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HUMB3001 > W10 - Thermoregulation in Hot Environments > Flashcards

W10 - Thermoregulation in Hot Environments Flashcards

1
Q

Where are central warm receptors primarily located?

A

Hypothalamus

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2
Q

Where in brain mediates responses that result in heat loss?

A

Pre-optic region & anterior hypothalamus

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3
Q

Which section of the brain is involved in heat production and conservation?

A

Cells in posterior hypothalamus

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4
Q

What causes rise in core temperature?

A

When heat gain from environment and metabolic processes exceed heat losses

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5
Q

How does body lose heat?

A

Evaporation of water from body surface

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6
Q

How much energy is required to transform water into water vapour?

A

2400 kJ/Litre

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7
Q

What is the clinical definition of hyperthermia in humans?

A

Core body temperature of 40 deg C or above

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8
Q

What are the main heat responses?

A

Decr heat gain by reduced activity, reduced feeding

Reduced heat gain from environment

Increase heat loss by body shape, sweating, panting/salivation, CVS responses

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9
Q

What are the 2 main ways to decrease heat gain?

A

Reduce activity, reduce feeding

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10
Q

What is impact of muscle activity on heat production?

A

Increased voluntary muscle activity increases heat production

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11
Q

What activity is synonymous with hot climates?

A

General apathy and inertia

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12
Q

What is an example of reduced activity to decrease heat gain?

A

Mexican siesta

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13
Q

What is aestivation?

A

Dormancy by mammals in summer months to reduce activity levels to zero and remain dormant in burrows

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14
Q

What is impact of food intake on heat production?

A

Increased food intake elevates metabolic heat production

Hot environment promotes anorexia

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15
Q

Describe food preference in hot environments.

A

High water content foods like fruit, salad, vegetables

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16
Q

What physiological methods are used to maximise heat loss?

A

Vasodilation of cutaneous BV
Increased sweating
Behavioural responses

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17
Q

What behavioural responses are used to maximise heat loss?

A

Use fans to incr convective heat loss
Water immersion to incr conductive heat loss
Stay out of sun to prevent radiant heat gain
Remove clothes

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18
Q

What behavioural methods are used to minimise heat production?

A

Diminish food intake to lessen obligatory heat production

Decrease physical activity

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19
Q

What is the purpose of clothing in dry desert areas?

A

Reduce heat gain from environment and increase heat loss from body

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20
Q

What type of clothing is required in dry desert areas and why?

A

Loose fitting, lightweight, light coloured materials

Reflect much of short wave solar radiation, permits circulation of air necessary to evaporate sweat

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21
Q

What type of clothing is required for humid tropical regions and why?

A

Minimal clothing to reduce body’s heat load and maximise surface area for sweat evaporation

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22
Q

What is the ideal body shape for hot desert conditions?

A

Tall with long lean limbs and low subcutaneous fat

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23
Q

How does tallness increase heat loss?

A

Maximises surface area to body weight ratio for evaporative cooling

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24
Q

How does leanness help increase heat loss?

A

Facilitates heat conduction from deep body tissues

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25
Which people have the optimal human form for desert environment?
Nilotic people of Sudan Masai of Kenya 2.14m average height
26
What is the primary means of increasing heat loss in hot environments?
Sweating
27
Describe sweat gland density and distribution on body.
120 sweat glands/cm2 | 50% of sweat from glands on back, chest
28
What is the maximum sweat rate in man?
3 L/ hour
29
What is an average sweat rate for man working in hot dry conditions?
12 L /day
30
Compare the sweat rates between male and females.
Women have lower sweat rates generally than men
31
Describe sweat as a solution.
Hypotonic solution
32
What glands produce sweat?
Eccrine glands in dermis of skin
33
Describe sweat production and secretion.
Primary isotonic secretion produced by proximal (coiled) region of eccrine gland Modified by solute reabsorption as fluid moves along duct towards skin surface - becomes hypotonic
34
Describe sodium content at different rates of sweat secretion.
Low rates of secretion Na content is low (5 mMol/L) High secretion rate, less time for reabsorption, Na is 10x higher
35
What is the latent heat of evaporation of water at 37 deg C?
2.4 kJ/mL
36
What is heat loss when there is a maximum sweat rate of 3L/h?
Heat loss of 7000 kJ/h or 20x BMR
37
How does hair impact sweating?
Lack of hair on surface of body facilitates evaporation of sweat
38
What significantly influences sweat evaporation?
The amount of water vapour in atmosphere
39
Why does human body appear dry in desert even when sweating?
Evaporation is so rapid that sweat evaporates as soon as reaches surface of skin
40
What happens when sweating in humid, tropical regions?
Humidity is approaching 100% | Little/no evaporation occurs
41
What happens to sweating when drinking water is unrestricted?
Balance is established between fluid intake and combined losses from sweating and urinary excretion
42
What happens when sweating and drinking water is restricted?
Dehydration occurs
43
What level of fluid loss triggers thirst mechanisms?
Fluid loss of 2% body weight
44
What is impact of excessive sweating?
May lead to negative salt balance unless salt intake increased
45
How does body counteract salt losses in sweating?
Increasing secretion of aldosterone to conserve sodium
46
How long does it take humans to acclimatise to desert conditions?
1-2 weeks
47
What change occurs in desert acclimatisation?
- Increased sweat rate - After 2 weeks; sweat loss/time may have doubled - Sweating starts at lower threshold core temperature - Decreased salt concentration in sweat
48
What are symptoms of salt deficiency heat exhaustion?
Muscular cramps
49
What mechanism do mammals use to increase heat loss?
Cool primarily by panting
50
Describe panting.
Increase evaporation from upper respiratory tract
51
What are the advantages of panting over sweating?
Provide own air currents to facilitate evaporation | Does not result in loss of Na
52
What is disadvantages of panting?
Increased ventilation Increases work of breathing Causes alkalosis
53
What CVS response occurs as ambient temperature rises?
Rapid cutaneous vasodilatation
54
What is purpose of cutaneous vasodilatation in heat loss?
Promotes transfer of heat from deep to superficial body tissues to environment
55
What is impact of increased core temperature on CVS?
Cardiac output increases with temperature = direct effect on sinoatrial node = increase HR
56
What happens to CVS when heat stress is prolonged?
Increased stroke volume
57
What happens to CVS during acclimatisation to heat?
Some CVS changes are reversed | HR will fall if core temperature is normalised
58
Describe tolerance of camels to hot environments.
No attempt to thermoregulate until core temp > 41 deg C
59
Describe tolerance of African gazelles to hot environments?
Can tolerate core temp of 46 deg for several hours
60
How can camels and gazelles tolerate higher temperature environments?
Brain temperature is kept at 40 deg C by special heat exchange network at base of brain
61
What is the significance of camel not thermoregulating until core temp > 41 ?
Camel at 500kg would have heat storage > 7000 kJ with 4 deg temp rise Equivalent to 3L of sweat saved = important if water supplies are restricted Night: stored heat can be unloaded by conduction, radiation with minimal fluid losses
62
What are 3 conditions that happen when failure to thermoregulate in extreme heat occurs?
Heat stroke Heat exhaustion Heat collapse
63
What is heat stroke and how does it result?
Most serious consequence of heat stress Resulting from complete loss of thermoregulatory control - typically failure of sweating mechanism
64
What are characteristics of heat stroke?
``` Loss of energy Irritability Serious neurological and mental disturbances Sweat diminishes/ceases Subject becomes comatosed Death ```
65
At what core temperature in humans does heat stroke cause coma?
42 degrees or above
66
What is cause of water deficiency heat exhaustion?
Insufficient water replacement of fluid losses
67
Describe tolerance of humans to water deficit.
Tolerate: 3-4% water deficit
68
What happens with water loss of 5-8%?
Fatigue and dizziness
69
What happens with water loss of >10%?
Physical and mental deterioration
70
What is the lethal limit of fluid loss for humans?
15-25% of body weight
71
What is impact of water deficiency on human body?
Affects ECF volume then intracellular dehydration Gradual decr in plasma volume (resp for initial symptoms)
72
What are the characteristics of heat collapse?
Fatigue, dizziness, temporary loss of consciousness Characteristic of unacclimatised indivudal in hot climate
73
What is heat syncope?
Heat collapse
74
What causes heat syncope?
Pooling of blood in dilated BV of skeletal muscles + skin in lower limbs Cerebral circulation becomes compromised, consciousness fades

HUMB3001 (8 decks)

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