• Glossary of Climatic and Environmental Words •
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Acclimation – Physiological or behavioral changes occurring within an organism, which reduces the strain or enhances endurance of strain caused by experimentally induced stressful changes in particular climatic factors. Note: The terms acclimation and acclimatization are etymologically indistinguishable. Both words have been assigned several and different meanings (Greenleaf & Greenleaf, 1970). The most useful of the assigned meanings, adopted here, would seem to be those of Hart (1957) and Eagan (1963) who use the term acclimation to describe the adaptive changes that occur within an organism in response to experimentally induced changes in particular climatic factors such as ambient temperature in a controlled environment, and the term acclimatization to describe the adaptive changes that occur within an organism in response to changes in the natural climate.
Acclimatization – Changes that reduce the physiological strain produced by stressful components of the total environment. This change may occur within the lifetime of an organism (phenotypic) or be the result of genetic selection in a species or subspecies (genotypic). Acclimation as defined in this Glossary relates to phenotypic adaptations to specified climatic components. In thermal physiology, the use of the term adaptation does not require specification of the climatic components of the total environment to which the organism adapts, but the most obvious component is often denoted (e.g., adaptation to heat). There are no distinct terms that relate genotypic adaptations to the climate or particular components of climate.
Adaptation – Changes that reduce the physiological strain produced by stressful components of the total environment. This change may occur within the lifetime of an organism (phenotypic) or be the result of genetic selection in a species or subspecies (genotypic). Acclimation as defined in this Glossary relates to phenotypic adaptations to specified climatic components. In thermal physiology, the use of the term adaptation does not require specification of the climatic components of the total environment to which the organism adapts, but the most obvious component is often denoted (e.g., adaptation to heat). There are no distinct terms that relate genotypic adaptations to the climate or particular components of climate.
Alliesthesia – Generally, the changed perception of a given peripheral stimulus resulting from the stimulation of internal sensors; in thermal physiology, the dependence of thermal sensation on both skin and core temperatures. Ambient temperature – The average temperature of a gaseous or liquid environment (usually air or water) surrounding a body, as measured outside the thermal and hydrodynamic boundary layers that overlay the body. Synonym: temperature, dry bulb (in a gaseous environment).
Area, solar radiation – The area of a body projected perpendicularly to the sun’s rays.
Area, total body – The area of the outer surface of a body, assumed smooth.
Area, wetted – The area of skin which, if covered with sweat (water), would provide the observed rate of evaporation under the prevailing condition.
Atmospheric pressure – The pressure due to the weight of the atmosphere as indicated by a barometer. Standard atmospheric pressure is the pressure 101.325 kilopascals (kPa) (or the weight of a 760 mm column of mercury at 0°C with density 13.5951 × 103 kg • m–3 under standard gravity of 9.80665 m • s–2)and is equivalent to 1,013.25 millibars or to 760 Torr.
Autonomic Temperature regulation – The regulation of body temperature by autonomic (i.e., involuntary) thermoeffector responses to heat and cold which modify the rates of heat production and heat loss (i.e., by sweating, thermal tachypnea, shivering, non-shivering thermogenesis,and adjustmentsof circulatory convection of heat to the surfaces of the body). Note: In this definition the term autonomic is used in its more general sense and does not imply that all responses are controlled by the Autonomic Nervous System (sympathetic and parasympathetic efferents). Autonomic temperature regulation is frequently described as physiological temperature regulation, a term which should be used for both autonomic and behavioral thermoregulatory processes.
Basal metabolic rate – Metabolic energy transformation calculated from measurements of heat production or oxygen consumption in an organism in a rested, awake, fasting* sufficiently long to be in post-absorptive state, and thermoneutral zone (a particular case of standard metabolic rate (SMR) Note: In these conditions, when the amount of work being done on an external system is negligible, the rate of heat production is equal to the rate of metabolism (metabolic energy production).* The period of fasting needs to be specified as this may be for days in large animals, and for much shorter periods for very small mammals and birds.
Behavioral temperature regulation – Any coordinated movement of an organism ultimately tending to establish a thermal environment that represents a preferred condition for heat exchange (heat gain, heat loss,or heat balance) of the organism with its environment. The distinction between thermoregulatory behavior and thermotropism is ill-defined. A plant may exhibit thermotropism but is not considered to be ther-moregulating. Some aquatic unicellular organisms move to a preferred ambient temperature,however, whether this is thermotropism or thermoregulatory behavior may be disputed. For these reasons the term is usually restricted in thermal physiology to patterns of behavior controlled by a nervous system. The complex patterns of somato-motor activities that serve as behavioral thermoeffector responses to heat and cold of temperature regulators in modifying their conditions of heat exchange with the environment, involve a wide variety of performances (e.g., moving to a different thermal ambiance, changes in posture, wetting of body surfaces, change of microclimate by nest building, parental behavior, huddling) and, in humans, also include voluntary exercise and cultural achievements (clothing, housing, air-conditioning etc.). In endothermic temperature regulators, the reduced demand for autonomic temperature regulation is a potential result of behavioral temperature regulation which, however, competes with other, nonthermal, behavioral drives (e.g., search for food). Note: Behavioral temperature regulation is characterized as operant, if it is acquired in an experimental condition only after training guided by an experimenter. Otherwise, it is defined as natural, but this may also involve learning. In humans the distinction between natural and operant behavioral temperature regulation becomes arbitrary in many instances.
Body heat balance – The steady-state relation in which total heat gain in the body equals its heat loss to the environment.
Body heat balance equation – A mathematical expression that describes the net rate at which a subject generates and exchanges heat with its environment (First Law of Thermodynamics). The dimension of the equation and its terms respectively, are in watts thereby constituting heat flows, but often are also expressed in relation to unit area of body surface, to unit body mass, or to unit body volume. S = M – (W) – (E) – (C) – (K) – (R). S = storage of body heat (positive = increase in body heat content; negative = decrease in body heat content). M = metabolic energy transformation (always positive in a living organism) = metabolic rate. W = work rate, positive (= useful mechanical power accomplished; negative = mechanical power absorbed by body: work rate, negative). E = evaporative heat transfer (positive = evaporative heat loss; negative = evaporative heat gain). C = convective heat transfer (positive = transfer to the environment; negative = transfer into the body). K= conductive heat transfer (positive = transfer to the environment; negative = transfer into the body). R = radiant heat exchange (positive = heat transfer to the environment; negative = heat absorption by the body.
Body heat content – The product of the body mass, its average specific heat, and the absolute mean body temperature.
Brown adipose tissue – A particular form of adipose tissue with selected distribution. It differs from white adipose tissue both structurally and functionally; its color is brownish (yellow to reddish), depending on its content of (mitochondrial) cytochromes and of fat which is distributed in multiple droplets (multilocular) as compared to white fat cells (monolocular) and also on its dense vascularization. Brown adipose tissue appears to be restricted to mammalian species, especially of smaller body size, including the hibernators (Hibernation), but is also present in newborns of larger species including humans. Regulatory heat production has been identified as its main but possibly not its only function. Its heat-generating capacity relates to the rate of fatty acid oxidation which is not constrained by cellular ATP catabolism. This property correlates with the presence of a specialized component in the inner mitochondrial membrane named UCP1 (the uncoupling protein or thermogenin, which establishes a proton shunt bypassing oxidative phosphorylation). The activity of brown adipose tissue is controlled by the sympathetic nervous system through release of adrenaline.
Calorimetry – The measurement of heat. In thermal physiology, the measurement of the heat transfer between a tissue, an organ, or an organism and its environment.
Circadian – Relating to the approximate 24-h periodicity of a free running biological rhythm, or to the exact 24-h periodicity of an environmentally synchronized biological rhythm that persists with an approximate 24-h periodicity when not environmentally synchronized.
clo – A unit to express the relative thermal insulation values of various clothing assemblies. 1 clo = 0.18°C•m2 •h • kcal–1 = 0.155°C •m2 •W–1 Note: The clo is a unit developed to express thermal insulation (thermal resistance) in practical terms and represents the insulation provided by the normal indoor clothing of a sedentary worker in comfortable indoor surroundings. The term is used in heating and ventilation engineering in the determination of environmental conditions for human comfort.
Cold tolerance – The ability to tolerate low ambient temperatures. This term comprises a variety of physiological properties. Certain homeotherms (tachymetabolic species) are described as cold tolerant, because they are able to balance heat production and heat loss in the state of cenothermy at particularly low ambient temperatures. This may be due to the development of insulation or to the efficiency of metabolic heat production and is often combined with an improved ability to protect appendages from freezing by effective vascular control of local heterothermy. In homeotherms, cold tolerance may also be achieved by general heterothermy, including hibernation, i.e., by controlled forms of hypothermia. Poikilotherms (bradymetabolic species) are characterized as cold tolerant, if they are able to survive low and even subfreezing body temperatures, either in the state of cryothermy, or because intracellular formation of ice crystals is avoided during freezing. All forms of cold tolerance may be subject to adaptation or acclimatization.
Cold-blooded – The thermal state of an animal in which core temperature remains close to ambient temperature when subjected to a low ambient temperature. Synonym: bradymetabolic, poikilothermic. Antonym: warm-blooded.
Conductive heat transfer – The net rate of heat transfer in a solid material or a non-moving gas or fluid (i.e., by conduction) down a thermal gradient, within an organism, or between an organism and its external environment. The latter is usually expressed in terms of unit area of the total body surface, i.e. as a heat flux, and is the quantity (K) in the body heat balance equation in which (–K) = heat gain, and (+K) = heat loss.
Convection, circulatory – Flow of blood in the vessels of the circulatory system. In thermal physiology, convective transfer of heat with the blood stream from the thermal core through the thermal shell to the heat dissipating surfaces of the body is a thermoeffector of autonomic temperature regulation. Its efficiency in transferring heat is a function of flow rate and the temperature gradient between core and heat dissipating surface, and may be modified by the degree of heat exchange by conduction between the vessels to and from these surfaces (e.g., due to the counter-current arrangement of arteries and veins).
Convective heat transfer – The net rate of heat transfer in a moving gas or fluid (i.e., by convection) between different parts of an organism, or between an organism and its external environment; it may develop and be amplified by thermal gradients (natural convection) and by forces such as wind, fans, pumps or body movement (forced convection) usually expressed in terms of unit area of the total body surface (Area, total body), i.e., as a heat flux. The quantity (C) in the body heat balance equation in which (–C) = heat gain and (+C) = heat loss.
Cutaneous evaporative heat loss – Rate of heat dissipated by evaporation from the skin. Cutaneous evaporative heat loss (CEHL) plus respiratory evaporative heat loss (REHL) constitute evaporative heat loss.
Cytokine – A class of immunoregulatory polypeptides produced by various cell types, but, at the outset, primarily by mononuclear phagocytes activated by various signals provided by exogenous pyrogens. Cytokines may act as endogenous mediators of fever (endogenous pyrogens). The most prominent among the fever producing cytokines are interleukins (IL)-1and -6, tumor necrosis factor (TNF)-α, and interferon (IFN)-α.
Dew-point temperature – The temperature at which condensation first occurs when an air-water vapor mixture is cooled at constant pressure.
Diet-induced thermogenesis – Increase of obligatory nonshivering thermogenesis occurring especially in rodents when transferred from standard food to a highly palatable cafeteria diet, of which the animals consume more but dissipate part of the surplus caloric intake by enhanced heat production. The brown adipose tissue (BAT) is considered as the main effector organ of diet-induced thermogenesis.
Direct calorimetry – The direct physical measurement of heat, usually the rate of transfer of heat (heat transfer) between a tissue, an organ, or an organism and its environment.
Dry bulb temperature – The temperature of a gas or mixture of gases indicated by a thermometer shielded from radiation. Synonym: temperature, ambient.
Dry heat loss – The sum of heat flows or heat fluxes by radiation, convection, and conduction from a body to the environment. Synonyms: sensible heat loss; heat loss, Newtonian. Note: In meteorological literature, sensible heat loss refers to convection only and does not include other forms of heat transfer.
Ectothermy – The pattern of temperature regulation of animals in which body temperature depends mainly on the behaviorally controlled exchange of heat with the environment. Autonomic thermoeffectors may be temporarily important in a few ectothermic species (panting in lizards, warming-up of insects). Antonym: endothermy.
Emissivity – The ratio of the total radiant energy emitted by a body to the energy emitted by a full radiator at the same temperature.
Endogenous pyrogens – Heat labile proteins, and lipids that cause fever. Their production is often stimulated by exogenous pyrogens (microorganisms and their products, e.g., antigens), but also by injury, trauma and stress. The best identified endogenous pyrogen is interleukin-1β (IL-1β). It is thought that this cytokine induces fever via the production of another cytokine, IL-6. Other endogenous pyrogens include interferon-α and tumor necrosis factor-α. The final common pathway for cytokine-mediated fever is generally thought to be the production of prostaglandins of the E series in or near the anterior hypothalamus.
Endothermy – The pattern of thermoregulation in which the body temperature depends on a high (tachymetabolic) and controlled rate of heat production. Behavioural responses are often used by endotherms. Antonym: ectothermy.
Endotoxin – Heat stable compounds that are intrinsic to the cell walls of Gram-negative bacteria. The term derives from the noxious effects on the infected host; in addition, endotoxins act as potent exogenous pyrogens and stimulators of the acute-phase response, respectively. Endotoxins contain lipopolysaccharides (LPS) of high molecular weight. Other cell wall components,e.g., peptidoglycans, may have partially similar effects.
Energy – Energy may occur either as chemical, electromagnetic, mechanical energy. Synonymous to mechanical energy (force times length, height or distance) is work, synonymous to thermal energy is heat. Work or heat per unit time (work rate or heat flow) constitute power, and are used as components in the body heat balance equation.
Energy metabolism – The sum of the chemical changes in living matter in which energy is transformed.
Estivation – A state of lethargy (often during the summer) with a reduction in body temperature and metabolism demonstrated by some animals that are temperature regulators when active. The term is usually, but not exclusively, applied to ectothermic temperature regulators.
Eurythermy – The tolerance by organisms of a wide range of environmental temperatures, or the accommodation to substantial changes in the thermal environment. Antonym: stenothermy.
Evaporative heat gain – Evaporative heat transfer from the ambiance to the body (i.e., gain of heat energy) due to condensation of vapor on the skin and/or the surfaces of the respiratory tract, usually expressed in terms of energy per unit time and unit area of total body surface. Note: evaporative heat transfer most frequently occurs by vaporization of water from body surfaces and is evaporative heat loss.
Evaporative heat loss – Evaporative heat transfer from the body to the ambiance (i.e., loss of heat energy) by evaporation of water from the skin and the surfaces of the respiratory tract, usually expressed in terms of heat flow or heat flux (energy per unit time and unit area of total body surface). Note: Evaporative heat loss comprises passive components, i.e., water vaporizing from respiratory surfaces at normal respiration and water diffusing through the skin and vaporizing at the surface (insensible perspiration); its thermoregulatory components are established by autonomic thermoeffectors, like thermal sweating and thermal panting, and behavioral thermoeffectors, like saliva spreading, wallowing, and other modes of behavioral surface wetting.
Exogenous pyrogen – A substance which, when entering the inner environment of a multicellular organism (host) will cause fever. Stimulation of the production and/or release of endogenous pyrogens is, at least in most instances, the humoral effect from which fever ultimately results. Note: The most potent exogenous pyrogens are heat-stable lipopolysaccharides intrinsic to the cell walls of gram-negative bacteria (→ endotoxin). Many exogenous pyrogens of different molecular composition exist; they comprise compounds intrinsic to bacteria, viruses, fungi, mycobacteria, and some protozoa, as well as foreign proteins and some steroids.
Full radiator – A radiator of uniform surface temper¬ature whose radiant exitance in all parts of the spectrum is the maximum obtainable. The emissivity of a full radiator is unity for all wavelengths. Synonym: blackbody.G
Geneotypic adaptation – A genetically fixed condition of a species or subspecies, or its evolution, which favors survival in a particular total environment – adaptation.
Globe temperature – The temperature of a blackened hollow sphere of thin copper (usually 0.15 m diameter) as measured by a thermometer at its center; Tg approximately equals temperature, operative.
Gular fluttering – The rapid oscillations of the hyoid apparatus and hence of the gular region of some birds as a characteristic form of thermal panting during exposure to high ambient temperature, by which means air is moved across the moist surfaces of the upper respiratory tract.
Habituation – Reduction of responses to or perception of a repeated stimulation.
Heat capacity – The product of the mass of an object and its specific heat.
Heat exhaustion – Muscular weakness, fatigue, and distress, resulting from prolonged exposure to heat. Core temperature is elevated and thermal sweating and cutaneous vasodilatation are commonly but not invariably reduced. This condition is aggravated by muscular exertion, dehydration and hyponatremia. Circulatory abnormalities may occur.
Heat flow – The amount of heat transferred per unit time between parts of a body at different temperatures, or between a body and its environment when at different temperatures. Physically it constitutes power.
Heat shock protein – Intracellular proteins present in every species, from bacteria to humans. Among the multiple functions ascribed to these proteins they protect nascent proteins, halt proteins de-folding etc. Their generation may alter the threshold for thermal injury.
Heat stroke – An acute syndrome caused by an excessive rise in core temperature as the result of overloading or failure of the thermoregulatory system during exposure to heat stress. It is characterized by a large variety of pathophysiological alterations of bodily functions, including mental disturbances, with a high incidence of permanent or fatal damage.
Heat syncope – Collapse, usually with loss of consciousness, during exposure to heat. The symptoms are similar to those of the vasovagal syndrome (fainting).
Heat tolerance – The ability to tolerate high ambient temperatures. This term comprises a variety of physiological characteristics. Homeotherms (tachymetabolic species) are often characterized as heat tolerant, if they remain comfortable or are able to balance heat production and heat loss at particularly high ambient temperatures. Homeothermic species are also characterized as heat tolerant, if they are capable to maintain normal functions, or to survive, at body temperatures exceeding their normal range; this may involve the facility of selective brain cooling. – Poikilotherms (bradymetabolic species) may also be specified as heat tolerant, if they are able to survive very high body temperatures. All forms of heat tolerance may be subject to adaptation or acclimatization. Synonym: heat endurance.
Heat transfer – Process of heat flow, i.e., the rate at which heat energy is transferred from one part of an organism to another especially between body core and shell. In this case the heat transfer is a combination of conduction and convection. A more frequently investigated topic of thermal physiology is heat transfer between an organism and its environment, by conduction, convection, radiation, evaporation, or a combination of these. Net transfer from the organism to the environment is heat loss, and from the ambiance into the organism is heat gain. Body heat balance equation.
Heat transfer coefficient – A parameter which determines the amount of heat flux (H) due to a temperature difference may be defined for conduction, convection, radiation (or the combination of these processes) and for evaporation.
Heliothermy – The regulation of the body temperature of an ectothermic animal by behavioral adjustments of its exposure to solar radiation.
Heterothermy – The pattern of temperature regulation in a tachymetabolic species in which the variation in core temperature, either nychthemerally or seasonally, exceeds that which defines homeothermy.
Hibernation – The state of winter (sometimes late fall or early spring) lethargy with a reduction in body temperature and metabolism of some animals that are homeothermic temperature regulators when active.
Homeostasis – General term characterizing the relative constancy of physicochemical properties of the internal environment of an organism as being maintained by regulation.
Homeothermy – The pattern of temperature regulation in a tachymetabolic species in which the cyclic variation in core temperature, either nychthemerally or seasonally, is maintained within arbitrarily defined limits despite much larger variations in ambient temperature, i.e., homeotherms regulate their body temperature within a narrow range. Synonym: homoiothermy.
Hyperthermia – The condition of a temperature regulator when core temperature is above its range specified for the normal active state of the species.
Hypothermia – The condition of a temperature regulator when core temperature is below its range specified for the normal active state of the species. Note: Hypothermia may be regulated (e.g., Torpor, Hibernation) or may be forced if heat loss exceeds the capacity for total heat production.
Indirect calorimetry – The measurement of the rate of transfer of a material involved in the transformation of chemical energy into heat between a tissue, an organ, or an organism and its environment. The process requires the calculation of the heat transfer from an empirically established relation between the material transfer and the heat transfer.
Induced hyperthermia – The state of hyperthermia produced purposefully by increase in heat load and/or inactivation of heat dissipation by physical and/or pharmacological means.
Induced hypothermia – The state of hypothermia produced purposefully by increasing heat loss from the body and/or inactivation of heat conservation and heat production by physical and/or pharmacological means. Note: Induced hypothermia is employed in surgery to reduce the oxygen demand of tissues that are particularly sensitive to circulatory arrest.
Infared thermography – The recording of the temperature distribution of a body from the infra-red radiation emitted by the surface.
Insensible heat loss – Same as evaporative heat loss. Note: The term insensible heat loss should be avoided in favor of evaporative heat loss, because of possible confusion with insensible water loss which implies evaporative heat loss but relates to fluid balance.
Insensible perspiration – The mass of water passing through the skin by diffusion per unit area in unit time. Synonym: passive cutaneous water vapor exchange. Insensible perspiration represents a fraction of the insensible water loss.
Insensible water loss – The sum of the water lost by diffusion through the skin and water lost in breathing, and excluding any water excreted (e.g., in sweat, urine, feces). Loss of water as vapor implies loss of heat according to the latent heat of vaporization of water.
Latent heat of vaporization – The quantity of heat absorbed (or released) by a volatile substance (fluid) per unit mass in the process of its reversible change of state by evaporation (or condensation) under isobaric and isothermal equilibrium conditions.
Local heterothermy – The pattern of temperature in those parts of the body which comprises the thermal shell of homeotherms.
Lower critical temperature – The ambient temperature below which the rate of metabolic heat production of a resting thermoregulating tachymetabolic animal must be increased by shivering and/or nonshivering thermogenesis in order to maintain thermal balance. Synonym: critical temperature for heat production.
Lower temperature survival limit – The environmental temperature below which thermal balance cannot be maintained for a long period and animals become progressively hypothermic. At this temperature basal metabolic rate (BMR) can be measured.
Mean radiant temperature – The temperature of an imaginary isothermal “black” enclosure in which a solid body or occupant would exchange the same amount of heat by radiation as in the actual nonuniform enclosure.
Mean skin temperature – The sum of the products of the area of each regional surface element (Ai) and its mean temperature (Ti) divided by the total body (surface) area. Note: Mean skin temperature can be used as a physical variable in the calculation of heat balance or of heat content of the body. It is, however, not necessarily a good estimate of what might be sensed as a mean skin temperature.
Metabolic body size – The function of an animal’s body size to which standard metabolic rate (SMR) (or basal metabolic rate (BMR)) is directly proportional. Synonym: metabolically effective body mass. The resting metabolic rate (RMR) of adult animals (both tachymetabolic and bradymetabolic) changes in proportion to the 3/4 power of body mass is an empirically established fact, and the use of W3/4 as the metabolic body size permits comparisons to be made between the metabolic levels of different animals. When W2/3 is used, this implies proportionality of metabolic body size to the animal’s surface area (surface rule). The relation between metabolic rate (M or MR) and body size (M = aW3/4) is a particular case of the general allometric equation (y = axb) which says that plotting a biological variable, y, against another biological variable, x, after logarithmic transformation (i.e., log y = log a + b • log x) will result in a straight line with slope b.
Metabolic heat production – Rate of transformation of chemical energy into heat in an organism, usually expressed in terms of unit area of the total body surface (total body area). H is equal to M – (W) in the body heat balance equation. Note: During positive work rate (+W) or in the absence of work (W = 0) metabolic heat production (H = M – (+W)), i.e. positive work rate subtracted from the rate of metabolic energy transformation (=metabolic rate), equals total heat production. When work is being done on the body by an external source (negative work), total heat production is the sum of metabolic rate (M or MR) (which is equal to metabolic heat production in this condition) and the heat liberated within the body due to negative work (M– (– W)).
Metabolic rate – The rate of transformation of chemical energy into heat and mechanical work by aerobic and anaerobic metabolic activities within an organism, usually expressed in terms of unit area of the total body surface, i.e., as a heat flux. The quantity M in the body heat balance equation.
Negative Work rate – The rate of work (power) done on an organism by an external force. The quantity (W) in the body heat balance equation. Antonym: work rate, positive.
Nocturnal – Occurring during the nighttime, as distinct from daytime. Antonym: diurnal. Non-shivering obligatory thermogenesis – That component of nonshivering thermogenesis (NST) (i.e., heat production unrelated to the contractions of voluntary muscles) that is independent of short-term changes in ambient temperature (Ta).
Non-shivering thermogenesis – Heat production due to metabolic energy transformation by processes that do not involve contractions of skeletal muscles, i.e., tone, microvibrations, tremor (shivering), or tonic or voluntary contractions. In thermal physiology this term is conventionally used to indicate thermoregulatory (cold-induced) nonshivering thermogenesis (NST).
Non-shivering thermoregulatory thermogenesis – The increase in nonshivering thermogenesis (NST) in response to acute cold exposure. The principal effector organ is the brown adipose tissue (BAT) which may adaptively increase its capacity for heat production in the course of acclimatization and adaptation to cold stress.
Nonthermal sweating – A response of the sweat glands to a nonthermal stimulus.
Operative temperature – The temperature of a uniform (isothermal) “black” enclosure in which a solid body or occupant would exchange the same amount of heat by radiation and convection as in the actual non¬uniform environment.
Optimal body temperature range – The range of body temperatures in which a species carries out its normal daily activity. Note: This “ecological optimum” integrates internal and external forces acting on a species; it applies mostly to ectotherms.
Passive cutaneous water exchange – The passage through the skin in either direction of water down an osmotic gradient per unit area in unit time. Note: Passive cutaneous water exchange occurs only when the skin is covered with water or an aqueous solution.
Passive cutaneous water vapor exchange – The passage through the skin in either direction of water vapor down a water vapor pressure gradient per unit area in unit time. Synonym: insensible perspiration.
Phenotypic Adaptation – Changes that reduce the physiological and/or emotional strain produced by stressful components of the total environment and occurring within the lifetime of the organism. Synonym: adaptation, nongenetic. Note: acclimation relates to phenotypic adaptation to individual climatic components of the total environment.
Poikilothermy – Large variability of body temperature as a function of ambient temperature in organisms without effective autonomic temperature regulation. As a rule, bradymetabolism implies poikilothermy with only temporary exceptions in some species (e.g., active warming-up of insects before flight). Synonym: Temperature conformer. Tachymetabolism excludes poikilothermy, except in pathological conditions (impairment of temperature regulation), but permits heterothermy or torpor to occur in a number of species. Antonym: homeothermy.
Positive work rate – The rate of work (power) done by an organism on an external system. The quantity (+W) in the body heat balance equation. Synonym: mechanical energy, work production, useful work ac-complished. Antonym: work rate, negative.
Postprandial heat production – The increase in metabolic heat production, relative to its post-absorptive resting level, in the hours following food intake. Its relationship to specific dynamic effect, or diet-induced thermogenesis is not clear.
Preferred ambient temperature – The range of ambient temperature, associated with specified radiation intensity, humidity, and air movement, from which an unrestrained human or animal does not seek to move to a warmer or colder environment.
Preferred body temperature – The range of core temperature within which an ectothermic animal seeks to maintain itself by behavioral means.
Pyrexia – The condition of being febrile. Note: Pyrogen: The generic term for any substance whether exogenous or endogenous that causes fever when introduced into or released in the body.
Q10 Effect – The ratio of the rate of a physiological process at a particular temperature to the rate at a temperature 10°C lower, when the logarithm of the rate is an approximately linear function of temperature.R
Radiant energy – Energy travelling in the form of electromagnetic waves. [J] Note: This term should be distinguished from the radi¬ant heat exchange (R) of the environment with the body. That part of the electromagnetic spectrum of sig¬nificance in thermal physiology is divided for conve¬nience into the wavebands: Ultraviolet: 0.25–0.38 µm Visible: 0.38–0.78 µm Infrared: 0.78–100 µm Microwave: 1–100 mm.
Radiant heat exchange – The net rate of heat ex¬change by radiation between an organism and its envi¬ronment, usually expressed in terms of unit area of the total body surface, i.e., as a heat flux. The quantity (R) in the body heat balance equation where (–R)= heat gain and (+R) = heat loss.
Relative humidity – The ratio of the mol fraction of water vapor present in a volume of air to the mol fraction present in saturated air, both at the same temperature and pressure; in thermal physiology, the ratio of the saturated vapor pressure at the dew point temperature of the enclosure to the saturated vapor pressure at its dry bulb temperature. When the relative humidity is expressed as a percentage, the symbol is RH.
Respiratory evaporative heat loss – Rate of heat dissipated by exhalation of air saturated with water vapor.
Resting metabolic rate – The metabolic rate (M or MR) of an animal that is resting in a thermoneutral environment but not in the post absorptive state. Note: A particular case of standard metabolic rate (SMR) used when the subject cannot be brought to a fasting condition, e.g., ruminant animals. The period of food deprivation should be stated.
Saliva spreading – The spreading of saliva on the body surface, often a deliberate (behavioral) thermoeffector action to cool the surface by evaporation. Sometimes inaccurately termed grooming.
Shivering – Involuntary tremor of skeletal muscles as a thermoeffector activity for increasing metabolic heat production (thermogenesis, shivering). (thermoregulatory muscle tone).
Shivering thermogenesis – An increase in the rate of heat production during cold exposure due to increased contractile activity of skeletal muscles not involving voluntary movements and external work.
Sickness behavior – A group of signs or symptoms which accompany cytokine-producing events (e.g., infection), usually with the elevation of body temperature. The behaviors include, among others, loss of appetite, lethargy, depression and decreased motor activity, and are thought to be mediated by the action of cytokines, not necessarily those which mediate the rise in body temperature, as a result of the infection.
Skin wettedness – The fraction of the total body area (Ab) that is covered by sweat.
Specific dynamic effect – Temporary increase in metabolic energy transformation following food intake. Originally described as “spezifisch-dynamische Wirkung” by Rubner, the term “Wirkung” has been translated as “action” as well as “effect”. The phenomenon is assumed to be related to the catabolism of food stuffs, particularly proteins. Note: The term is becoming obsolete, because it cannot be distinguished precisely from other factors that stimulate metabolic energy transformation in the hours following food intake and, with the inclusion of the specific dynamic effect, account for postprandial (excess) heat production. Synonyms: specific dynamic action; diet-induced thermogenesis.
Specific heat – The quantity of heat required to raise the temperature of unit mass of a substance by one degree Celsius. Note: For gases, it is necessary to specify whether the pressure (cp)orthe volume (cv) is held constant during its determination. The specific heat of body tissue is usually taken to be 3.48 kJ • kg–1 •°C–1 (i.e., 0.83 kcal •kg–1 •°C–1).
Standard metabolic rate – metabolic energy transformation calculated from measurements of heat production or oxygen consumption in an organism under specified standard conditions*. Note: The conditions are usually such that the amount of work being done on an external system is negligible. The rate of heat production is then an acceptable index of the rate of metabolism. * The specified standard conditions are usually that the organismis rested (or as near to rested as is possible), fasting (if possible), awake, and in a thermoneutral environment. The extent to which standard conditions can be achieved varies with species. Metabolic rate, minimum observed (MOMR).
Steady state – The state of body heat balance in which there is no positive or negative heat storage when heat gain and heat loss between the body and the environment are equivalent. The body temperatures reached in a steady state depend on the extent of external or internal thermal load. If the thermoeffector capacities are exceeded due to excessive load or pathological processes, a steady state will not be reached, leading to hypo-or hyperthermia.
Storage of body heat – The rate of increase (+) or decrease (–) in the heat content [Ws = J] of the body caused by an imbalance between heat production (metabolic heat transformation)and heat loss, usually expressed in terms of unit area of total body surface (Area, total body). The quantity S in the body heat balance equation.
Sunstroke – An acute and dangerous reaction to heat exposure caused by a failure of the heat regulating mechanisms of the body. It is characterized by high core temperature, usually above 40.6°C, cessation of thermal sweating; headache, numbness, tingling and confusion prior to sudden delirium or coma, fast pulse, rapid respiratory rate and elevated blood pressure.
Surface rule – A statement that the basal metabolic rate (BMR) is proportional to the 2/3 power of body mass. Note: The rule is based on the proposition that basal metabolic rate (BMR) is related to surface area and that surface area varies with the 2/3 power of body mass. However, this is not experimentally verifiable, for when basal metabolic rate (BMR) is expressed per 2/3 power of body mass it increases systematically with body size (Kleiber, 1947). Basal metabolic rate (BMR) is more nearly proportional to the 3/4 power of body mass (metabolic body size).
Temperature – A measure of the mean kinetic energy of the molecules in a volume.
Temperature Humidity Index – The combination of temperature and humidity that is a measure of the degree of discomfort experienced by an individual in warm weather; it was originally called the discomfort index.
Temperature regulation – The maintenance of the temperature or temperatures of a body within a restricted range under conditions involving variable internal and/or external heat loads. Biologically, the existence of body temperature regulation to some extent by autonomic or behavioral means. Antonym: temperature conformity.
Temperature regulator – An organism which regulates its body temperature to some extent by autonomic and/or behavioral processes. Antonym: temperature conformer. Note: Tachymetabolism is the property of endothermic temperature regulators in which core temperature appears as the controlled variable, with arbitrarily defined degrees of thermostability (homeothermy, heterothermy). Bradymetabolism does not exclude that such species may be ectothermic temperature regulators, although mainly by means of behavioral temperature regulation, because thermoeffectors of autonomic temperature regulation are vestigial or absent.
Thermal conductance – The rate at which heat is transferred between unit area of two parallel surfaces in a medium when unit temperature difference is maintained between them, or the rate of heat transfer during steady state when a temperature difference of 1°C is maintained across a layer of tissue, either expressed per unit area.
Thermal conductivity – A property of a material defined by the flow of heat by conduction through unit thickness of the material per unit area and per unit temperature difference maintained at right angles to the direction of heat flow.
Thermal core – Those inner tissues of the body whose temperatures are not changed in their relationship to each other by circulatory adjustments and changes in heat dissipation to the environment that affect the thermal shell of the body. Note: The term is usedmainly to describe the deep body tissues of homeotherms whose temperatures are represented by the core temperature. The brain is generally a section of the thermal core: however, in several mammalian and avian species its temperature may deviate to some extent from that of the remaining thermal core due to selective brain cooling.
Thermal hyperpnea – An increase in tidal volume associated with an increase in alveolar ventilation occurring during severe heat stress which has caused a large rise in core temperature. In animals capable of thermal panting the phase of thermal hyperpnea with its slower, deeper breathing is also named second phase panting, since it is usually preceded by a phase of typical panting (rapid, shallow breathing).
Thermal panting – Increased respiratory evaporative heat loss (REHL) due to increased respiratory minute volume. Note: Thermal panting in animals can occur both with a closed and open mouth.
Thermal shell – The skin and mucosal surfaces of the body engaged directly in heat exchange with the environment and, in addition, those tissues under these surfaces whose temperatures may deviate from core temperature, due to heat exchange with the environment and to changes in circulatory convection (convection, circulatory) of heat from the core to the heat exchanging surfaces. Antonym: thermal core. Note: The temperature of the shell of temperature regulators depends on ambient temperature, heat loss to the environment, and on rate and geometry of blood flow to and from the skin.
Thermal strain – In temperature regulators:1.Any deviation of body temperature induced by sustained thermal stress that cannot be fully compensated by temperature regulation; 2. Any activation of thermoeffector activities in response to thermal stress that cause sustained changes in the state of other, nonthermal, regulatory systems.
Thermal stress – Any change in the thermal relation between a temperature regulator and its environment which, if uncompensated by temperature regulation, would result in hyper-or hypothermia.
Thermal sweating – A response of the sweat glands to a thermal stimulus.
Thermal tachypnea – A rapid respiratory frequency accompanied by an increase in respiratory minute volume and, commonly, a decrease in tidal volume, in response to a thermoregulatory need to dissipate heat.
Thermoeffector – An organ system and its action, respectively, that affect heat balance in a controlled manner as part of the processes of temperature regulation. Note: A multitude of thermoeffectors is involved in both autonomic and behavioral temperature regulation.
Thermoeffector threshold – The level of activity of a potential thermoeffector that is transgressed when it becomes actively involved in temperature regulation. Note: For some of the thermoeffectors, thresholds can be precisely determined, e.g., as the level of basal metabolic rate (BMR) or resting metabolic rate (RMR) of a tachymetabolic animal above which metabolic heat production (H) will increase in response to a sufficiently strong cold exposure. The thresholds of other thermoeffectors are arbitrarily defined, or agreed upon by convention, because basal or resting levels are difficult to define, e.g., in case of circulatory convection of heat to the skin and the underlying cutaneous vasomotor tone.
Thermoneutral zone – The range of ambient temperature at which temperature regulation is achieved only by control of sensible heat loss, i.e., without regulatory changes in metabolic heat production (H) or evaporative heat loss. The thermoneutral zone (TNZ) will therefore be different when insulation, posture or basal metabolic rate (BMR) vary. The term thermoneutral zone (TNZ) does not apply to ectotherms. The thermoneutral zone (TNZ) should be distinguished from thermoeffector threshold zone.
Thermoreceptor – Thermosensitive neural element for which both its afferent function and its response characteristics are electrophysiologically identified (thermosensor).
Thermosensor – Neural element or circuitry of neural elements for which it is established by psychophysical criteria or analysis of thermoeffector responses or changes of core temperature that they transduce temperature in such a way that thermal sensation is elicited and/or temperature regulation is adequately stimulated.
Thermotolerance – A rapid, short acting molecular process associated with the synthesis of several families of heat shock proteins (HSP) of different molecu¬lar weights elicited as a result of acute short sub-lethal heat injury. It is thought to protect cells from noxious stimuli as well as to accelerate their repair. It is also defined as heat shock response (HSR).The time course of heat shock proteins (HSP) vary in different cells but, on the average, heat shock proteins (HSP) in the intact body seem to operate several hours following the stress and retains its activity for a few days. The response is not heat-specific and can be elicited subsequent to subjection to several other stressors (e.g., ischemia, some chemicals, etc.). Synonym: heat shock response (HSR).
Torpor – A state of inactivity and reduced responsiveness to stimuli (e.g., during hibernation, hypothermia, or estivation).
Total heat production – The rate of transformation of chemical energy into heat in an organism (metabolic heat production (H)) plus any heat flow liberated within the body resulting from work done on the organism by an external force (negative work rate).
Tympanic temperature – The temperature of the tympanic membrane.
Upper critical temperature – The ambient temperature above which the rate of evaporative heat loss of a resting thermoregulating animal must be increased (e.g., by thermal tachypnea or by thermal sweating) in order to maintain thermal balance. Synonym: critical temperature for evaporative heat loss.
Upper temperature survival limit – The environmental temperature above which thermal balance cannot be maintained for a long period and animals become progressively hyperthermic.
Wallowing – The thermoregulatory increase in evaporative heat loss by spreading an aqueous fluid (e.g., water, mud, urine) on the body surface.
Warm-blooded – The thermal state of an animal that maintains its core temperature considerably higher than that of the environment when subjected to a low ambient temperature (Ta). Synonym: tachymetabolic (preferred). Antonym: cold-blooded. Note: This maintained temperature gradient between the organism and its environment is dependent on the relatively high rate of metabolic heat production (H) (tachymetabolism) of warm-blooded animals compared with the low rate of heat production (bradymetabolism) of cold-blooded animals. Thus, the terms tachymetabolic and bradymetabolic are preferred to the terms warm-blooded and cold-blooded because the first pair of terms relates to a more basic physiological distinction and because the second pair of terms has been used with various meanings not all of which are consistent with the definitions given here. Warm-blooded is not a synonym of homeothermic, because the definition of warm-blooded does not specify the degree of temperature stability consistent with homeothermy: the core temperatures of some warm-blooded animals vary considerably either nychthemerally or seasonally.
Water vapor pressure – The pressure exerted by water vapor. If water vapor is confined over its liquid so that the vapor comes into equilibrium with the liquid, and the ambient temperature Tof the medium is held constant, the vapor pressure approaches a maximum value called the saturated vapor pressure (Ps,Ta)or (P). The term vapor pressure (water) is always synonymous with a saturated vapor pressure at a temper¬ature T. [Pa, millibar, Torr] Note: The water vapor pressure (→ vapor pressure (water)) of an enclosure may be calculated from the observed wet bulb and dry bulb temperatures and the atmospheric pressure, by using standard steam or meteorological tables and formulas. The water vapor pressure in an enclosure is equal to the saturated vapor pres¬sure at its dew-point temperature (Ps,Tdp)ortothe product of the relative humidity (N) and the saturated vapor pressure at its dry bulb temperature (NPs,Tdp).
Wet-bulb temperature – The thermodynamic wet bulb temperature of a sample of air is the lowest temperature to which it can be cooled by evaporating water adiabatically. This measurement is compared to that read by an ordinary thermometer, or dry bulb thermometer, to estimate the ambient humidity. [°C] Note: The term is usually applied to the temperature recorded by an aspirated thermometer covered with a wet sleeve that is approximately equal to the thermodynamic wet bulb temperature when the bulb is shielded from radiation. Thermal comfort: Subjective indifference to the thermal environment.
Zone of thermal comfort – The range of ambient temperatures, associated with specified mean radiant temperature, humidity, and air movement, within which a human in specified clothing expresses indifference to the thermal environment for an indefinite period.