Download Temperature Regulation

Thomas A. Pressley
TTUHSC Physiology
Temperature Regulation (Lecturer Notes)
This combination of PowerPoint Presentation, handout, and lecturer notes represents a self-contained
lecture on thermoregulation appropriate for first- or second-year medical students, as well as
graduate students. As a “hook” to hold student interest, it takes several examples from the cinema.
The material can be easily covered in two 50-minute presentations, or condensed, as appropriate,
into a single 50-minute presentation.
I.
Introduction
A.
Temperature regulation (Slide)
1.
Body Heat was a psychological thriller from Lawrence Kasdan in the
tradition of film noir
a.
A heat wave in Florida and the reactions of the local inhabitants are
a major contributor to the plot
2.
Derangements in our ability to control body heat decrease our quality of life,
when mild, and produce medical problems, when severe
B.
Mammals and birds are endotherms
1.
Heat generated from within, i.e., body heat
2.
High metabolic rate provides most of that heat
a.
Heat production at rest is about 80 kcal/h
3.
Body temperature is relatively constant (Slide)
a.
Changes little despite changes in external environment
(1)
Nude individual exposed to dry air for several hours
(2)
Prescriptive zone is the external temperature range in which
we can avoid major changes in internal temperature (Slide)
b.
Heat capacity of water resists rapid changes in temperature
C.
But are we homeotherms?
1.
On a large scale, we are, but
2.
Temperature changes with time and activity
a.
Circadian rhythm of about 1o C
(1)
Temperature is often lowest in the morning
b.
Menstrual cycle
(1)
Slight elevation in temperature after ovulation
c.
Exercise
(1)
Temperature may increase several degrees Celsius
(2)
Heat production may increase more than tenfold
3.
Temperature of skin and extremities may vary drastically with changes in
environment and subsequent thermoregulation
II.
Processes of Thermal Transfer (Slide)
A.
All can occur in both directions, with heat moving from the warm to cold
B.
Radiation
1.
Infrared light
a.
Not dependent on contact with external material
2.
Not involved in internal heat transfer
Thomas A. Pressley
TTUHSC Physiology
3.
C.
D.
E.
F.
III.
Visible by IR photography (Slide)
a.
In a scene from John McTiernan’s Predator, the alien is able to see
the heat loss from Arnold Schwarzenegger’s character in the infrared
b.
Later, Schwarzenegger is able to elude the alien by covering his
extremities with cool mud, thus masking temporarily the infrared
Conduction (Slide)
1.
Flow of heat by contact
a.
No bulk movement of fluid
2.
Important within the body and between body and environment
a.
Immersion in water can greatly increase rate of heat transfer
(1)
As we’ll see later, this can lead to hypothermia in cold water,
hyperthermia in hot water
3.
Reduced by insulators such as fat, hair, and clothing
a.
e.g., a wet suit can minimize heat loss when diving in cold water
Convection
1.
Bulk movement of fluid
2.
Natural convection
a.
Hot fluids rise
3.
Forced convection
a.
Wind and fans improve heat transfer between body and environment
(1)
Every movie set in the South will have a scene with old men
and women sitting on the front porch, fanning themselves
b.
Circulation improves heat transfer within body
Evaporative heat loss
1.
High heat of vaporization for water
a.
Effective means of removing heat
2.
Insensible evaporation
a.
Skin and respiratory tract
b.
About 600 ml per day
3.
Sensible evaporation
a.
Sweat
Typical consequence of excessive sun exposure (Slide)
Mechanisms of Temperature Regulation
A.
Behavior
1.
Most potent mechanism
2.
Active avoidance of temperature extremes (Slide)
a.
Extremely hot object elicits withdrawal response
b.
Notice the reaction of Peter Boyle’s character to heat in Mel Brook’s
Young Frankenstein
3.
Changes in posture to increase or decrease exposed surfaces
B.
Changes in peripheral circulation
1.
Adjusts relative thickness of insulating shell (Slide)
a.
Surrounds thermogenic core
2.
Countercurrent heat exchange (Slide)
Thomas A. Pressley
TTUHSC Physiology
a.
b.
3.
IV.
Shift in blood flow between superficial and deep veins of extremities
Regulates transfer of heat between warm arterial blood and cooler
venous blood
c.
Similar to countercurrent mechanism in kidney
Cutaneous blood vessels (Slide)
a.
Blood flow to the skin is greater than metabolic requirements
b.
Transfers heat to surface
c.
Venous plexus
(1)
Elaboration of the veins to form large reservoir for blood
(2)
Can fill with blood, maximizing its exposure to surface
temperature
d.
Sympathetic innervation of arterioles regulates blood flow
e.
Ears, hands, and feet also have arteriovenous shunts
(1)
When these shunts open, the venous plexus can become
engorged with blood
Response to a Cold Environment
A.
Strategy is to minimize heat loss and maximize heat production
1.
Most important response is behavioral (Slide)
a.
In David Lean’s Dr. Zhivago, Omar Sharif’s character displays many
of the behavioral adaptations to cold
(1)
Closed posture
(a)
Minimizes surface area by keeping extremities close
to the body
(2)
Increased clothing
(3)
Desire to be near external heat sources
B.
Increased sympathetic activity
1.
Shunt blood away from the skin
a.
If extreme, the resulting ischemia can lead to increased flow
b.
Origin of the flush seen during cold exposure
(1)
Often a precursor to frost bite
2.
Increase thickness of insulating shell
3.
Stimulates nonshivering thermogenesis
a.
Newborns are at thermoregulatory risk because of their large
surface:volume ratio and immature thermostat
b.
Brown adipose tissue acts as thermogenic organ
(1)
Interscapular pads of fat produce heat by uncoupling of
mitochondrial oxidative phosphorylation
c.
Not very important in adults
(1)
Found in many small mammals
4.
Initiates piloerection
a.
“Goosebumps”
b.
Raise hair to increase insulating layer
(1)
Not very effective in humans
c.
Useful as a diagnostic tool
Thomas A. Pressley
TTUHSC Physiology
C.
D.
V.
Shivering thermogenesis
1.
Asynchronous contractions of antagonistic muscles
a.
Nonvoluntary
b.
Dorsomedial portion of the posterior hypothalamus contains the
primary motor area for shivering
(1)
Center becomes activated with decrease in body temperature
(2)
Signals to motoneurons increase muscle tone
c.
Actual shivering is probably caused by a feedback oscillation of the
spindle stretch reflex mechanism
2.
Very effective source of heat
a.
May increase heat production 4-5-fold
3.
Rarely seen in movies because it is difficult to simulate and not very
photogenic
Long-term cold exposure
1.
Robert Flaherty’s Nanook of the North was the first theatrical documentary
a.
It showed the daily lives of Arctic natives
2.
Stimulation of TRH release
a.
Demonstrated in animals
b.
Not as clear how important this may be in humans
(1)
Some reports of higher resting metabolic rates in Inuit tribes
and other long-term inhabitants of higher latitudes
Response to Warm Environment (Slide)
A.
Strategy is to maximize heat loss
1.
Most important response is behavioral
a.
Open posture
(1)
Holding extremities away from body maximizes surface area
b.
Minimal activity
c.
Avoidance of heat sources
B.
Decreased sympathetic activity
1.
Dilation of the cutaneous circulation
a.
A-V shunts open
C.
Sudomotor response
1.
Sweating
a.
In Body Heat, the dialog contains many references to sweat
b.
Only way to transfer heat to an environment warmer than the body
(1)
Efficiency depends on relative humidity
c.
Evaporative heat loss
(1)
Specific increase in cholinergic pathways of sympathetic
system
(2)
Stimulates secretion by sweat glands
(a)
Perspiration is hypotonic relative to most body fluids
(b)
Slower rates of sweating (long transit time in duct)
allow reabsorption of Na+ and Cl(c)
Greater rates of salt loss with increased sweating
Thomas A. Pressley
TTUHSC Physiology
i)
D.
E.
VI.
Loss of calcium can become a problem with
chronic bouts of sweating
(3)
May also promote release of local vasodilator
Decreased heat production
1.
Thermogenic mechanisms discussed earlier will be inhibited
Long-term exposure (4-6 wk)
1.
Enhanced ability to sweat
a.
Increased aldosterone levels produce a more copious, dilute sweat
(1)
Minimizes salt loss by increasing Na+ reabsorption
b.
Can achieve 2 l/h, or about 1200 kcal/h
c.
Avoids heat-induced hypovolemia
Control of Thermoregulatory Mechanisms
A.
Two classes of thermoreceptors
1.
Cutaneous sensors
a.
Detect external temperature
b.
Allows anticipation of thermal regulatory burden
2.
Internal sensors
a.
Located in the CNS and viscera
b.
Probably monitor the thickness of the insulating shell
B.
The hypothalamic “thermostat” (Slide)
1.
Produces appropriate thermoregulatory actions in response to deviations from
the set-point temperature
a.
Preoptic area integrates signals from the various thermoreceptors
b.
Integrated signals are compared with set-point
c.
Resulting error signal triggers appropriate actions by the control
centers in the posterior hypothalamus
2.
Anesthetics and alcohol can disable the thermostat
a.
Surgery patients can become poikilothermic
C.
Fever
1.
Elevation of core temperature that results from an increase in set-point
temperature
a.
Not equivalent to hyperthermia
b.
Normal thermoregulatory mechanism are intact
c.
Large impact on energy balance and metabolic requirements
2.
Typical time course of a fever (Slide)
a.
There are several movies in which a sick-bed scene with a febrile
patient is a key plot device
b.
At onset, set-point increases
c.
Normal response to decreased body temperature
(1)
Shivering, vasoconstriction
(2)
The “chill”
d.
When the fever ends, there is a normal response to an elevated
temperature
(1)
Sweating, vasodilation
Thomas A. Pressley
TTUHSC Physiology
(2)
3.
4.
VII.
The “flush”
(a)
Sign that crisis is past
Mechanism (Slide)
a.
Pyrogens are fever producing substances
b.
Pathogenic organisms and their products act as exogenous pyrogens
(1)
Stimulate phagocytic cells of the immune system to secrete
cytokines
(a)
Cytokines stimulate host defense response
(b)
Act as endogenous pyrogens
i)
Includes the interleukins, tumor necrosis
factors, and interferons
ii)
Stimulate endothelial cells of the blood-brain
barrier to secrete prostaglandins such as PGE2
iii)
Aspirin works as anti-pyretic by inhibiting
prostaglandin synthesis
c.
Hypothalamus responds with increased set-point
Function of fever
a.
Many studies suggest that elevated temperatures may increase
efficiency of immune system
(1)
Increased rate of T-lymphocyte proliferation?
b.
Another suggestion, that higher temperatures impair the growth of
pathogens, is probably not realistic
(1)
Bacteria are fully capable of growing at higher-than-normal
body temperatures
c.
Sociobiology
(1)
Identifies ill individual
(a)
Produces drastic changes in behavior
i)
Febrile individual goes into seclusion
a)
Reduces metabolic demands
ii)
Others in community will respond with care
and protection
Thermoregulatory Abnormalities
A.
Heat exhaustion (Slide)
1.
Circulatory shock brought on by heat reduction mechanisms
a.
Excessive sweating and cutaneous dilation
b.
Hypovolemia
c.
Decrease in cardiac output and MAP
2.
Body temperature may not be elevated very much
B.
Heat stroke (Slide)
1.
Extreme exposure to heat
a.
Strenuous exercise
b.
Hot, humid environment
c.
Alcohol
2.
Heat reduction mechanisms are inadequate
Thomas A. Pressley
TTUHSC Physiology
C.
D.
a.
Elevated body temperature impairs CNS
b.
Inhibitors thermoregulatory response
c.
Heat-stroke cycle
3.
Elevated body temperature causes organ and tissue damage
a.
May reach temperatures above 42o C
4.
“Hot tub syndrome”
a.
Immersion in hot water can easily lead to heat stroke
(1)
High conductivity of water facilitates transfer of heat to body
(2)
Often exacerbated by depression of thermoregulatory system
after consumption of alcohol
Frostbite
1.
Actual freezing of tissue
2.
Rewarming can minimize damage, if not too severe
3.
Severe frostbite results in necrosis, gangrene, and eventual loss of limb
Hypothermia (Slide)
1.
James Cameron’s Titanic includes a graphic demonstration of immersion
hypothermia
a.
As with the hot tub, the high conductivity of water contributes to the
problem
(1)
In this case, loss of heat from body is accelerated
b.
One inaccuracy in the movie may be that the characters do not
display sufficient shivering, given the temperature of the air and
water in the North Atlantic
2.
Reduced body temperature impairs CNS
3.
Eventually impairs organ function
a.
Depolarization of cardiac pacemaker cells increases likelihood of
bradycardia and arrhythmia
b.
Death usually caused by ventricular fibrillation
Revised August 24, 2004
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