 


21
• Describe the thermal balance and explain its importance
• Explain the homeostatic regulation of core body temperature
• Describe the body changes in pyrexia (fever), hyperthermia, acclimatization to heat and
hypothermia

 
  

• Humans are homoiothermic as the core body temperature is maintained within a narrow
range and is relatively constant (37 ± 0.5 oC)
 The speed of chemical reactions varies with the temperature
 The enzyme systems of the body have narrow temperature ranges in which their
function is optimal
 enzyme activities or metabolic rate remain more or less constant independent of the
change in external temperature
• The balance between heat production and heat loss determines the core body temperature
• Core body temperature displays diurnal rhythm: lowest in predawn hours and rises in the
afternoon
• The rectal temperature (average 37.2oC) is representative of the temperature at the core
of the body and varies least with changes in environmental temperature
• The surface temperature fluctuates widely depending on environmental temperatures

 
  

• Thermal balance is the balance between heat gain and heat loss which determines the core
body temperature

A. Heat gain
Heat Gain = Heat production + Heat transferred from environment
• The process of heat production in organisms is called thermogenesis
• Heat production of the body includes the following:
1 Basal rate of metabolism1
2 Muscle activity including exercise, contractions or shivering
3 Extra metabolism due to:
 effect of thyroxine2 on the cells
 effect of epinephrine, norepinephrine and sympathetic stimulation on the
cells
 increased temperature of the body (due to heat gain from environment)

B. Heat loss
• Heat transferred to the surrounding environment, including conduction, convection,
radiation and evaporation of sweat in the skin and water in the respiratory tract
• The rate of heat loss depends on
1 the temperature difference between the body and the surroundings
 affects how rapidly heat is lost from surface of skin to surroundings
2 the amount of insulation by clothing
3 the velocity of air movement (affects convection)
4 environmental humidity (affects evaporation)
5 how rapidly heat is conducted from core to skin by blood vessels
 blood flow controlled by vasoconstrictor sympathetic nervous system
 The amount of sympathetic activity3 is inversely proportional to the blood
flow to skin and heat loss

1 Heat production is one of the principal by products of metabolism

2 Thyroxine's principal function is to stimulate the consumption of oxygen and thus the metabolism of all cells
and tissues in the body.
3 These include pupil dilation, increased sweating, increased heart rate, and increased blood pressure.

 
  

C. Heat transfer
Radiation • Heat transfer via electromagnetic heat waves through air
Conduction • Direct heat transfer from one object to another when they are in contact.
• Air is a poor conductor of heat, so air trapped in clothes makes a good
insulator
Convection • Dissipation of heat by air currents (convection currents)
Evaporation • Evaporation of sweat or water from lungs results in loss of latent heat of
vaporization as the water vaporizes.
• Its rate depends on air movement and humidity.

 
  

A. Overview
• The temperature of the body is regulated by feedback mechanisms operating through
temperature-regulating centres located in the hypothalamus
• The superficial and deep thermoreceptors provide temperature signals to the
thermoregulatory centres
• after analysing the information, they will activate effector mechanisms either to increase
or decrease the temperature of the body

B. Temperature Sensors
• There are two types of receptors:
Type of Receptor Description Area includes
Peripheral Detects skin • The skin
thermoreceptors temperature
Central Detects core body • Anterior hypothalamic-preoptic area
thermoreceptors temperature • the deep tissues4

Area of the body Description

Anterior hypothalamic- Contains more heat-sensitive neurons which will increase
preoptic area their firing rate as temperature rises

The skin and the deep Contains more cold-sensitive neurons which will increase
tissues their firing rate as temperature falls
 to prevent hypothermia

4 E.g. in the spinal cord, abdominal viscera and great veins

 
  

C. Controller
• Temperature signals are integrated and compared to a reference or “set-point” value in
the posterior hypothalamus
 E.g. stimulation of sympathetic nervous system in the posterior hypothalamus
is the cause of constriction of cutaneous arterioles in cold weather
• If there is any difference between the set-point and the actual body temperature, then
appropriate heat-conserving or heat-losing mechanisms will be activated.

 
  

Type of Receptor Response to heat Response to cold

Physiological • Decrease heat production • Increase heat production (thermogenesis)
response  Decrease muscle tone5 and voluntary  Shivering thermogenesis: increase skeletal muscle
activity (inertia) tones.8
 Decrease secretion of epinephrine6  Non-shivering thermogenesis: increase metabolism
(minimal in adults) rate by hormones such as TRH, TSH and thyroxine.9
 Decrease food appetite (anorexia)  Increase secretion of epinephrine (minimal in adults).
• Increase heat loss  Increase food appetite.
 Sympathetic vasoconstrictor centres are • Reduce heat loss
7
inhibited  Vasodilation of cutaneous  Sympathetic vasoconstrictor centers are activated 
arterioles  Increase blood flow near skin vasoconstriction10 of cutaneous arterioles  Decrease
surface  Increase heat loss by radiation, blood flow near skin surface  Decrease heat loss by
conduction and convection. radiation, conduction and convection
 Sweat production increases  increase  Piloerection11: contraction of arrector pili muscles to
heat loss by evaporation of sweat. improve insulation.

Behavioural changes in clothing (e.g. be shirtless), choice of changes in clothing, choice of surroundings, increase
response surroundings, decrease voluntary activities, etc. voluntary activities, curling up etc.

5 muscle tone (residual muscle tension or tonus) is the continuous and passive partial contraction of the muscles
6 Epinephrine, more commonly known as adrenaline, is a hormone secreted by the medulla of the adrenal glands. Strong emotions such as fear or anger cause
epinephrine to be released into the bloodstream, which causes an increase in heart rate, muscle strength, blood pressure, and sugar metabolism.
7 the dilatation of blood vessels, which decreases blood pressure.
8 The primary motor centre for shivering are located in the dorsomedial portion of the posterior hypothalamus and signals are ultimately transmitted to the

anterior motor neurons.

9 The increase in metabolism by thyroxine does not occur immediately but requires several weeks.
10 the constriction of blood vessels, which increases blood pressure.
11 involuntary erection or bristling of hairs due to a sympathetic reflex usually triggered by cold, shock, or fright or due to a sympathomimetic agent.

 
 

• Pyrexia is an increased core body temperature to around 38oC or above caused by an

elevation of the thermal set point.
• The human is a homeothermic animal. The temperature regulatory center
integrates nervous and chemical signals to produce a thermal set point and an
estimate of body temperature, compare them and engage in effector mechanisms
that compensate any deviation of the body temperature from the thermal setpoint.
PGE2 is one of those signals. It will try to increase the thermal set point by shivering.
• Interleukin-1 is an endogenous pyrogen
• Flow chart:

• As the set point is raised, there are symptoms of chills and rigors, which mean shaking
occurring during a high fever.
• Besides, vasoconstriction, piloerection and increased epinephrine secretion also take
place.

 
  

• Hyperthermia refers to an increase in body temperature above the thermal set-point

• Heat stroke and heat exhaustion may result due to hyperthermia

Heat exhaustion Heat stroke

Cause Dehydration • Strenuous physical exertion in high and
humid condition
 Profuse12 sweating
 Salt and water depletion
• complete breakdown of thermoregulatory
system so that the body temperature
keeps increasing
• Dehydration and decreased blood
pressure resulted
 Decreased blood flow (hypotension)
to kidneys, splanchnic region and
brain
 Decreased cerebral13 blood flow14
• Increased brain temperature
Symptoms • Weakness • Fatigue
• Nausea • Confusion
• fainting • Unconsciousness
• Core temperature reaches 41 – 42 oC
Measures • increase fluid
intake
• shade

12 abundant
13 cerebrum of the brain
14 One of the Central Nervous System symptom, along with increased brain T

 
  

• Hypothermia is a fall in the core temperature to below 35 oC

• It results in a reduced metabolic rate which can lead to brain damage, coma and death.
• It is more likely to occur in very young babies, old people, mountaineers and swimmers.

• When a person is placed in a hot environment for several days

 he will gradually adapted to the environment and will tolerate heat better.
• Changes in physiological processes include:
 Maximum rate of sweating increases.
 Plasma volume increases.
 NaCl concentration in sweat and urine decreases as a result of the increase in aldosterone
secretion.

 