Temperature regulation: acclimatisation and fever Flashcards
Define acclimatisation and adaptation
Acclimatisation: short-term response
Individual responses to environmental changes
Adaptation:
Traits heritable through generations
What is hyperthermia and hypothermia?
Hyperthermia:
Core body temperature higher than 40 ºC (variability)
Hypothermia:
Core body temperature lower than 35 ºC association with peripheral cold injury
What is peripheral cold injury: the trench foot?
The name “trench foot” comes from soldiers who fought in soggy trenches during World War I.
The trench foot is caused by the cooling of peripheral nerve and muscle, which cause sensory and motor malfunction.
= water in shoes –> much colder feet as heat transfer is greater than air
What is peripheral cold injury: frostbite?
Frostbite results from freezing of tissues
Polar explorer Sir Ranulph Fiennes lost the fingertips of his left hand to frostbite during a 2000 expedition
What are the 2 types of surgically induced hypothermia?
Induced hypothermia may provide benefits that outweigh the risks.
For neurosurgery on inaccessible aneurysms the body can be cooled by extracorporeal circulation so that blood flow can be stopped for ~15 minutes.
For cardiac surgery (e. g. valve replacement) cold “cardioplegia” is used to stop the heart for ~60 minutes.
How are we constantly shifting the thermoneutral zone (TN) and ambient temperature?
TN = by altering our clothing
Ambient temp = by heating or cooling our homes, cars, etc.
What can lead to hypothermia?
Immersion in cold water (with high thermal conductivity).
Subcutaneous fat provides some insulation, but fur and clothing offer little protection.
A woman got trapped under a layer of ice for 80 minutes in freezing water. She suffered cardiopulmonary arrest after 40 minutes of submersion and was not rescued for a further 40 minutes.
What was she treated with?
Treatment with cardiopulmonary bypass and extracorporeal blood warming meant that 4 hours after the incident her heart started to beat again. She made an almost full recovery.
Her core body temperature was 13.7 ºC on arrival at the hospital. At this temperature, cellular oxygen demand was reduced, so cells could survive despite a lack of effective circulation.
What is an advantage and disadvantage of skin vasoconstriction in response to adapting to prolonged cold exposure?
- Helps maintain core temperature
- Can cause other problems, e.g. loss of manual dexterity and frostbite.
Adaptation to prolonged cold exposure?????
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Night-time lows in the central Australian desert reach 0 ºC.
In the 1950s the central Australian Aborigines were nomadic people who lived outdoors and wore no clothing. They slept on bare ground and their only protection from the cold was a small fire at their feet and wind-break made from light bush.
Shivering helps maintain core temperature,
but keeps you awake at night.
Aborigines have reduced shivering, and their
core temperature falls… but they sleep well.
What is a “hunting” reaction?
When blood flow “hunts” to go back towards the baseline skin temp (well developed in Inuit)
What does paradoxical vasodilation do?
Improves manual dexterity, and decreases the risk of frostbite (well developed in Inuit)
How does heat production increase in response to low temp?
1) Voluntary muscle activity: hand rubbing, foot stamping, moving around quickly
2) Involuntary muscle activity: increased muscle tone, tremor, shivering - can increase heat production x5 briefly but raises convective heat loss
3) Brown fat metabolism: newborns can increase heat production x2-3 under sympathetic control. About 4% of body weight.
What is the body’s response to cold exposure?
Skin vasoconstriction - helps maintain core temperature
But, can cause other problems e.g. loss of manual dexterity and frostbite
What is malignant hyperthermia?
- triggered by drugs
- DNP = originally used as an explosive and later intro in 30’s to stimulate metabolism and promote weight loss
- concerns about hyporexia led to DNP being banned as a dietary aid in 1938
How does thermoregulation in the elderly occur?
- thermoregulatory mechanisms less efficient
- mobility greatly reduced, as can shivering and vasocon in the cold
- heat-producing lean body mass diminishes with age
- financial difficulties = heating left on low in winter
- in heat = show reduced sweating and vasodilation
What is non-shivering thermogenesis?
Metabolic process located primarily in brown adipose tissue and controlled by the activity of the sympathetic nervous supply of this tissue
(Himms-Hagen and Bull, 1984)
What is the role of adipose tissue?
- brown fat can generate large amounts of heat
- ox phos in the mitochondria of brown fat is mainly uncoupled: when cells are stimulated by sympathetic nerves, the mitochondria produce a large amount of heat, but almost no ATP
What is the thermoregulation in neonates like?
Linear VS area VS vol change
- heat loss = proportional to SA
- heat production = prop to volume
Neonates have a greater SA to vol ratio than adults, so produce relatively little heat and lose relatively more.
Neonates can be prone to a drop in Tc in response to cold stimuli
Neonates can vasoconstrict and vasodilate skin blood vessels and sweat
What are the 2 responses to low temperatures?
1) increased heat production
2) reduced heat loss
What are the 2 responses to high temperatures?
1) increased heat loss
2) reduced heat production
What are the responses within increased heat production?
1) Voluntary muscle activity: hand rubbing, foot stamping
2) Involuntary muscle activity: increased muscular tone, tremors, shivering (increases HP x5 briefly – but raises convective heat loss)
3) Brown-fat metabolism: new-borns increase heat production x2-3 under sympathetic control = about 4% of body weight
What are the responses within reduced heat production?
1) Inactivity, inertia: humans rest in hot conditions; some animals adapt to a dormancy called aestivation rather like hibernation in cold weather
2) Anorexia: food intake reduced; items w/ high water content are preferred
What are the responses within increased heat loss?
1) Behavioural: response to moderate heat = vary clothing, housing and degree of exposure
2) Vasodilation of skin vessels: varies mainly in TZ in apical areas but at higher temps in non-apical areas; important in mild, moderate, & severe heat exposure
3) Sweating: in man, eccrine glands (apocrine) have evolved to produced sweat (they are foot/palm glands only in most animals). Sweating = main response to severe heat but is wasteful of water & NaCl. Sympathetic cholinergic control. Animals pant.
