Temperature regulation Flashcards
Normal body temperature
37 degrees+/- 0.5
What are normal variations in body temperature?
Person to person variations
Circadian rhythm
Menstrual cycle
How do circadian rhythms affect body temperature?
Daily variation (0.5 degrees) with a minimum in early hours and peak in late afternoon
How does the menstrual cycle affect body temperature?
Rise of 1 degree C after ovulation
Above what temperature are proteins damaged?
42 degrees
Energy expenditure falls …. for every 1 degree drop in core temperature
13%
The thermoneutral zone is
The temperature range over which there is no “active” heat generation or loss
Naked human thermoneutral zone
25-30 degrees
What is a calorie?
Energy required to heat 1g of water by 1°C
Thermogenic effect of food is?
Describes the heat generated by the chemical reactions of digestion, absorption and storage of food
What food type increases BMR by the most (greatest thermogenic effect)?
Protein
What hormone raises BMR?
Thyroid hormone
How much can thyroid hormone increase BMR by?
50-100% above normal
Total loss of thyroid secretion will decrease BMR by…
40-60% of normal levels
What hormones other than TR have an effect on BMR?
Testosterone (~10-15% increase in BMR) and growth hormone (~15-20% increase in BMR)
What activates BAT?
Low temperatures
Two means of thermogenesis
Shivering and BAT
How does BAT promote thermogenesis?
Rich in mitochondria and uncoupling protein 1 (UCP-1). Heat is generated through the dissipation of the proton gradient across the inner mitochondrial membrane via UCP-1
Radiation
Results from the emission of a spectrum of electromagnetic radiation due to an objects temperature
Convection
Removal of heat from the body by air currents
Magnitude of convection heat loss depends on?
The movement of air over the surface of body
Conductive heat loss ∝ √wind velocity
Conduction
Transfer of heat through direct contact with objects
Why is conduction a small factor for heat loss in humans?
Air is a poor conductor
Evaporation
Conversion of water from a liquid to a gas requires energy to overcome the latent heat of vaporization.
Typical water loss per day (evaporation)
600-700ml
Body Heat=
Metabolic heat production ±
Conductive, convective and radiant heat exchange − Evaporative heat loss
In high temperatures, how do we lose heat?
evaporation is the only way to lose heat as conduction, convection and radiation all lead to heat gain.
What parts of the hypothalamus function as a thermostat?
Preoptic and anterior hypothalamus operates as a thermostat for the body (with greater sensitivity to heat than cold).
Do central temp regulators have greater sensitivity to hot or cold?
Hot
Do peripheral temp regulators have greater sensitivity to hot or cold?
Cold
Where are peripheral temp regulators located?
Spinal cord, abdominal viscera and great veins (deep body sensors) and the skin.
How do peripheral receptors respond to changes in temperature?
Sensors respond to change in temperature by changing discharge frequency
Where is central and peripheral sensor information combined?
Posterior hypothalamus
Arteriovenous shunts allow?
Blood supply to the periphery to be controlled
How are shunts centrally mediated?
alpha-1-adrenergic receptors
Shivering can increase heat production by %?
200%
Exercise induces heat production increase by how many fold?
10 fold
What is shivering?
Rapid (~50Hz) and unsynchronized muscular activity
How is shivering centrally controlled?
Signalled from the posterior hypothalamus and translated to the anterior motor neurons
What fibres lead to production of sweat?
Cholinergic sympathetic nerve fibres elicit the secretion of a primary fluid into the sweat duct.
The glands can also be stimulated by adrenaline/noradrenaline (e.g. during exercise)
Is sweat reabsorbed?
Yes, some along the duct
Light sweat is consistent of? Why?
Concentrated solution of urea, lactic acid and potassium
Water, NaCl reabsorbed
How much sweat can be produced by athletes?
Up to 2L/hour
Hypothermia arises when
Continued exposure to cold temperatures causes the core body temperature to drop below 35°C
Loss of consciousness at what temperature?
30 degrees
Why are neonates susceptible to hypothermia?
Reduced behavioural adaptations and greater surface area to volume ratio (although they do possess BAT for thermogenesis)
What causes hyperthermia?
Pyrogens (LPS or cytokines)
Elevate prostaglandin E2 synthesis in the hypothalamus
Infection can lead to..
Elevation in the regulatory set-point of core body temperature
Heat exhaustion/heat syncope occurs due to
Hypovolaemia from excessive sweating
Heat stroke occurs when?
Body temperature is greater than 40.6 degrees
