A brief desription of temperature regulation in the human body, from a physiologists point of view.

1. Temperature Regulation
Dr Elizabeth J
Prof & HOD
Dept of Physiology
AIMS &RC , Bangalore
4 / 09 /14

2. Scheme
• Introduction
• General Considerations
• Mechanisms of heat loss & heat gain
• Control of heat production, by radiation,
conduction & evaporation
• Skin &Temperature regulating reflexes
• Thermoneutral Zone
• Control mechanisms of temperature regulation
• Fever & Hyperthermia
• Heat exhaustion, heat stroke & hypothermia

3. Introduction
• Homeothermic- Humans capable of maintaining
their body temperatures within narrow limits.
• Poikilothermic- (Cold blooded)
• Biochemical reactions do not fluctuate due to the
constant & high temperatures.
• 410 C (1060F) – 430C convulsions are seen
• Nerve malfunction & protein denaturation seen
with higher temperature.

4. General Considerations
Temperature can be expressed as 0C or 0F.
C = ( F - 32) x 5/9 and F = (C x 9/5) + 32
Normal is 370C or 98.60F
• Measured under tongue, axilla or rectum
• Oral temp is 0.50C less than core body temperature (rectal temp).
• Internal temp varies with activity pattern and changes in ext temp.
• Circadian fluctuation of about 10C - lowest at night and highest during
the day.
• Women show higher temp during second half of menstrual cycle

5. General Considerations
Considering temp as a measure of heat concentration
Total heat content gained / lost by the body
is determined by NET DIFFERENCE
in heat produced and lost
STEADY STATE- heat production must equal heat loss

6. Mechanisms of heat loss / heat gain
• Radiation- Surface of all objects emit heat in form of
Electromagnetic radiation.
Rate of emission α Temperature
• Conduction- Loss/ gain by transfer of thermal
energy during collisions between adjacent molecules
– Heat is conducted from Molecule to Molecule

7. Mechanisms of heat loss / heat gain
• Convection- Conductive heat loss or gain is aided by
movement of air / water next to body.
-Cool air replacing warm air.
• Evaporation- Water evaporates from skin &
membrane lining the respiratory tract MAJOR
PROCESS for Loss of body heat.
• 600Kcal/L reqd to transfer water from fluid to
gaseous state.
- Heat reqd to evaporate water from the surface is conducted
from the surface cooling it.

8. SKIN & ITS ROLE
• Skin is not a perfect insulator
• Temp of outer surface of skin is between external temp &
core temp. (net conduction = 0)
• Skin acts as insulator by change in blood flow.
• The heat from within is brought out by the blood and lost
to the outside.
• Vasoconstrictor sympathetic nerves firing rates
– in response to cold &
– in response to heat

9. Temperature – Regulating reflexes
• Thermoreceptors detect changes in the balance
between heat loss & production due to metabolic rate
(exercise).
• Two types
– Peripheral on Skin
– Central in hypothalamus (integrating center), spinal cord,
abdominal organs.
• Output from hypothalamus is sent to effectors via
– sympathetic nerves to sweat glands,
– skin arterioles & adrenal medulla.
– Motor neuron to skeletal muscles.
• Core temp is maintained relatively constantly.
• Peripheral thermoreceptors help identify heat & cold.

10. Thermoneutral zone(TNZ)
• All the mechanisms are Graded and not All or None
responses.
• 250C to 300 C or 750F to 860F is known as a
thermoneutral zone.
• At temps lower than this TNZ max vasoconstriction
cannot prevent heat loss from exceeding heat production
& this requires the body to increase heat production.
• At temps above this TNZ max vasodilation cannot
eliminate the heat as fast as it is produced & this requires
another heat loss mechanism ‘SWEATING’

11. Control Mechanisms of Temperature Regulation
• Nervous Mechanisms
– Thermoregulatory centers
• Direct Action
• Reflex Mechanisms
• Efferent Nerves
– Endocrine and Temperature Control
• Adrenal Medulla
• Adrenal Cortex
• Thyroid
• Behavioral & Voluntary control

12. Scheme
• Introduction
• General Considerations
• Mechanisms of heat loss & heat gain
• Control of heat production, by radiation,
conduction & evaporation
• Skin &Temperature regulating reflexes
• Thermoneutral Zone
• Control mechanisms of temperature regulation
• Fever & Hyperthermia
• Heat exhaustion, heat stroke & hypothermia

13. Hypothalamus control
• Warming of anterior hypothalamus (Heat loss)
– Vasodilatation
– Sweating
– Hyperpnoea
– Injury abolishes the heat loss responses to hot environment.
• Stimulation of Posterior hypothalamus (Heat production)
– Vasoconstriction
– Injury abolishes responses to cold & interferes with the
responses to heat.
• Preoptic region of Ant. Hypothalamus is regarded as
the thermostat.
• SET POINT is maintained by this region

14. Hypothalamic temperature Regulation (contd)
• Direct action:
When environmental temp
– Is high, warm blood flowing through hypothalamus
causes HEAT LOSS responses.
– Is low, cool blood causes
HEAT PRODUCTION / CONSERVATION responses.

15. Hypothalamic temperature Regulation (contd)
• Reflex Mechanisms
– Sensitive thermoreceptors
• in the skin carry information via cutaneous nerves
and hypothalamus
– Efferent Nerves
• Autonomic
– Sympathetic adrenergic vasomotor nerves
(cutaneous vasoconstriction & vasodilatation)
– Sympathetic cholinergic nerves to sweat glands
• Somatic
– Nerves to skeletal muscle ( tone, activity, shivering)
– Nerves to respiratory muscles

16. Endocrine control
• Adrenal medulla-
– immediate adrenaline release (Calorigenic)
– Exposure to cold leads to
• Cutaneous vasoconstriction the heat loss
• metabolic rate & heat production
• Adrenal Cortex
– BMR is low in adrenal cortical insufficiency
– Patients do not tolerate cold well & body temp is subnormal.
• Thyroid
– Calorigenic
– Permissive action on adrenaline calorigenesis
– Hyperthyroidism-skin is warm

17. Behavioral and Voluntary Control
• Animals move from warm to cold
regions
• Curling up of body in cold
conditions
• Clothing in woolens in winter and
thin cotton clothing in summer.
• Fans, air conditioners, heaters &
central heating

19. Control of heat production
Response to exposure to cold
 Shivering thermogenesis
- This contains rhythmical
oscillatory muscle contractions.
- No external work is involved, all
the energy liberated by the
metabolic machinery appears as
internal heat.
 Non shivering Thermogenesis
- Increase in (metabolic rate) heat
production not due to muscular
activity.
- Increased epinephrine,
sympathetic activity to adipose
tissue & contribution of thyroid
hormone.

20. Temperature regulating mechanisms
Skin temperature
Cerebral cortex
“ Voluntary” motor responses
Core temperature
Peripheral
thermoreceptors
Hypothalamus
Central
thermoreceptors
Via sympathetic nerves
Adrenal medulla Sweat glands Skin arterioles Skeletal muscles
Epinephrine
“Involuntary” motor responses

21. Effector mechanisms in Temperature Regulation
STIMULATED BY COLD
Desired Effect Mechanism
Decreased Heat
Loss
Vasoconstriction of skin vessels
Reduction of surface area (curling up)
Behavioral responses( warm clothes)
Increased Heat
Production
Increased muscle tone
Shivering & increased voluntary activity
Epinephrine secretion (minimal)
Increased appetite

22. Effector mechanisms in Temperature
Regulation
STIMULATED BY HEAT
Desired effect Mechanism
Increased Heat
Loss
Vasodilation of skin vessels
Sweating
Behavioral response
Decreased Heat
Production
Decreased muscle tone
Decreased secretion of epinephrine (minimal)
Decreased Appetite

23. Fever & Hyperthermia
• Fever is an elevation of body temperature due to a
“resetting of the thermostat”
• > 990F
• Bacterial / Viral Infections, trauma,lesions of CNS,
exposure to high temperatures & drug induced.
• Increased heat production by shivering (rigor) &
increased metabolism
• Diminished heat loss by vasoconstriction
• Skin is warm & flushed
• Subsides by sweating

24. INFECTION
LIVER
MACROPHAGES
IL-1, IL-6
Firing of neural
receptors
MULTIPLE
ORGANS
MACROPHAGES
IL-1, IL6
HYPOTHALAMUS
TEMP SET POINT
Skeletal Muscles
Shivering Curling up
SKIN ARTERIOLES
VASOCONSTRICTION
HEAT PRODUCTION HEAT LOSS
Heat production greater than heat loss
Heat retention
BODY TEMPERATURE

25. Heat Exhaustion
• Heat waves in a humid atmosphere
• Increased sweating with water and salt loss.
• Headache, dizziness, cramps, vomiting and
rise in temperature.
• Skin is cold & clammy
• Fall in BP, circulatory collapse & stupor.
• Treated by fluids & electrolytes

26. Heat Stroke
• Serious Condition,high environmental temp
• Overheating of body, impaired sweating
• Headache, restlessness & mental confusion
• Hyperpyrexia (410C or 1060F)
• Delirium, convulsions,CV collapse & COMA
• Death results if untreated
• Temp to be brought down to 1020C with ice
packs

27. Hypothermia
• Reduction in temp < 350C
• At 270C the metabolism is greatly reduced
(<280C unable to correct by itself)
• HR,BP & RR are decreased
• Unconscious state
• Exposure to low temps
• Cardiac surgery where heart is stopped