Internal core temperature is the temperature of the deep tissues of the body

• Core temperature normally varies about 1.8 F (1 C) during the day
• Women experience a monthly rhythm in core temperature
• Core temperature increases during exercise
• Older is colder
• Core temperature may vary slightly with exposure to extremes of temperature

Sites for monitoring
body temperature

• Oral
• Axillary
• Rectal (0.5-0.6 higher than oral, more accurate)
• Tympanic (0.5-0.6 higher than oral)
• Temporal artery

Sweating

• Sweating is a regulated evaporative heat loss process
  
  • Under sympathetic nervous control
  • Rate of heat loss can be adjusted
  • Sweat must be evaporated for heat loss to occur

Central/Core Thermoreceptors

• Detect change in body core temperature
• Located in
  
  • Hypothalamus
  • Spinal cord
  • Abdominal viscera
  • Great veins in the upper abdomen and thorax
• More receptors that detect cold than warmth

Increasing Heat Production

• Shivering is the primary involuntary means of increasing heat production
  
  • Adjustments in heat production by skeletal muscles
  • Shivering: rhythmic, oscillating skeletal muscle contractions that occur at a rapid rate of 10 to 20 per second
• Non shivering thermogenesis by brown fat
  
  • Increased heat production stimulated by brown adipose tissue, which is capable of converting chemical energy from food into heat

Heat Loss Mechanisms

• The magnitude of heat loss can be adjusted by varying the flow of blood through the skin
  
  • Vasoconstriction, vasodilation
  • Heat loss mechanisms are subject to control, largely by the hypothalamus

Increased Heat Production

• Increased muscle tone
• Shivering
• Increased voluntary exercise*
• Nonshivering thermogenesis

Decreased Heat Loss

• Skin vasoconstriction
• Postural changes to reduce exposed surface area (hunching shoulders, etc.)*
• Warm clothing*

Radiation
The transfer of heat energy from a warmer object to a cooler object in the form of electromagnetic waves (“heat waves”), which travel through space.

Conduction
The transfer of heat from a warmer to a cooler object that is in direct contact with the warmer one. The heat is transferred through the movement of thermal energy from molecule to adjacent molecule

Convection
The transfer of heat energy by air currents. Cool air warmed by the body through conduction rises and is replaced by more cool air. This process is enhanced by the forced movement of air across the body surface.

Evaporation
Conversion of a liquid such as sweat into a gaseous vapor, a process that requires heat (the heat of vaporization), which is absorbed from the skin

Adjustments in muscle activity (in metabolic heat production)

Adjustment in loss or conservation of heat

Adjustments in heat production or heat loss

Hypothalamic Thermoneutral Zone

• Core temperature can fluctuate within this range without triggering thermoregulatory effectors
• 36.8 C to 37.2 C

• The higher the skin temperature, the lower the set point
• The lower the skin temperature, the higher the set point
• Anticipatory mechanism to moderate the need for large responses

Process

• Hypothalamic set point is raised
• Chills / feels cold / shiver
• Body temperature reaches the new set point
• Cessation of fever
• Hypothalamic set point reduced to normal level
• Vasodilatation / sweating
• Temperatures falls

Exercise Hyperthermia

• At the onset of exercise, the rate of heat production initially exceeds the rate of heat loss so the core temperature rises.
• When heat loss mechanisms are reflexly increased sufficiently to equalize the elevated heat production, the core temperature stabilizes slightly above the resting point for the duration of the exercise.

• Heatstroke
  
  • Beyond 40.5 C, heatstroke is likely to develop
  • Thermoregulation overwhelmed and become seriously impaired at this temperature
  • Lack of compensatory heat loss measure such as sweating
  • Damage to brain, liver, kidney etc
• Acclimatization
  
  • Increased tolerance to heat with exposure to hot and humid conditions in 1 3 weeks

Hypothermia

• Lowered core temperature without reset of set point
• Temperature regulation is impaired below 35 C
• Artificial hypothermia
  
  • Therapeutic hypothermia
  • Hypothalamic activity depressed with drugs followed by cooling the patient down

Decreased Heat Production

• Decreased muscle tone
• Decreased voluntary exercise*