1. Temperature Regulation

2. Why temperature regulation?

3. Why temperature regulation?
All organisms are limited by their ability to survive in different temperatures
Some, like reptiles and amphibia are poikilothermic
Others like humans are homeothermic

4. Core temperature
The core temperature of the human body is 37°C ± 0.6°C
The core of the human body includes the organs of the thorax, abdomen and the head
This is where the vital organs are located
Their enzyme systems must operate in optimum conditions
The periphery of the body can withstand some deviation from the core temperature

5. Core Temperature

6. Mechanisms of Heat Transfer
1) Radiation, especially infrared
2) Conduction: heat transfer by direct contact
conductors – high conductance
insulators – low conductance (high resistance)
 Convection:  conduction due to movement of a liquid or gas

7. 3) Evaporation
0.58 Cal required to evaporate 1g of H2O from the surface of the skin.
heat of vaporization = 0.54 Cal
When environmental temperature > body temperature, evaporation is the only mechanism to cool the body.

8. Balance Body Skin Environment Basal metabolic rate Muscle activity
Shivering
Vasomotor
Sweating
Piloerection
Conduction
Convection
Radiation
Evaporation
Body
Skin
Environment

9. Set-point hypothesis So how is it regulated? Reference Response
Controlled
system
Feedback
Set-point hypothesis

10. Response
Controlled
system
Feedback
Balance hypothesis
warm
cool

11. Anterior Hypothalamus
Inputs
Outputs
Anterior Hypothalamus
Central
thermosensors
warm
cool
Peripheral
Neural
sweating
shivering
vasoconstriction
vasodilation
Hormonal
adrenaline
TRH

12. Central Control of Body Temperature
Controlled Variable
temperature of body core
Set Point
37oC
Receptors
central thermoreceptors: in hypothalamus of brain
peripheral thermoreceptors: in skin (mostly affect behavior, very little role in regulating core temperature)
Control Center
hypothalamus
Effectors
dermal blood vessels
skeletal muscle
endocrine glands
brown fat
sweat glands

13. Hypothalamic Control of Body Temperature
When core body temperature drops below the set point, the following mechanisms are activated to increase body temperature:
 heat loss
vasoconstriction
[piloerection]
 heat production
shivering
muscle contraction
generates heat
hormone secretion
e.g., thyroxine, epinephrine (adrenaline)
activation of brown fat
e.g., in human infants mitochondria generate heat instead of ATP

14. Hypothalamic Control of Body Temperature
When core body temperature rises above the set point, the following mechanisms are activated to decrease body temperature:
 heat loss
vasodilation
(inhibit vasoconstriction)
sweating
 heat production
 voluntary activity
 fuel intake
 thyroxine secretion

15. Muscles of skin arteriole walls relax
NEGATIVE FEEDBACK
Blood temperature
Muscles of skin arteriole walls relax
Core body temperature >37°C
Hypothalamus
Sweat glands increase secretion
nerves
Muscles
reduce activity
Thermoreceptors
Thermoreceptors
Body loses heat
Return to 37°C

16. Muscles of skin arteriole walls constrict
NEGATIVE FEEDBACK
Blood temperature
Muscles of skin arteriole walls constrict
Core body temperature <37°C
Sweat glands decrease secretion
nerves
Muscles
shivering
Thermoreceptors
Hypothalamus
Thermoreceptors
Body loses less heat
Return to 37°C
Body gains heat

17. Babies Babies have an extra mechanism (cant shiver) Brown fat.
Suprascapular deposits
Rich in Iron containing mitochondria

18. And the menstrual cycle
Rectal
temp OC
37.4
36.8
36.2 12 18 6 24
Time
Postovulatory
Preovulatory

19. Fever and hyperthermia
An elevation of body temperature that exceeds the normal daily variation and occurs in conjunction with an increase in the hypothalamic set point----fever
Fever due to a disturbance of thermal regulatory control----hyperthermia

20. Development of fever Pyrogenic activator
Macrophage, monocyte, lymphocyte, etc.
Endogenous pyrogen
Thermoregulatory Centre
Mediators in regulation of body temperature
Rise in set point
fever

21. Pyrogenic activator
A substance that can manufacture and release endogeneous pyrogen which produces a fever.
1．Exogenous pyrogen
bacteria
virus
other microorganism
2. internal metabolic product
antigen-antibody complex
steroid
inflammatory substance

22. Fever
monocytes and phagocytes release endogenous pyrogen (Interleukin-1, IL-1).
The anterior hypothalamus is sensitive to IL-1
Hypothalamic sensitivity to temperature is altered.

23. The body temperature then becomes regulated at a new, higher level.
So…
The body temperature then becomes regulated at a new, higher level.
There is some evidence that the raised body temperature enables the fight against the infection.

24. In acute infection, the ability to mobilise fat stores is inhibited.
But...
Every 1OC rise in temperature increases basal metabolic rate and oxygen consumption by about 14%,
In acute infection, the ability to mobilise fat stores is inhibited.

25. Consequently
Skeletal muscle is broken down and the amino acids are used in gluconeogenesis.
This can be debilitating.

26. In addition to the increased demand for energy
And even worse
In addition to the increased demand for energy
Temperatures (above 42OC) damage nerve cells
impair thermoregulation
have more serious consequences.