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 37ºC 37ºC 37ºC
temperature. Core Core
• Total body heat content 32ºC
is not regulated.
• In general, the body Shell
surface and extremities are
cooler than the “core.” 28ºC
• The magnitude of the
differences between the
body surface and 34ºC
extremities and the “core”
varies with environmental
temperature. 31ºC
Temperature regulatory systems act
to maintain the “core temperature”
at, or near, a “set point.”
Cold Warm
Peripheral
Central Receptors Skin
Anterior Hypothalamus/Pre-optic Area Receptors
Warm Warm
Cold Cold
Other Central Receptors
Midbrain and Spinal Cord Posterior
Hypothalamic
Warm Temperature-
Cold Regulating Center
Integration
Other Central Receptors
Abdominal Visceral
Receptors Efferent Signals
Controlling the
Warm only 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 Moderate exercise
temperature
(ºC)
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 Heat Loss Core
“Set Point” Heat Production 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
-
NSAIDs
cting at
Fever cessation
Fever increases decreases
hypothalamic temperature hypothalamic
set point temperature
set point
Heat gain increased
and heat loss
reduced Heat Loss
1. Skin vasoconstriction increased
2. shivering 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 Heat Loss Core
“Set Point” Heat Production Temperature
Temperature Regulatory System(s)
Fever
Core Temperature Heat Loss Core
“Set Point” Heat Production 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
Work Done
Energy = on External + Total Heat
Expenditure Environment 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
Exposed Body Area (Ap) heat loss by sweat
• Behavior may be altered evaporation.
• 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 V (Ts - Ta)
Radiation R = kr (Ts - TW)
Vaporization 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 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).
Patterns of Heat Loss from the Body during Different
Environmental Conditions and Levels of Physical Activity
TABLE 1
CONDITION AMBIENT HEAT LOSS BY HEAT LOSS BY HEAT LOSS BY
TEMPERATURE CONVECTION RADIATION VAPORIZATION
0
At rest, lying in 30 C 5 – 25 % 50 – 75 % 25 %
still dry air (thermoneutral)
0
At rest, lying in 22-28 C increase increase decrease
still dry air (cold)
0
Shivering, lying 22-28 C greater increase greater increase same decrease
in still dry air (cold)
0 0
At rest, lying in > 30 37 C 0 0 greater increase
still dry air (very hot)
0
Exercise 30 C increase increase graded increase
(thermoneutral)
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.
RATE OF
SKIN TEMPERATURE HEAT LOSS
RATE OF
SKIN TEMPERATURE HEAT LOSS
SKIN TEMPERATURE AND HEAT LOSS
• The transfer of body heat to the R = kr (Ts - TW)
environment via conduction, convection,
C = 10 V (Ts - Ta)
or radiation requires a favorable
temperature gradient between the skin and D = k(T1 - T2)
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.
• The transfer of body heat to the E = Ein + Esw
environment via vaporization E = Ein + he (Pws - faPWa)Aw/Ap
requires a difference in saturated water vapor pressures at the skin
and air temperatures
• 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 Ambient Air Temperature = 200C
Pws = 35.66 mmHg Pwa = 17.535 mmHg
Relative Humidity = 50%
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
Scenario #2 skin and the ambient air.
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 V (Ts - Ta)
Radiation R = kr (Ts - TW)
Vaporization 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 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)
15 Vasodilated
Heat transfer from core to skin
(ml/min per 100 g tissue)
Forearm blood flow
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 Decreased sympathetic Increased
core adrenergic outflow to Vasodilation blood
temperature resistance vessels flow
Increased sympathetic
cholinergic outflow to
sweat glands
Increased
local
bradykinin
Increased
Rate of Heat
Loss
Vasomotor responses to
changes in ambient
37ºC 37ºC 37ºC
temperature are greatest
in the extremities. Core Core
32ºC
Range of
Shell
Blood Flow
Rates
28ºC
(ml/min/100
ml tissue
Fingers 0.5 to 90
34ºC
Hands 1 to 20
Arms and Legs Much 31ºC
smaller
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)
Excretory duct Epidermis
Absorption, During muscular
mainly Na+ and exertion in a hot
Cl- ions dry environment,
the sweat
secretion rate
Secretory duct may reach as
Secretion,
Dermis high as 1600
mainly protein ml/h.
free filtrate
Sympathetic
Cholinergic
Post-Ganglionic 928 kcal
Nerve
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
• Increased sweating Esw = he (Pws - faPWa)Aw/Ap
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
Peripheral
Central Receptors Skin
Anterior Hypothalamus/Pre-optic Area Receptors
Warm Warm
Cold Cold
Other Central Receptors
Midbrain and Spinal Cord Posterior
Hypothalamic
Warm Temperature-
Cold Regulating Center
Integration
Other Central Receptors
Abdominal Visceral
Receptors Efferent Neural Signals Controlling
the Rates of Heat Loss
Warm only 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 Panting Rectal
Warming of Vasodilation Temperature
Hypothalamus Sweating
Experimental Shivering Rectal
Cooling of the Vasoconstriction Temperature
Hypothalamus
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.
Peripheral
Central Receptors Skin
Anterior Hypothalamus/Pre-optic Area Receptors
Warm Warm
Cold Cold
Other Central Receptors
Midbrain and Spinal Cord Posterior
Hypothalamic
Warm Temperature-
Cold Regulating Center
Integration
Other Central Receptors
Abdominal Visceral
Receptors Efferent Signals
Controlling the
Warm only Rates of Heat Loss
and Heat Production