cracked, scaly, and soft.[14]
A man in a sweat-drenched shirt, after some physical exertion. Diaphoresis is a non-specific symptom or sign, which means that it has many possible causes. Some causes of diaphoresis include physical exertion, menopause, fever, ingestion of toxins or irritants, and high environmental temperature. Strong emotions (anger, fear, anxiety) and recall of past trauma can also trigger sweating.[citation needed] The vast majority of sweat glands in the body are innervated by sympathetic cholinergic neurons.[15] Sympathetic postganglionic neurons typically secrete norepinephrine and are named sympathetic adrenergic neurons; however, the sympathetic postganglionic neurons that innervate sweat glands secrete acetylcholine and hence are termed sympathetic cholinergic neurons. Sweat glands, piloerector muscles, and some blood vessels are innervated by sympathetic cholinergic neurons. Pathological sweatingDiaphoresis may be associated with some abnormal conditions, such as hyperthyroidism and shock. If it is accompanied by unexplained weight loss or fever or by palpitations, shortness of breath, or chest discomfort, it suggests serious illness. Diaphoresis is also seen in an acute myocardial infarction (heart attack), from the increased firing of the sympathetic nervous system, and is frequent in serotonin syndrome. Diaphoresis can also be caused by many types of infections, often accompanied by fever and/or chills. Most infections can cause some degree of diaphoresis and it is a very common symptom in some serious infections such as malaria and tuberculosis. In addition, pneumothorax can cause diaphoresis with splinting of the chest wall. Neuroleptic malignant syndrome and other malignant diseases (e.g. leukemias) can also cause diaphoresis.[16] Diabetics relying on insulin shots or oral medications may have low blood sugar (hypoglycemia), which can also cause diaphoresis. Drugs (including caffeine, morphine, alcohol, antidepressants and certain antipsychotics) may be causes, as well as withdrawal from alcohol, benzodiazepines, nonbenzodiazepines or narcotic painkiller dependencies. Sympathetic nervous system stimulants such as cocaine and amphetamines have also been associated with diaphoresis. Diaphoresis due to ectopic catecholamine is a classic symptom of a pheochromocytoma, a rare tumor of the adrenal gland. Acetylcholinesterase inhibitors (e.g. some insecticides) also cause contraction of sweat gland smooth muscle leading to diaphoresis. Mercury is well known for its use as a diaphoretic, and was widely used in the 19th and early 20th century by physicians to "purge" the body of an illness. However, due to the high toxicity of mercury, secondary symptoms would manifest, which were erroneously attributed to the former disease that was being treated with mercurials.[citation needed] Infantile acrodynia (childhood mercury poisoning) is characterized by excessive perspiration. A clinician should immediately consider acrodynia in an afebrile child who is sweating profusely. Some people can develop a sweat allergy.[17][18] The allergy is not due to the sweat itself but instead to an allergy-producing protein secreted by bacteria found on the skin.[19] Tannic-acid has been found to suppress the allergic response along with showering.[17] HyperhidrosisIn some people, the body's mechanism for cooling itself is overactive—so overactive that they may sweat four or five times more than is typical.[20] Millions of people are affected by this condition, but more than half never receive treatment due to embarrassment, lack of awareness, or lack of concern. While it most commonly affects the armpits, feet, and hands, it is possible for someone to experience this condition over their whole body. The face is another common area for hyperhidrosis to be an issue. Sweating uncontrollably is not always expected and may be embarrassing to people with the condition. It can cause both physiological and emotional problems in patients. It is generally an inherited problem that is found in each ethnic group. It is not life-threatening, but it is threatening to a person's quality of life.[21] Treatments for hyperhidrosis include antiperspirants, iontophoresis, and surgical removal of sweat glands. In severe cases, botulinum toxin injections or surgical cutting of nerves that stimulate the excessive sweating (Endoscopic thoracic sympathectomy) may be an option.[22] Night sweatsNight sweats, also known as nocturnal hyperhidrosis, is the occurrence of excessive sweating during sleep. The person may or may not also perspire excessively while awake. One of the most common causes of night sweats in women over 40 is the hormonal changes related to menopause and perimenopause. This is a very common occurrence during the menopausal transition years. While night sweats might be relatively harmless, it can also be a sign of a serious underlying disease. It is important to distinguish night sweats due to medical causes from those that occur simply because the sleep environment is too warm, either because the bedroom is unusually hot or because there are too many covers on the bed. Night sweats caused by a medical condition or infection can be described as "severe hot flashes occurring at night that can drench sleepwear and sheets, which are not related to the environment". Some of the underlying medical conditions and infections that cause these severe night sweats can be life-threatening and should promptly be investigated by a medical practitioner.[citation needed] The evaporation of sweat on the skin cools the body. Sweating allows the body to regulate its temperature. Sweating is controlled from a center in the preoptic and anterior regions of the brain's hypothalamus, where thermosensitive neurons are located. The heat-regulatory function of the hypothalamus is also affected by inputs from temperature receptors in the skin. High skin temperature reduces the hypothalamic set point for sweating and increases the gain of the hypothalamic feedback system in response to variations in core temperature. Overall, however, the sweating response to a rise in hypothalamic ('core') temperature is much larger than the response to the same increase in average skin temperature.[citation needed] Sweating causes a decrease in core temperature through evaporative cooling at the skin surface. As high energy molecules evaporate from the skin, releasing energy absorbed from the body, the skin and superficial vessels decrease in temperature. Cooled venous blood then returns to the body's core and counteracts rising core temperatures. There are two situations in which the nerves will stimulate the sweat glands, causing perspiration: during physical heat and during emotional stress. In general, emotionally induced sweating is restricted to palms, soles, armpits, and sometimes the forehead, while physical heat-induced sweating occurs throughout the body.[23] People have an average of two to four million sweat glands, but how much sweat is released by each gland is determined by many factors, including sex, genetics, environmental conditions, age and fitness level. Two of the major contributors to sweat rate are an individual's fitness level and weight. If an individual weighs more, sweat rate is likely to increase because the body must exert more energy to function and there is more body mass to cool down. On the other hand, a fit person will start sweating earlier and more readily. As someone becomes fit, the body becomes more efficient at regulating the body's temperature and sweat glands adapt along with the body's other systems.[24] Sweat is not pure water; it always contains a small amount (0.2–1%) of solute. When a person moves from a cold climate to a hot climate, adaptive changes occur in the sweating mechanisms of the person. This process is referred to as acclimatization: the maximum rate of sweating increases and its solute composition decreases. The volume of water lost in sweat daily is highly variable, ranging from 100 to 8,000 mL/day. The solute loss can be as much as 350 mmol/d (or 90 mmol/d acclimatised) of sodium under the most extreme conditions. During average intensity exercise, sweat losses can average up to 2 litres of water/hour. In a cool climate and in the absence of exercise, sodium loss can be very low (less than 5 mmol/d). Sodium concentration in sweat is 30–65 mmol/L, depending on the degree of acclimatisation. Beads of sweat emerging from eccrine glands Sweat is mostly water. A microfluidic model of the eccrine sweat gland provides details on what solutes partition into sweat, their mechanisms of partitioning, and their fluidic transport to the skin surface.[25] Dissolved in the water are trace amounts of minerals, lactic acid, and urea. Although the mineral content varies, some measured concentrations are: sodium (0.9 gram/liter), potassium (0.2 g/L), calcium (0.015 g/L), and magnesium (0.0013 g/L).[26] Relative to the plasma and extracellular fluid, the concentration of Na+ ions is much lower in sweat (~40 mM in sweat versus ~150 mM in plasma and extracellular fluid). Initially, within eccrine glands sweat has a high concentration of Na+ ions. In the sweat ducts, the Na+ ions are re-absorbed into tissue by epithelial sodium channels (ENaC) that are located on the apical membrane of epithelial cells that form the duct (see Fig. 9 of the reference).[2] Many other trace elements are also excreted in sweat, again an indication of their concentration is (although measurements can vary fifteenfold) zinc (0.4 milligrams/liter), copper (0.3–0.8 mg/L), iron (1 mg/L), chromium (0.1 mg/L), nickel (0.05 mg/L), and lead (0.05 mg/L).[27][28] Probably many other less-abundant trace minerals leave the body through sweating with correspondingly lower concentrations. Some exogenous organic compounds make their way into sweat as exemplified by an unidentified odiferous "maple syrup" scented compound in several of the species in the mushroom genus Lactarius.[29] In humans, sweat is hypoosmotic relative to plasma[30] (i.e. less concentrated). Sweat is found at moderately acidic to neutral pH levels, typically between 4.5 and 7.0.[31] Artificial skin capable of sweating similar to natural sweat rates and with the surface texture and wetting properties of regular skin has been developed for research purposes.[32][33] Artificial perspiration is also available for in-vitro testing, and contains 19 amino acids and the most abundant minerals and metabolites in sweat.[citation needed] DiagnosticsThere is interest in its use in wearable technology. Sweat can be sampled and sensed non-invasively and continuously using electronic tattoos, bands, or patches.[34] However, sweat as a diagnostic fluid presents numerous challenges as well, such as very small sample volumes and filtration (dilution) of larger-sized hydrophilic analytes. Currently the only major commercial application for sweat diagnostics is for infant cystic fibrosis testing based on sweat chloride concentrations.
|
What property of water is illustrated when your body cools down when we perspire
Postagens relacionadas
direito autoral © 2024 comojogaruno Inc.
