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The Thermometer • 1592 • -- Galileo produces the first thermometer Early instruments contained water, then wine, and finally, in 1670, mercury. • 1614 -- Italian physician, Sanctorio Santorius, published results of studies in which he used his own clinical thermometer to determine body temperature. • He concludes that man’s temperature remains remarkably constant, except during illness, when it rises. The Thermometer • 1714 -- German physicist, Gabriel Fahrenheit, constructs a mercury thermometer but chooses a rather arbitrary reference point for zero and the boiling point of water. • Zero was the lowest temperature observed in his hometown during a particular winter. This was not the air temperature, but the temperature of a mixture of snow and sal ammoniac! • • The boiling point of water was set at 212o (Why???) Measured body temperature and found it to be constant at 96o. • At about the same time, a Swedish astronomer, Anders Celsius, constructed a thermometer choosing the freezing point of water as 0o and the boiling point as 100o. The Thermometer • Whatever the scale, the thermometer provided the means of measuring temperature of the air as well as of the living body. • Where to place the instrument, on, or in, the body was still to be resolved. • At first, investigators pressed it against the skin, or in the armpit, or between the thighs. • 1774 -- Dr. George Fordyce first suggests that the bulb of the thermometer be placed under the tongue. • 1778 -- John Hunter, and English surgeon and anatomist, using relatively small thermometers inserted them everywhere: • In humans in the male urethra and the rectum, and • In experimental animals in the body cavities and a variety of organs. • Hunter reported that humans and animals could generate heat as well as dissipate heat. The Thermometer • 1775 -- Charles Blagden, a Scottish physician, published the results of his work that contains the origins of much of our knowledge of the physiology of temperature regulation. • For example, in an atmosphere of high temperature, “The external circulation was greatly increased; the veins had become very large, and a universal redness had diffused itself over the body.” • “…it appears beyond all doubt, that the living powers were very much assisted by the perspiration, that cooling evaporation is a further provision of nature for enabling animals to support great heats.” • “Perhaps no experiments hitherto made furnish more remarkable instances of the cooling effect of evaporation than these last facts; a power which appears to be much greater than hath commonly been suspected.” The Thermometer • Using the thermometer, the abilities of the body to generate heat in a cold environment, and to dissipate heat when the ambient temperature rises were revealed. Temperature regulation is a fundamental homeostatic process. Poikilothermic vs. Homeothermic Vertebrates Poikilotherms (“cold-blooded”) • Body temperature fluctuates over a considerable range with changing environmental temperature. • Behavioral temperature regulation. • Reptiles, amphibia, and fish Homeotherms (“warm-blooded”) • Body temperature regulated within a narrow range in spite of wide variations in environmental temperature. • Temperature Regulatory System(s) Temperature Regulatory System(s) What does the system regulate? • Core temperature • varies little with changes in environmental temperature. • Total body heat content is not regulated. 37ºC 37ºC 37ºC Core Core 32ºC • In general, the body surface and extremities are cooler than the “core.” • The magnitude of the differences between the body surface and extremities and the “core” varies with environmental temperature. Shell 28ºC 34ºC 31ºC Temperature regulatory systems act to maintain the “core temperature” at, or near, a “set point.” Cold Warm Central Receptors Anterior Hypothalamus/Pre-optic Area Peripheral Skin Receptors Warm Cold Warm Cold Other Central Receptors Midbrain and Spinal Cord Warm Cold Posterior Hypothalamic TemperatureRegulating Center Integration Other Central Receptors Abdominal Visceral Receptors Warm only Efferent Signals Controlling the Rates of Heat Loss and Heat Production Variations in Core Temperature • Normal Range: Rectal 97-1000 F (36.1 - 37.8 OC) • Different organs within the core may differ in temperature • Organ-specific metabolic activity • Temperature of perfusing blood • Temperature gradient to surrounding tissues • e.g., liver > rectum • Diurnal Rhythm • Regular daily fluctuation of 0.90 - 1.300 F (0.5 - 0.70 C) • On normal L:D and activity • Lowest approximately 6-7 AM • Highest approximately 5-7 PM Variations in Core Temperature: • Monthly Rhythm in females • Associated with ovulation • Progesterone-induced increase (0.5 - 0.60 C or 10 F) in body temperature • Maintained during the luteal phase of the menstrual cycle. • During Exercise • Body temperature rises • Elevation of body temperature “set point.” • Heat produced exceeds heat dissipation. • Rectal Temperature may rise as high as 1040 F (400 C) • Rise in body temperature is limited by thermoregulatory systems which increase heat dissipation. Heavy exercise Core temperature (ºC) Moderate exercise Mild exercise Time (min) Begin exercise Fig. 27-16, pg: 840 Temperature Regulatory System(s) Variations in Core Temperature • During Fever • Increase in the “set point” for body core temperature induced by • Pyrogens • Hypothalamic lesions FEVER Core Temperature “Set Point” Heat Loss Heat Production Core Temperature Pyrogens Released from toxic bacteria or from degenerating body tissues. Some pyrogens act directly and immediately on the hypothalamic termperature regulating center to increase the set point for body core temperature. Other pyrogens (e.g., endotoxins from gram-negative bacteria) function indirectly and may require several hours to cause effects. Bacteria or breakdown products are phagocytized by leukocytes, tissue macrophages, and large granular killer lymphocytes. These cells digest the bacterial products and then release interleukin-1 (IL-1) and interleukin-6 (IL-6) IL-1 and IL-6, acting at the hypothalamus, stimulate the production of PGE2, that acts to elicit fever. Antigens recognized as foreign - infectious - autoimmune - neoplastic Activated immune response cells - leukocytes - mesangial cells - vascular endothelial cells - astrocytes Production of interleukins 1 and 6 Increased prostaglandin E2 synthesis in the hypothalamus Elevation of hypothalamic temperature set point Increased heat production, reduced heat loss - vasoconstriction - shivering - behavior Elevation of hypothalamic temperature to a new set point fever cting at NSAIDs Fever cessation decreases hypothalamic temperature set point Fever increases hypothalamic temperature set point Heat gain increased and heat loss reduced 1. Skin vasoconstriction 2. shivering Heat Loss increased 1. Skin vasodilation 2. sweating Days Fig. 27-15, pg: 837 Temperature Regulatory System(s) Variations in Core Temperature • Hypothalamic lesions • Brain surgery in region of the hypothalamus may alter the hypothalamic temperature “set point” and induce fever (sometimes hypothermia) • Compression due to brain tumor may do the same. FEVER Core Temperature “Set Point” Heat Loss Heat Production Core Temperature Temperature Regulatory System(s) Fever Core Temperature “Set Point” Heat Loss Heat Production Core Temperature “Chills” • Skin vasoconstriction ( Heat Loss) • Shivering ( Heat Production) • Until the new higher “set point” is reached. The Crisis or “Flush” • If the factor that elevated the “set point” is removed, then the “set point” returns to normal. • Patient reports feeling “hot.” • Intense sweating • Skin vasodilation Heat Loss Energy Balance, Energy Expenditure, and Total Heat Production Energy Balance Chemical Work Done Chemical Energy Total Heat Energy = on External + - of New Tissues + Production of Food Environment and Fat Stores Energy Expenditure Energy Expenditure Work Done = on External + Environment Total Heat Production The energy expended on work done on the external environment averages no more than about 1% of the total energy expenditure of the body Energy Expenditure Total Heat Production Physical Laws Governing Heat Exchange between Living Organisms and the Environment Evaporation to air Radiation Evaporation to air Convection to air Conduction to seat Conduction to handle bar CONDUCTION ≡ Heat exchange between objects or substances that are in contact with each other. • Heat transferred from one molecule to another (solids, liquids, gases) • The rate of heat transfer (D; watts/m2) is proportional to the temperature difference (i.e., thermal gradient) D = k(T1 - T2) k = conductance = thermal conductivity divided by length of conducting pathway and multiplied by area of contact T1, T2 = temperatures of warm and cool surfaces • • Air is a poor conductor Not much heat is lost or gained by body contact unless the bare skin is in contact with a good conductor CONVECTION ≡ Movement of molecules away from the area of contact • Aids conduction in liquids and gases • Liquid or gas in contact with surface of different temperature is heated or cooled by conduction, altering its specific gravity. • The rate of heat transfer (C; watts/m2) is proportional to the velocity of the air (V; m/sec.), as well as, the temperature difference between skin and air (Ts - Ta) C = 10 • • V (Ts - Ta) Heat loss by convection increases when cooler air replaces air that has been warmed during contact with the skin. When wind, fans, or movement of the body through the air increases the velocity of air (“forced convection”), the rate of heat loss can be increased dramatically. THERMAL RADIATION ≡ Exchange of thermal energy between objects in space through a process that depends only on the absolute temperature and the nature of the radiating surfaces. • Energy will pass from a hot object to a cooler one. • Does not require an intervening medium. • Speed of light transmission • Electromagnetic waves from an emitting object carry heat away to an absorbing object. • Electromagnetic waves absorbed by the absorbing object are converted to heat. THERMAL RADIATION •The net transfer of heat is the difference between the radiation emitted by a surface and that which it receives. Stefan-Boltzmann Law R = s e1, e2 (T4 - TW4) where: R = radiant heat transfer in W/m2 s = 5.75 X 10-8 W/m2 0K4 (Stefan-Boltzmann constant) T, TW = Temperatures of hot object and surface of absorbing object (0K), respectively e1, e2 = Emissivities of radiator surface and absorbing surfaces, respectively In the equation above, the surface quality or emissivity (e) of a surface is an important factor. Thermal Radiation An object with an emissivity (e) = 1 An ideal absorber of radiant energy (i.e., a “black body”) Such an hypothetical surface absorbs all incident radiation on one side and reflects nothing (e.g., an open window). An ideal absorber of radiant energy is also an ideal emitter of radiant energy. An ideal absorber of thermal radiation (i.e., an ideal thermal “black body”) is also an ideal emitter of thermal radiant energy. Emissivity (e) = 0 A perfect reflector of radiant energy Such an hypothetical surface reflects all incident radiation and absorbs none (e.g., highly polished metallic surfaces). Many surfaces are almost “black body” absorber/radiators for some wavelengths of radiation (with e’s close to 1) , but reflect other wavelengths quite well (with e’s close to 0) . Thermal Radiation Human Skin Colors The emissivity (e) of skin varies with the wavelength of the radiant energy. In the visible spectrum, skin colors vary due to differences in the absorbance and reflectance (i.e., variations in emissivity coefficient (e)) for light of various wavelengths. All human skin, regardless of color, is an excellent absorber/radiator in the infrared wavelengths (e is close to 1) . For thermal radiation, human skin is a “black body absorber/radiator” All skin is black to infrared radiation! Radiation Stefan-Boltzmann Law R = s e1, e2 (T4 - TW4) Rate of heat transfer by thermal radiation to and from the body: Human Skin: 97% perfect infrared “black body” absorber/radiator • The temperatures of surfaces in the environment are usually lower than body temperature. • Surfaces in the environment are highly absorbing for infrared radiation • The equation above assumes that all surfaces are “black” (e1 = e2 = 1) • If the mean skin temperature (TS) and the environmental temperature are not very different (i.e., within 200C), then the equation above can be simplified: R = kr (Ts - TW) Kr = 4sTS3 • For a man dressed in shorts and sitting quietly in an environment at 250C, R equals about 50 - 70 % of the heat lost from the body (about 30 W/m2). Radiation R = kr (Ts - TW) Heat transfer by radiation to and from the body: • Not all of the body surface is effective in radiation exchange with the environment. • Between the legs, under the arms, and between fingers, radiant heat lost from one area is absorbed by the opposite skin surface and no net loss occurs to the environment. Effective radiating area (% of total body area) Standing man with arms at his side 75 Standing man with arms and legs extended 85 Man in tightly curled-up position 50 Vaporization • Heat of Vaporization • Vaporization of 1.0g H2O removes 0.58 kcal. • The total rate of heat transferred away from the body by vaporization (E) is proportional to the rate of evaporative moisture lost via two different routes: • “Insensible evaporation” (Ein) •Not subject to physiological control. • Sweat evaporation (Esw) •Some aspects under physiological control •Other aspects depend on environmental factors. Rate of heat loss by vaporization = E = Ein + Esw Vaporization E = Ein + Esw • Insensible Evaporation (Ein) • Ein is not controlled in the regulation of body temperature. • Ein occurs at all times, even in a cold environment • Two components of Ein: • Evaporation of water after its transudation through the skin (not sweat). • Evaporation of water from the respiratory tract. At 30 0C, • Ein = 12-15 ml/m2/h X 0.58 kcal/ml = 6.96 - 8.70 kcal/m2/h • Transudation of H2O through the skin (~50% of Ein) • Evaporative H2O loss from the respiratory tract (~50% of Ein) • 20-25% of total heat loss Vaporization E = Ein + Esw • Sweat Evaporation (Ein) Esw = he (Pws - faPWa)Aw/Ap where: Pws = water vapor pressure of saturated air at skin temperature Pwa = water vapor pressure saturated air at ambient air temperature Aw = area of wet skin fa = relative humidity Ap = body area he = water vaporization heat transfer coefficient that depends on the air velocity • Sweat Evaporation (Ein) Esw = he (Pws - faPWa)Aw/Ap where: Pws = water vapor pressure of saturated air at skin temperature Pwa = water vapor pressure saturated air at ambient air temperature Aw = area of wet skin fa = relative humidity Ap = body area he = water vaporization heat transfer coefficient that depends on the air velocity Evaporation of Sweat (ESW) Skin temperature is controlled. Ambient temperature, Thus, PWS is variable Relative humidity, and The rate of sweating is controlled. Air velocity Thus, AW is variable. also affect the efficacy of heat loss by sweat Exposed Body Area (Ap) evaporation. • Behavior may be altered • e.g., Clothing Vaporization E = Ein + Esw At 30 0C • Evaporative heat loss is fairly constant (12 -15 g/m2/h) • Approximately 25% of total heat loss. • 50% of evaporative heat loss due to Ein • 50% of evaporative heat loss due to Esw • Remaining 75% of heat loss is by other means Above 30 0C • Evaporative heat loss increases linearly with increased ambient temperature. Rectal Temperature Skin Temperature Vaporization Heat Loss Physical Laws Governing Heat Exchange between Living Organisms and the Environment Conduction D = k(T1 - T2) Convection C = 10 Radiation R = kr (Ts - TW) Vaporization V (Ts - Ta) E = Ein + he (Pws - faPWa)Aw/Ap N.B. When the environmental temperature is equal to or above the skin temperature, then • No heat is lost by conduction, convection, or radiation because the thermal gradient is zero or positive. • All heat must be lost by evaporation Physical Laws Governing Heat Exchange between Living Organisms and the Environment SUMMARY Where: S = M - E + (R + C + D)] S = rate of body heat storage M = total metabolic rate (i.e., total heat production) E = evaporative heat loss rate R + C + D = rates of heat gain (or loss) by radiation, convection, or conduction If the rate of body heat storage (S) is zero, then M = - E + ( R + C + D)] At all environmental temperatures, heat is lost by evaporation (Ein + Esw). If the environmental temperature is less than body temperature, then R, C, and D are negative quantities (i.e., heat is lost by these mechanisms). If the environmental temperature is equal to or greater than body temperature, then R, C, and D are positive (i.e., heat is gained by these mechanisms); heat may be lost only by evaporation (E). Patterns of Heat Loss from the Body during Different Environmental Conditions and Levels of Physical Activity TABLE 1 CONDITION At rest, lying in still dry air At rest, lying in still dry air Shivering, lying in still dry air At rest, lying in still dry air At rest, lying in still dry air Exercise AMBIENT TEMPERATURE 0 30 C (thermoneutral) 0 22-28 C (cold) 0 22-28 C (cold) 0 0 > 30 < 37 C (hot) 0 > 37 C (very hot) 0 30 C (thermoneutral) HEAT LOSS BY CONVECTION 5 – 25 % HEAT LOSS BY RADIATION 50 – 75 % HEAT LOSS BY VAPORIZATION 25 % increase increase decrease greater increase greater increase same decrease decrease decrease increase 0 0 greater increase increase increase graded increase Temperature Regulation Patterns of Heat Loss SKIN TEMPERATURE AND HEAT LOSS • Transfer of heat from the body to the environment via conduction, convection, and radiation depends on the temperature gradient between skin and the environment. • Transfer of heat from the body to the environment via vaporization depends on the difference in saturated water vapor pressures at skin and air temperatures. SKIN TEMPERATURE RATE OF HEAT LOSS SKIN TEMPERATURE RATE OF HEAT LOSS SKIN TEMPERATURE AND HEAT LOSS • The transfer of body heat to the environment via conduction, convection, or radiation requires a favorable temperature gradient between the skin and the environment. R = kr (Ts - TW) C = 10 V (Ts - Ta) D = k(T1 - T2) • If a favorable temperature gradient exists, then increasing the skin temperature will increase this gradient and increase the rate of heat loss via conduction, convection and radiation. E = Ein + Esw E = Ein + he (Pws - faPWa)Aw/Ap environment via vaporization requires a difference in saturated water vapor pressures at the skin and air temperatures • The transfer of body heat to the • As relative humidity increases and the value of the product faPwa approaches Pws, then evaporative cooling becomes less effective. • At higher skin temperatures, the amount of water vapor that can be held in air in contact with the skin (indicated by increased Pws) is greater. Thus the vapor pressure gradient (Pws - faPWa) may also be increased, increasing the efficiency of sweat evaporation. E = Ein + he (Pws - faPWa)Aw/Ap Scenario #1 Skin Temperature = 320C Pws = 35.66 mmHg Esw Esw Ambient Air Temperature = 200C Pwa = 17.535 mmHg Relative Humidity = 50% = he (35.66 mmHg - 0.5[17.535 mmHg]) Aw/Ap = he (26.89 mmHg) Aw/Ap Positive value indicates a favorable water vapor pressure gradient between the skin and the ambient air. Scenario #2 Same as #1, but raise relative humidity to 95% Esw = he (35.66 mmHg - 0.95[17.535 mmHg]) Aw/Ap Esw = he (19.00 mmHg) Aw/Ap Water vapor pressure gradient less favorable than in Scenario #1 Scenario #3 Same as #2, but raise skin temperature to 350 C and, consequently, raise Pws Esw = he (42.175 mmHg - 0.95[17.535 mmHg]) Aw/Ap Esw = he (25.52 mmHg) Aw/Ap Raising skin temperature increases the water vapor pressure gradient. Mechanisms by which Homeotherms increase Heat Dissipation • Increased skin temperature • Improves the rate of heat loss to the environment by Conduction D = k(T1 - T2) Convection C = 10 Radiation R = kr (Ts - TW) Vaporization V (Ts - Ta) E = Ein + he (Pws - faPWa)Aw/Ap How can body core temperature be kept constant in a warm environment? Mechanisms by which Homeotherms increase Heat Dissipation Mechanisms by which Homeotherms increase Heat Dissipation Control of Skin Temperature • Blood Flow • Arterial blood leaving the core is identical to body core temperature (370 C). • Tissues receiving a high blood perfusion rate have temperatures close to the core temperature. • Also true for skin • Because the skin is in contact with the environment, changing the blood flow to the skin also changes the temperature of the skin. • By changing the temperature of the skin, the temperature gradient between the body surface and the environment can be altered. • Via conduction, convection, radiation, and vaporization. Mechanisms by which Homeotherms increase Heat Dissipation • Mechanism by which skin temperature is increased • Vasodilation of skin vessels • A reflexive decrease in sympathetic discharge occurs in response to an increase in the temperature of blood perfusing the temperatureregulating center in the hypothalamus and/or stimulation of cutaneous temperature (warmth) receptors. • Opening of arterio-venous anastomoses in skin while venous flow through the venae comitantes (deep veins) decreases. • Arterial blood perfuses superficial skin veins (“flushing”). • Warm arterial blood perfuses the skin of the extremities. • Increased conduction and convection of heat from “core” to skin • Increased skin temperature • Increased heat dissipation by convection, radiation, and evaporation (Esw + Ein) Vasodilated Heat transfer from core to skin Forearm blood flow (ml/min per 100 g tissue) 15 10 5 Vasoconstricted 0 37 37.5 38 Environmental temperature (ºC) Core temperature (oC) Fig. 27-6, pg: 831 Role of the cutaneous circulation in thermoregulation Direct effect of increased temp. on resistance vessels Increased core temperature Decreased sympathetic adrenergic outflow to resistance vessels Vasodilation Increased blood flow Increased sympathetic cholinergic outflow to sweat glands Increased local bradykinin Increased Rate of Heat Loss Vasomotor responses to changes in ambient temperature are greatest in the extremities. Fingers Hands Arms and Legs 37ºC 37ºC 37ºC Core Core 32ºC Range of Blood Flow Rates (ml/min/100 ml tissue 0.5 to 90 Shell 28ºC 34ºC 1 to 20 Much smaller 31ºC Cold Warm Mechanisms by which Homeotherms increase Heat Dissipation • Increased Vaporization • Increased insensible water loss • Increased transudation of water through the skin due to increased cutaneous blood flow and skin temperature. • Increased sweating 2.5 X 106 sweat glands in humans • Reflexive increase in sympathetic discharge to the sweat glands via cholinergic post-ganglionic sympathetic neurons. • Occurs in response to • An increase in the temperature of blood perfusing the temperature-regulating center in the hypothalamus. • An increase in the temperature of cutaneous (skin) temperature (“warmth”) receptors • Some segmental reflex control by spinal centers (e.g., quadriplegics) Epidermis Excretory duct Absorption, mainly Na+ and Cl- ions Secretory duct Dermis Secretion, mainly protein free filtrate Sympathetic Cholinergic Post-Ganglionic Nerve During muscular exertion in a hot dry environment, the sweat secretion rate may reach as high as 1600 ml/h. 928 kcal dissipated per hour (0.58 kcal/g X 1600g/h) Sweat gland Mechanisms by which Homeotherms increase Heat Dissipation • Increased Vaporization • Increased insensible water loss Esw = he (Pws - faPWa)Aw/Ap • Increased sweating N.B. • The relative amount of heat dissipated by sweating depends on: • Skin Temperature • Area of wet skin/body surface area • Environmental temperature • When the body temperature is equal to or lower than the environmental temperature, heat can only be lost by evaporation (i.e., heat loss by conduction, convection, and radiation is zero or negative) • Relative humidity • If Esw must be maintained despite increasing humidity, then skin temperature and/or the area of wet skin must be increased. • Air movement • The value of he (water vaporization heat transfer coefficient) depends on air movement Mechanisms by which Homeotherms increase Heat Dissipation Panting • In animals with no sweat glands (e.g., dogs) • Rapid, shallow breathing • Increases water vaporization from the mouth and respiratory passages • Air moved primarily in respiratory “dead spaces” • Relatively little change in the composition of alveolar air Behavioral Mechanisms • Alter posture to expose more body surface area • Remove clothing • Move to area of lower environmental temperature • Increase air movement (e.g., fan) • Lower the environmental temperature (e.g., air conditioning) How can body core temperature be kept constant in a cold environment? Mechanisms by which Homeotherms decrease Heat Dissipation Mechanisms by which Homeotherms increase Heat Production Mechanisms by which Homeotherms decrease Heat Dissipation Control of Skin Temperature • Decrease skin temperature Vasoconstriction of skin vessels A direct effect of cold on vasculature (transient). A reflexive increase in sympathetic discharge occurs in response to: a fall in the temperature of blood perfusing the temperature-regulating center in the hypothalamus, and/or stimulation of cutaneous (cold) receptors. Closure of arterio-venous anastomoses in skin and shunting of venous blood to venae comitantes Mechanisms by which Homeotherms decrease Heat Dissipation • Decrease skin temperature Vasoconstriction of skin vessels results in: Decreased conduction and convection of heat from “core” to skin Decreased skin temperature Decreased heat dissipation by conduction, convection, radiation, and evaporation Tips of the extremities remain cold, but “core” body heat is conserved. 37ºC 37ºC 37ºC Core Core 32ºC Shell 28ºC 34ºC 31ºC Cold Warm Fig. 27-5, pg: 831 Mechanisms by which Homeotherms decrease Heat Dissipation Piloerection Contraction of microscopic bundles of smooth muscle cells attached at one end to hair follicles and at the other end to the surface of the basal layer of the epidermis. Reflexive increase in sympathetic discharge in response to: a fall in the temperature of blood perfusing the temperature-regulating center in the hypothalamus and/or stimulation of cutaneous (cold) receptors. Entraps an insulating layer of air next to the skin. Decreases the convective loss of heat from skin to air. Humans have a paucity of hair which limits the effectiveness of piloerection. Mechanisms by which Homeotherms decrease Heat Dissipation Abolition of Sweating Cooling of the temperature-regulating center in the hypothalamus below 36.8 0C (98.2 0F) completely abolishes sweating. Remember: Heat loss by insensible evaporation (Ein) continues. Behavioral Mechanisms Postural changes Decrease surface area Addition of clothing Take shelter from air movement Increase environmental temperature Move to an area of higher temperature Mechanisms by which Homeotherms increase Heat Production As the environmental temperature is lowered, the body heat losses by conduction, convection, and radiation become progressively greater. Periphery becomes cooler Mean body temperature may fall despite Maximal vasoconstriction Maximal piloerection Altered behavior If body “core” temperature is to be preserved in the face of an increase in the rate of heat loss,then heat production must be increased. Mechanisms by which Homeotherms increase Heat Production Increased muscle contractile activity Increased muscle tension Stimulation of “cold” receptors in the skin and spinal cord results in Reflexive activation of the primary motor center for shivering in the posterior hypothalamus. Prior to the onset of shivering, there occurs: an increased sensitivity of muscle spindle stretch reflex an increased tone of skeletal muscle, and increased heat production from skeletal muscle When muscle tone exceeds a critical level, then shivering begins due to a feedback oscillation of the stretch reflex mechanism. Maximal shivering Increase body heat production 2-5X Mechanisms by which Homeotherms increase Heat Production Increased muscle contractile activity Exercise Increases body heat production Increased body temperature Shivering and/or Exercise The resulting increased body temperature increases the difference between the body and the environmental temperatures. The rate of heat loss by conduction, convection, radiation, and vaporization is increased (compared to the rate if muscle activity did not occur). Rectal Temperature Skin Temperature Vaporization Heat Loss Mechanisms by which Homeotherms increase Heat Production • Endocrine Mechanisms • Adrenal Medulla • Epinephrine • Chemical Thermogenesis • Immediate, but short duration, increase in “faculative” or non-shivering thermogenesis • 10-15% increase in heat production in adults; as much as 100% in infants. • Brown Fat (uncouple oxidative phosphorylation) • Increased rate of catabolism of body fuels • Thyroid Gland • Thyroid hormones (T4 and T3) • Slow onset (weeks), but more prolonged, increase in metabolism and body heat production. • Increased “set point” for thyroid hormone feedback with increased circulating T4 and T3. • In addition, T4 and T3 potentiate effects of catecholamines. Mechanisms by which Homeotherms increase Heat Production • Endocrine Mechanisms • Adrenal Medulla • Epinephrine •Thyroid Gland • Thyroid hormones (T4 and T3) • Acclimation to Cold • Requires several weeks • Thyroid hormones, epinephrine, and other hormones interact to increase body heat production. Mechanisms by which Homeotherms increase Heat Production • Change in Composition of the Diet • Thermic Effect of Food (TEF) • Chemical energy is converted to heat during digestion and assimilation of food. protein > carbohydrate or fat Increase food intake Consume a diet high in protein Mechanisms by which Homeotherms decrease Heat Dissipation • Decrease skin temperature • Vasoconstriction of skin vessels; close venous anastomoses • Return venous blood in venae commitantes; counter-current cooling of blood perfusing the skin • Piloerection • Abolition of Sweating • Behavioral Mechanisms Mechanisms which increase Heat Production • Increased muscle contractile activity • Increased muscle tension • Shivering • Exercise • Endocrine Mechanisms • Adrenal Medulla • Epinephrine • Thyroid Gland • Thyroid hormones (T4 and T3) • Increase food intake • Change in Composition of the Diet Mechanisms by which Homeotherms increase Heat Dissipation • Increase skin temperature • Vasodilation of skin vessels • Decreased counter-current cooling of blood perfusing the skin • Increased Vaporization • Increased insensible water loss • Increased sweating • Behavioral Mechanisms Mechanisms which decrease Heat Production • Decreased muscle contractile activity • Decreased exercise • Change in Composition of the Diet • Decrease food intake Neural Regulation of Body Temperature • Body temperature is “regulated” almost entirely by nervous feedback control mechanisms. • Temperature-sensitive neurons are found in the following locations: • Hypothalamus (warmth and cold receptors), • Anterior hypothalamus • Hypothalamic preoptic area •Monitor temperature of blood perfusing these areas • Midbrain and spinal cord (warmth and cold receptors), • Abdominal viscera (warmth receptors only), • Skin (warmth and cold receptors). • Posterior Hypothalamic “Temperature-Regulating Center” • Integrates sensory information from temperature-sensitive neurons. • Generates efferent signals for controlling • Rate of heat loss • Rate of heat production Central Receptors Anterior Hypothalamus/Pre-optic Area Peripheral Skin Receptors Warm Cold Warm Cold Other Central Receptors Midbrain and Spinal Cord Warm Cold Posterior Hypothalamic TemperatureRegulating Center Integration Other Central Receptors Abdominal Visceral Receptors Warm only Efferent Neural Signals Controlling the Rates of Heat Loss and Heat Production Neural Regulation of Body Temperature Importance of the Sympathetic Nervous System • Required for the control of the following: • • • • Sweat gland secretion Control of blood vessel diameter Epinephrine secretion Piloerection Sympathectomy Loss of control of skin temperature Loss of ability to control the rate of loss of body heat Central Temperature Receptors Hypothalamic Temperature Experimantal Warming of Hypothalamus Experimental Cooling of the Hypothalamus Panting Vasodilation Sweating Rectal Temperature Shivering Vasoconstriction Rectal Temperature Interaction of Inputs from Central and Peripheral Receptors Threshold Core Temperatures for Sweating and Shivering • Sweating • There is a core temperature (36.8 0C) below which no sweating will occur regardless of skin temperature. • Shivering • There is a core temperature (37.10C) above which no shivering will occur regardless of skin temperature. Central Receptors Anterior Hypothalamus/Pre-optic Area Peripheral Skin Receptors Warm Cold Warm Cold Other Central Receptors Midbrain and Spinal Cord Warm Cold Posterior Hypothalamic TemperatureRegulating Center Integration Other Central Receptors Abdominal Visceral Receptors Warm only Efferent Signals Controlling the Rates of Heat Loss and Heat Production