Temperature Regulation

1
Temperature Regulation

• When studying temperature regulation the body can
  be divided into two regions
• the core and the shell.
• Core (internal organs)
• temperature highly regulated (high gain)
• traditional values 37C, 98.6F
• ? 1F within normal range
• small variations with time of day, exercise and
  environmental temperature

Fig. 14-22 G
2

• Shell (skin and limbs)
• temperature not highly regulated (low gain)
• varies with changes in environmental temperature
• Cold Environment
• body heat conserved for the core
• shell allowed to become cold
• Warm environment
• excess heat leaves body via the shell

Fig. 14-21 G
3
Physics of Heat

• heat molecular motion
• cold absence of heat
• downhill movement
• from region of higher temperature
• to region of lower temperature
• Flow of heat temperature gradient
• X conductance

4
Mechanisms of Heat Transfer
From Guyton, Medical Physiology

• 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

5
Mechanisms of Heat Transfer

• 3) Evaporation
• 0.58 Cal required to evaporate 1g of H2O from the
  surface of the skin.
• heat of vaporization 0.54 Cal
• 1 Cal 1 kcal
• 1 Cal (nutritional calorie) 1000 cal
  (physics calorie)
• When environmental temperature gt body
  temperature, evaporation is the only mechanism to
  cool the body.

6
Roles of the Skin
evaporative cooling
insulation
vasodilation and vasoconstriction
Fig. 6.1
7
Roles of the Skin
Radiation Conduction Convection Modulated by
vasodilation and vasoconstriction
From Guyton, Medical Physiology
8
Roles of the Skin
Fig. 6.1
maximum rate of perspiration 2-3 L
H2O/hr Na, Cl-, K, urea, and lactic acid are
also excreted in sweat.
evaporative cooling
9
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

10
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

11
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