The Thermometer

1. 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.

2. 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.

3. 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.

4. 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.”

5. 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.

6. 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)

7. 37ºC 37ºC 37ºC Core Core 32ºC Shell 28ºC 34ºC 31ºC Cold Warm • 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. • 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. Temperature regulatory systems act to maintain the “core temperature” at, or near, a “set point.”

8. Central Receptors Anterior Hypothalamus/Pre-optic Area Warm Cold Other Central Receptors Midbrain and Spinal Cord Warm Cold Posterior Hypothalamic Temperature-Regulating Center Integration Other Central Receptors Abdominal Visceral Receptors Warm only Peripheral Skin Receptors Warm Cold Efferent Signals Controlling the Rates of Heat Loss and Heat Production

9. 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

10. 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.

11. Heavy exercise Core temperature (ºC) Moderate exercise Mild exercise Time (min) Begin exercise Fig. 27-16, pg: 840

12. FEVER Core Temperature Core Temperature “Set Point” Heat Loss Heat Production • Temperature Regulatory System(s) Variations in Core Temperature • During Fever • Increase in the “set point” for body core temperature induced by • Pyrogens • Hypothalamic lesions

13. 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.

14. 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

15. IL-1 & IL-6 at the hypothalamus - NSAIDs Acting at

16. 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 Core temperature (ºC) Heat Loss increased 1. Skin vasodilation 2. sweating Days Fig. 27-15, pg: 837

17. FEVER Core Temperature Core Temperature “Set Point” Heat Loss Heat Production • 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.

18. Temperature Regulatory System(s) Fever Core Temperature Core Temperature “Set Point” Heat Loss Heat Production • “Chills” • Skin vasoconstriction ( Heat Loss) • Shivering ( Heat Production) • Until the new higher “set point” is reached. • Intense sweating • Skin vasodilation Heat Loss • 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.”

19. Energy Balance,Energy Expenditure, and Total Heat Production

20. Energy Balance - Chemical Energy of Food Work Done on External Environment Chemical Energy of New Tissues and Fat Stores Total Heat Production = + + Work Done on External Environment Energy Expenditure Total Heat Production = +  Energy Expenditure Total Heat Production Energy Expenditure The energy expended on work done on the external environment averages no more than about 1% of the total energy expenditure of the body

21. Physical Laws Governing Heat Exchange between Living Organisms and the Environment Evaporation to air Radiation Convection to air Evaporation to air Conduction to seat Conduction to handle bar

22. 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

23. C = 10 V (Ts - Ta) 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) • 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.

24. 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.

25. 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 THERMAL RADIATION • The net transfer of heat is the difference between the radiation emitted by a surface and that which it receives. In the equation above, the surface quality or emissivity (e) of a surface is an important factor.

26. 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’sclose to 1), but reflect other wavelengths quite well (with e’s close to 0).

27. 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!

28. Radiation Stefan-Boltzmann Law R = s e1, e2 (T4 - TW4) R = kr (Ts - TW) Kr = 4sTS3 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: • 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).

29. 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 R = kr (Ts - TW) Radiation 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.

30. Vaporization • Heat ofVaporization • 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

31. 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

32. 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 Vaporization E = Ein + Esw • Sweat Evaporation (Ein) Esw = he (Pws - faPWa)Aw/Ap

33. 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. • Thus, PWS is variable • The rate of sweating is controlled. • Thus, AW is variable. • Ambient temperature, • Relative humidity, and • Air velocity also affect the efficacy of heat loss by sweat evaporation. • Exposed Body Area (Ap) • Behavior may be altered • e.g., Clothing • Sweat Evaporation (Ein) Esw = he (Pws - faPWa)Aw/Ap

34. 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.

35. Rectal Temperature Skin Temperature Vaporization Heat Loss

36. Physical Laws Governing Heat Exchange between Living Organisms and the Environment Conduction D = k(T1 - T2) Convection Radiation Vaporization E = Ein + he (Pws - faPWa)Aw/Ap C = 10 V (Ts - Ta) R = kr (Ts - TW) • 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

37. Physical Laws Governing Heat Exchange between Living Organisms and the Environment SUMMARY S = M - E + (R + C + D)] Where: 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).

38. Patterns of Heat Loss from the Body during Different Environmental Conditions and Levels of Physical Activity

39. RATE OF HEAT LOSS SKIN TEMPERATURE RATE OF HEAT LOSS SKIN TEMPERATURE 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.

40. The transfer of body heat to the environment via conduction, convection, or radiation requires a favorable temperature gradient between the skin and the environment. E = Ein + Esw E = Ein + he (Pws - faPWa)Aw/Ap • The transfer of body heat to the environment via vaporization requires a difference in saturated water vapor pressures at the skin and air temperatures C = 10 V (Ts - Ta) R = kr (Ts - TW) D = k(T1 - T2) SKIN TEMPERATURE AND HEAT LOSS • 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. • 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.

41. Scenario #1 Ambient Air Temperature = 200C Pwa = 17.535 mmHg Relative Humidity = 50% Skin Temperature = 320C Pws = 35.66 mmHg 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 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 E = Ein + he (Pws - faPWa)Aw/Ap Esw = he (35.66 mmHg - 0.5[17.535 mmHg]) Aw/Ap Esw = he(26.89 mmHg) Aw/Ap Positive value indicates a favorable water vapor pressure gradient between the skin and the ambient air. Water vapor pressure gradient less favorable than in Scenario #1 Raising skin temperature increases the water vapor pressure gradient.

42. Conduction D = k(T1 - T2) Convection Radiation Vaporization E = Ein + he (Pws - faPWa)Aw/Ap C = 10 V (Ts - Ta) R = kr (Ts - TW) Mechanisms by which Homeotherms increase Heat Dissipation • Increased skin temperature • Improves the rate of heat loss to the environment by

43. How can body core temperature be kept constant in a warm environment? Mechanisms by which Homeotherms increase Heat Dissipation

44. 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.

45. 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 temperature-regulating 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)

46. Vasodilated 15 Heat transfer from core to skin Forearm blood flow (ml/min per 100 g tissue) 10 5 Vasoconstricted Environmental temperature (ºC) Core temperature (oC) 0 37 37.5 38 Fig. 27-6, pg: 831

47. Increased core temperature Increased blood flow Increased Rate of Heat Loss Role of the cutaneous circulation in thermoregulation Direct effect of increased temp. on resistance vessels Decreased sympathetic adrenergic outflow to resistance vessels Vasodilation Increased sympathetic cholinergic outflow to sweat glands Increased local bradykinin

48. Vasomotor responses to changes in ambient temperature are greatest in the extremities. 37ºC 37ºC 37ºC Core Core 32ºC Shell 28ºC 34ºC 31ºC Cold Warm

49. 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)

50. 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) Epidermis Excretory duct Absorption, mainly Na+ and Cl- ions Secretory duct Dermis Secretion, mainly protein free filtrate Sympathetic Cholinergic Post-Ganglionic Nerve Sweat gland