53.4 Body Temperature Control Flashcards
What are some normal variations in body temperature?
*Circadian rhythms
*Menstrual cycle.
What is the normal range of body temperature?
36oC-37.5oC
How does body temp change in the circadian rhythm?
- Variations of around 1oC
- Lowest temp from 3-6am
- Highest temp from 3-6pm
How does body temp change in the menstrual cycle?
- Rise of ~0.5 - 1 oC seen after ovulation
What are the two compartments of the body in temp regulation?
- Core
- Peripheral shell
What is the difference between the core and peripheral temperatures?
- Important for core temp to stay constant - for optimal enzymatic reactions
- Whereas peripheral temp is highly variable - changes depdendent on environmental conditions. Can drop as low as 21oC in fingers and toes
Why is (core) body temperature so tightly regulated?
- To provide optimal conditions for cellular enzymatic reactions
- When body temperature exceeds 42*C, cellular proteins are damaged
- If the temperature drops, these enzymes and proteins become less efficient/effective
- Energy expenditure falls 13% for every 1*C drop in core temperature (as drop in body temperature results in decreased enzymatic rates)
What type of organ is brown adipose tissue?
Thermogenic
How does brown adipose tissue underlie thermogenesis?
- Rich in mitochondria and uncoupling protein 1
- UCP-1 dissipates the proton gradient across the inner mitochondrial membrane
- Energy released as heat
When is brown adipose tissue thermogenesis important?
For neonates that cannot shiver yet
What is the definition of hyperthermia?
Sustained high body temp above 40.6oC
What is the definition of hypothermia?
Sustained low temp below 35oC
What are some causes of hyperthermia?
- Febrile crisis
- Malignant hyperthermia (e.g. suxamethonium)
- Overheating
What are consequences of hyperthermia?
- Heat exhaustion, heat stroke
- Fibrinolysis –> disseminated intravascular coagulation (DIC), haemorrhage
- Damage to membranes –> rhabdomyolosis (muscle cells release contents), myocardial necrosis
- High brain temp –> CNS disturbances
- Loss of consciousness, death
What are some causes of hypothermia?
More to do with environment
- Cold water
- Cold weather
What are consequences of hypothermia?
- Death and decay of tissue (gangrene) - interrupted blood flow
- Freezing of body tissue (frostbite)
What are the peripheral injuries caused by extreme heat and cold respectively?
- Frostbite
-Burns
What are the four processes that cause heat loss?
- Convection
- Evaporation
- Conduction
- Radiation
What is conduction?
Transfer of heat through direct contact with objects
What is convection?
Removal of heat from body by air currents
What is evaporation?
Heat loss through conversion of water in sweat as a liquid to a gas (requires energy to overcome latent heat of vaporisation, which is released to environment)
What is radiation?
Heat loss through emission of infrared rays
Which process accounts for least heat loss?
Conduction
Which process accounts for most heat loss?
Radiation
How do neurons in the anterior/preoptic hypothalamus respond to cold? What are the effects?
- Fall in temp detected by cold-sensitive neurons –> TSH released to anterior pit, stimulates thyroxine release to increase metabolism
- Vasoconstriction in skin and piloerection (goose bumps)
- Shivering
- Behavioural response - seek warmth, more clothes
How do neurons in the anterior/preoptic hypothalamus respond to hot temp? What are the effects?
- Rise in temp detected by warm-sensitive neurons –> reduces TSH, metabolism slowed
- Blood shunted to periphery to dissipate heat
- Sweating
- Behavioural response - seek shade
How are changes in temperature sensed?
- Primarily through nervous feedback systems from the periphery to the CNS
- Centre in the brain: temperature regulating centers in the hypothalamus
Where is temperature centrally controlled?
Hypothalamus
What is the ‘thermostat’ for the body?
- The preoptic and anterior hypothalamus operates as the body’s thermostat
- This part of the brain has greater sensitivity to heat than to cold
Where are the peripheral temperature sensors located and what are they more sensitive to?
- Spinal cord, abdominal viscera and great veins (deep body sensors)
- Skin
- Greater sensitivity to cold than hot
How can heat loss be increased without the presence of a thermal gradient?
Through sweat production and its subsequent evaporation facilitates heat loss from the body, particularly in hot conditions
How can sweat production be increased? What properties does this secretion have?
- Sweat production can be increased by cholinergic sympathetic nerve fibres
- The glands can also be stimulated by adrenaline/noradrenaline, e.g. during exercise to increase heat dissipation
- This elicits the secretion of a primary fluid into the sweat duct
- This fluid has a composition similar to that of plasma, just without plasma proteins
- As regulation/reabsorption occurs along the tubule, if the demand for secretion increases and therefore the transit time decreases, then the time over which this secretion can be modified is rapidly decreased
Are the following more sensitive to hot or cold stimuli?
- Preoptic and Anterior Hypothalamus
- Peripheral temperature receptors
- Hot
- Cold
Which part of the hypothalamus controls temperature?
Anterior hypothalamus and preoptic area (sometimes grouped together)
What is the thermoneutral zone and its values?
- The temp range over which there is no ‘active’ heat generation or loss
- 25-30oC
How does light clothing affect the thermoneutral zone?
- Reduces loss of heat from body
- Reduces thermoneutral zone range to 18-22oC
What are some adaptations seen in cold acclimatisation?
- Increased thermogenesis with less shivering
- Blunted cutaneous vasoconstriction response
- Behavioural: clothing, shelters, posture, activity
- piloerection
What are some adaptations seen in exercise heat acclimatisation?
- Improved sweating
- Improved skin blood flow
- Lowered body temp
- Reduced cardiovascular strain
- Enhanced cellular protection
How do NSAIDs and paracetamol act as antipyretics (reduce fever)?
Reduce production of prostaglandins
So they cannot signal to hypothalamus to raise set-point
How does fever occur following infection?
- Body releases pyrogens - e.g. cytokines, prostaglandins, thromboxane
- Pyrogens induce COX2 to convert arachidonic acid to prostaglandin E2 (PGE2)
- PGE2 binds to E2 receptors in the anterior hypothalamus - raises set-point
- Elevated set-point means body works harder to achieve a higher internal temperature
- Sweating, vasodilation, hot skin
What is the definition of the set-point for temperature control?
The point above or below which there are intense changes that bring about heat loss or production
What is the mechanism by which PGE2 binding to its receptors raises the set-point?
PGE2 binds to GqPCR in warm neurons –> increases cAMP sythesis and release
cAMP acts as a neurotransmitter –> activates thermostatic neurons
Increases set point
Which drugs can act as antipyretics?
- NSAIDs
- Paracetamol (Acetaminophen)
