the thermometer 1592 -- galileo produces the first thermometer early instruments contained water,...

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. HomeothermicPoikilothermic vs. HomeothermicVertebratesVertebrates

PoikilothermsPoikilotherms (“cold-blooded”)

• Body temperature fluctuates over a considerable range with changing environmental temperature.

• Behavioral temperature regulation.

• Reptiles, amphibia, and fish

HomeothermsHomeotherms (“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)

Cold

31ºC

34ºC

28ºC

37ºC

32ºC

37ºC37ºC

Warm

Shell

CoreCore• 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 System(s)Temperature Regulatory System(s)

What does the system regulate?• Core temperature

• varies little with changes in environmental temperature.

Temperature regulatory systems act to maintain the “core temperature”

at, or near, a “set point.”

Central ReceptorsAnterior Hypothalamus/Pre-optic Area

Warm Cold

Other Central ReceptorsMidbrain and Spinal Cord

WarmCold

Other Central ReceptorsAbdominal Visceral

Receptors

Warm only

Posterior Hypothalamic Temperature-

Regulating Center

Integration

PeripheralSkin

Receptors

WarmCold

Efferent Signals Controlling the

Rates of Heat Loss and Heat Production

Variations in Core Temperature

• Normal Range: Rectal 97-100F (36.1 - 37.8 C)

• 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 - 1.30F (0.5 - 0.7C)

• 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 104F (40C)

• Rise in body temperature is limited by thermoregulatory systems which increase heat dissipation.

Fig. 27-16, pg: 840

Time (min)Beginexercise

Coretemperature

(ºC)

Mild exercise

Moderate exercise

Heavy exercise



• During Fever

• Increase in the “set point” for body core temperature induced by

• PyrogensPyrogens

• Hypothalamic lesionsHypothalamic lesions

Core Temperature“Set Point”

Heat LossHeat Production

CoreTemperature

FEVERFEVER

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.

Increased heat production, reduced heat loss - vasoconstriction

- shivering- behavior

Elevation of hypothalamic temperature set point

Elevation of hypothalamic temperature to a new set point fever

Increased prostaglandin E2 synthesis in the hypothalamus

Production of interleukins 1 and 6

Activated immune response cells- leukocytes- mesangial cells- vascular endothelial cells- astrocytes

Antigens recognized as foreign- infectious- autoimmune- neoplastic

NSAIDs

-

IL-1 & IL-6IL-1 & IL-6at the at the

hypothalamushypothalamus

Acting at Acting at

Fig. 27-15, pg: 837

Fever cessationdecreaseshypothalamictemperatureset point

Heat Lossincreased1. Skin vasodilation2. sweating

Heat gain increasedand heat lossreduced1. Skin vasoconstriction2. shivering

Fever increaseshypothalamic temperatureset point

Co

re t

emp

erat

ure

(º C

)

Days



• 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.



CoreTemperature

FEVERFEVER




CoreTemperature

FeverFever

“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.”

Heat Loss• Intense sweating• Skin vasodilation

Energy Balance,Energy Balance,Energy Expenditure, Energy Expenditure,

and and Total Heat ProductionTotal Heat Production

Energy ExpenditureEnergy Expenditure

Energy BalanceEnergy Balance

EnergyExpenditure

=Work Doneon 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

EnergyExpenditure

Total HeatProduction

ChemicalEnergyof Food

=Work Doneon External

Environment+

Chemical Energyof New Tissuesand Fat Stores

+Total HeatProduction--

Physical Laws Governing Heat Exchange Physical Laws Governing Heat Exchange between Living Organisms and the between Living Organisms and the EnvironmentEnvironment

Evaporation to air

Conduction tohandle bar

Radiation

Convection to air

Conduction to seat

Evaporation to air

CONDUCTIONCONDUCTION

D = k(T1 - T2)

≡ 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)

• 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

k = conductance = thermal conductivity divided by length of conducting pathway and multiplied by area of contact

T1, T2 = temperatures of warm and cool surfaces

≡ 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)

CONVECTIONCONVECTION

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.

≡ 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 RADIATIONTHERMAL RADIATION

•The net transfer of heat is the difference between the radiation emitted by a surface and that which it receives.

THERMAL RADIATIONTHERMAL RADIATION

R = 1, 2 (T4 - TW4)

Stefan-Boltzmann LawStefan-Boltzmann Law

where: R = radiant heat transfer in W/m2

= 5.75 X 10-8 W/m2 0K4 (Stefan-Boltzmann constant)

T, TW = Temperatures of hot object and surface of absorbing object (0K), respectively

1, 2 = Emissivities of radiator surface and absorbing surfaces, respectively

In the equation above, the surface quality or In the equation above, the surface quality or emissivity (emissivity () of a surface is an important factor.) of a surface is an important factor.

Thermal RadiationThermal Radiation An object with an emissivity () = 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 () = 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 ’sclose to 1) , but reflect other wavelengths quite well (with ’s close to 0) .

Thermal RadiationThermal Radiation

Human Skin Colors

The emissivity () 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 ()) for light of various wavelengths.

All human skin, regardless of color, is an excellent absorber/radiator in the infrared wavelengths ( is close to 1) .

For thermal radiation, human skin is a “black body absorber/radiator”

All skin is black to infrared radiation!

RadiationRadiationR = 1, 2 (T4 - TW

4)Stefan-Boltzmann LawStefan-Boltzmann Law

Human Skin: 97% perfect infrared “black body” absorber/radiator

Rate of heat transfer by thermal radiation to and from the body:

• 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” (1 = 2 = 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 = 4TS3

• 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).

Standing man with arms at his side 75

Standing man with arms and legs extended 85

Man in tightly curled-up position 50

Effective radiating area(% of total body area)

Heat transfer by radiation to and from the body:

RadiationRadiation R = kr (Ts - TW)

• 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.

•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

VaporizationVaporization

E = Ein + EswVaporizationVaporization

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

• 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.

• Insensible Evaporation (Ein)

E = Ein + EswVaporizationVaporization

• Sweat Evaporation (Ein)

Pws = water vapor pressure of saturated air at skin temperaturePwa = water vapor pressure saturated air at ambient air

temperatureAw = area of wet skin

a = relative humidity

Ap = body area hwater vaporization heat transfer coefficient that

depends on the air velocity

where:

Esw = h (Pws - aPWa)Aw/Ap

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

Evaporation of Sweat (ESW) Skin temperature is controlled.

Thus, PWS is variable The rate of sweating is controlled.

Thus, AW is variable.

Pws = water vapor pressure of saturated air at skin temperaturePwa = water vapor pressure saturated air at ambient air

temperatureAw = area of wet skin

a = relative humidity

Ap = body area hwater vaporization heat transfer coefficient that

depends on the air velocity

where:

Esw = h (Pws - aPWa)Aw/Ap

• Sweat Evaporation (Ein)

VaporizationVaporization E = Ein + Esw

At 30 C

• 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.

Heat Loss

Vaporization

Rectal Temperature

Skin Temperature

Physical Laws Governing Heat Exchange between Physical Laws Governing Heat Exchange between Living Organisms and the EnvironmentLiving Organisms and the Environment

VaporizationVaporization E = Ein + h (Pws - aPWa)Aw/Ap


ConvectionConvection C = 10 V (Ts - Ta)

ConductionConduction D = k(T1 - T2)

N.B.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

If the rate of body heat storage (S) is zero, thenIf the rate of body heat storage (S) is zero, then

M = - E + ( R + C + D)]

Where: S = rate of body heat storageM = total metabolic rate (i.e., total heat production)E = evaporative heat loss rateR + C + D = rates of heat gain (or loss) by radiation, convection, or conduction

SUMMARYSUMMARY S = M - E + (R + C + D)]

Physical Laws Governing Heat Exchange between Physical Laws Governing Heat Exchange between Living Organisms and the EnvironmentLiving Organisms and the Environment

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).

TABLE 1CONDITION AMBIENT

TEMPERATUREHEAT LOSS BYCONVECTION

HEAT LOSS BYRADIATION

HEAT LOSS BYVAPORIZATION

At rest, lying instill dry air

300 C

(thermoneutral)5 – 25 % 50 – 75 % 25 %


22-280C

(cold)increase increase decrease

Shivering, lyingin still dry air

22-280C

(cold)greater increase greater increase same decrease


> 300 < 37

0 C

(hot)decrease decrease increase


> 370 C

(very hot)0 0 greater increase

Exercise 300 C

(thermoneutral)increase increase graded increase

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

Temperature RegulationTemperature RegulationPatterns of Heat LossPatterns of Heat Loss

SKIN TEMPERATURE AND HEAT LOSSSKIN 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 TEMPERATURERATE OFHEAT LOSS

SKIN TEMPERATURERATE OFHEAT LOSS

• The transfer of body heat to the environment via vaporization requires a difference in saturated water vapor pressures at the skin and air temperatures

E = Ein + Esw

E = Ein + h (Pws - aPWa)Aw/Ap

SKIN TEMPERATURE AND HEAT LOSSSKIN TEMPERATURE AND HEAT LOSS

R = kr (Ts - TW)

C = 10 V (Ts - Ta)

D = k(T1 - T2)

• The transfer of body heat to the environment via conduction, convection, or radiation requires a favorable temperature gradient between the skin and the environment.

• 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 aPwa

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 - aPWa) may also be increased, increasing the efficiency of sweat evaporation.

Scenario #1Skin Temperature = 320CPws = 35.66 mmHg

Ambient Air Temperature = 200CPwa = 17.535 mmHgRelative Humidity = 50%

Esw = h (35.66 mmHg - 0.5[17.535 mmHg]) Aw/Ap

Esw = h(26.89 mmHg) Aw/Ap



Scenario #2

Same as #1, but raise relative humidity to 95%



Scenario #3

Same as #2, but raise skin temperature to 350 C and, consequently, raise Pws

E = Ein + h (Pws - aPWa)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.

Mechanisms by which Homeotherms Mechanisms by which Homeotherms increase increase HeatHeat Dissipation Dissipation

• Increased skin temperature• Improves the rate of heat loss to the

environment by

VaporizationVaporization E = Ein + h (Pws - aPWa)Aw/Ap


ConvectionConvection C = 10 V (Ts - Ta)

ConductionConduction D = k(T1 - T2)

Mechanisms by which Mechanisms by which Homeotherms Homeotherms

increase increase HeatHeat Dissipation Dissipation

How can body core temperature be How can body core temperature be kept constant in a warm environment?kept constant in a warm environment?

• 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.

Control of Skin TemperatureControl of Skin Temperature



• 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)

Fig. 27-6, pg: 831

Hea

t tr

ansf

er f

rom

co

re t

o s

kin

Vasodilated

Vasoconstricted

Environmental temperature (ºC)Core temperature (oC)

Fo

rear

m b

loo

d f

low

(ml/

min

per

100

g t

issu

e)

0

5

10

15

37 37.5 38

Increasedcore

temperature

Direct effect of increasedtemp. on resistance vessels

Decreased sympatheticadrenergic outflow to resistance vessels

Increased sympatheticcholinergic outflow to sweat glands

Increasedlocalbradykinin

VasodilationIncreased

bloodflow

Role of the cutaneous circulation in thermoregulation

Increased Rate of Heat

Loss

Cold

31ºC

34ºC

28ºC

37ºC

32ºC

37ºC37ºC

Warm

Shell

CoreCore

Vasomotor responses to changes in ambient temperature are greatest in the extremities.

Range of Blood Flow

Rates(ml/min/100

ml tissueFingers 0.5 to 90

Hands 1 to 20

Arms and Legs Much smaller


• 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)

Sweat gland

Dermis

Epidermis

Secretion,mainly protein free filtrate

Absorption,mainly Na+ andCl- ions

Secretory duct

SympatheticCholinergicPost-GanglionicNerve

Excretory ductDuring muscular During muscular exertion in a hot exertion in a hot dry environment, dry environment, the sweat the sweat secretion rate secretion rate may reach as may reach as high as 1600 high as 1600 ml/h.ml/h.

928 kcal928 kcal dissipated dissipated

per hourper hour(0.58 kcal/g X 1600g/h)(0.58 kcal/g X 1600g/h)

• Increased Vaporization• Increased insensible water loss• Increased sweating Esw = h (Pws - aPWa)Aw/Ap

N.B.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 h (water vaporization heat transfer coefficient)

depends on air movement



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)

Mechanisms by which Homeotherms decrease Heat Dissipation

How can body core temperature be How can body core temperature be kept constant in a cold environment?kept constant in a cold environment?

Mechanisms by which Homeotherms increase Heat Production Heat Production

Mechanisms by which Homeotherms Mechanisms by which Homeotherms decrease decrease Heat Heat DissipationDissipation

• 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

Control of Skin TemperatureControl of Skin Temperature


• 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.

Fig. 27-5, pg: 831Cold

31ºC

34ºC

28ºC

37ºC

32ºC

37ºC37ºC

Warm

Shell

CoreCore

Mechanisms by which Homeotherms Mechanisms by which Homeotherms decrease decrease HeatHeat DissipationDissipation

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.


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 Mechanisms by which Homeotherms increaseincrease Heat ProductionHeat Production

As the environmental temperature is lowered, the body heat losses by conduction, convection, and radiation become progressively greater.Periphery becomes coolerMean 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 productionheat production must be increased.


Increased muscle contractile activity Increased muscle tension

Stimulation of “cold”“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


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).

Increased muscle contractile activity Exercise

Increases body heat production Increased body temperature

Heat Loss

Vaporization

Rectal Temperature

Skin Temperature


• Endocrine Mechanisms • Adrenal MedullaAdrenal Medulla

• EpinephrineEpinephrine• 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 GlandThyroid Gland• Thyroid hormones (TThyroid hormones (T44 and T and T33))

• 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.



• EpinephrineEpinephrine•Thyroid GlandThyroid Gland

• Thyroid hormones (TThyroid hormones (T44 and T and T33))

• Acclimation to ColdAcclimation to Cold• Requires several weeksRequires several weeks

• Thyroid hormones, epinephrine, and other hormones interact to increase body heat production.


• Change in Composition of the Diet • Thermic Effect of Food (TEF)Thermic Effect of Food (TEF)

• Chemical energy is converted to heat during Chemical energy is converted to heat during digestion and assimilation of food. digestion and assimilation of food.

protein > carbohydrate or fatprotein > carbohydrate or fat

Increase food intakeIncrease food intake Consume a diet high in proteinConsume a diet high in protein

Mechanisms which increaseMechanisms which increase Heat ProductionHeat Production

• Increase food intake• Change in Composition of the Diet


• EpinephrineEpinephrine• Thyroid GlandThyroid Gland

• Thyroid hormones (TThyroid hormones (T44 and T and T33))

• Increased muscle contractile activity• Increased muscle tension • Shivering• Exercise

• Decrease skin temperature• Vasoconstriction of skin vessels; close venous anastomoses• Return venous blood in venae commitantes; counter-current

cooling of blood perfusing the skin

• Abolition of Sweating• Behavioral Mechanisms

Mechanisms by which Homeotherms decreaseMechanisms by which Homeotherms decrease Heat DissipationHeat Dissipation

• Piloerection

Mechanisms which decrease Mechanisms which decrease Heat ProductionHeat Production

• Change in Composition of the Diet• Decrease food intake

• Decreased muscle contractile activity• Decreased exercise

• Increase skin temperature• Vasodilation of skin vessels• Decreased counter-current cooling of blood perfusing the skin

Mechanisms by which Homeotherms increaseMechanisms by which Homeotherms increase Heat DissipationHeat Dissipation

• Increased Vaporization• Increased insensible water loss• Increased sweating

• Behavioral Mechanisms

Neural Regulation of Body TemperatureNeural Regulation of Body Temperature

• Body temperature is “regulated” almost entirely by nervous feedback control mechanisms.

• Temperature-sensitive neurons are found in the following locations:

• HypothalamusHypothalamus (warmth and cold receptors),• Anterior hypothalamus• Hypothalamic preoptic area

•Monitor temperature of blood perfusing these areas

• Midbrain and spinal cordMidbrain and spinal cord (warmth and cold receptors),

• Abdominal visceraAbdominal viscera (warmth receptors only),

• SkinSkin (warmth and cold receptors).

• Posterior Hypothalamic “Temperature-Regulating Center”

• Integrates sensory information from temperature-sensitive Integrates sensory information from temperature-sensitive neurons.neurons.

• Generates efferent signals for controllingGenerates efferent signals for controlling• Rate of heat loss• Rate of heat production


Warm Cold


WarmCold


Receptors

Warm only


Regulating Center

Integration

PeripheralSkin

Receptors

WarmCold

Efferent Neural Signals Controllingthe Rates of Heat Loss

and Heat Production

Neural Regulation of Body TemperatureNeural 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 ability to control the rate of

loss of body heat

Loss of control of skin temperature

Hypothalamic Temperature

Experimantal Warming of

Hypothalamus

RectalTemperature

PantingVasodilation

Sweating

Experimental Cooling of the Hypothalamus

ShiveringVasoconstriction

RectalTemperature

Central Temperature 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.

Interaction of Inputs from Central and Interaction of Inputs from Central and Peripheral ReceptorsPeripheral Receptors


Warm Cold


WarmCold


Receptors

Warm only


Regulating Center

Integration

PeripheralSkin

Receptors

WarmCold

Efferent Signals Controlling the

Rates of Heat Loss and Heat Production

the thermometer 1592 -- galileo produces the first thermometer early instruments contained water,...

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