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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
Q

Which people have the optimal human form for desert environment?

A

Nilotic people of Sudan
Masai of Kenya
2.14m average height

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26
Q

What is the primary means of increasing heat loss in hot environments?

A

Sweating

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27
Q

Describe sweat gland density and distribution on body.

A

120 sweat glands/cm2

50% of sweat from glands on back, chest

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28
Q

What is the maximum sweat rate in man?

A

3 L/ hour

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29
Q

What is an average sweat rate for man working in hot dry conditions?

A

12 L /day

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30
Q

Compare the sweat rates between male and females.

A

Women have lower sweat rates generally than men

31
Q

Describe sweat as a solution.

A

Hypotonic solution

32
Q

What glands produce sweat?

A

Eccrine glands in dermis of skin

33
Q

Describe sweat production and secretion.

A

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
Q

Describe sodium content at different rates of sweat secretion.

A

Low rates of secretion Na content is low (5 mMol/L)

High secretion rate, less time for reabsorption, Na is 10x higher

35
Q

What is the latent heat of evaporation of water at 37 deg C?

A

2.4 kJ/mL

36
Q

What is heat loss when there is a maximum sweat rate of 3L/h?

A

Heat loss of 7000 kJ/h or 20x BMR

37
Q

How does hair impact sweating?

A

Lack of hair on surface of body facilitates evaporation of sweat

38
Q

What significantly influences sweat evaporation?

A

The amount of water vapour in atmosphere

39
Q

Why does human body appear dry in desert even when sweating?

A

Evaporation is so rapid that sweat evaporates as soon as reaches surface of skin

40
Q

What happens when sweating in humid, tropical regions?

A

Humidity is approaching 100%

Little/no evaporation occurs

41
Q

What happens to sweating when drinking water is unrestricted?

A

Balance is established between fluid intake and combined losses from sweating and urinary excretion

42
Q

What happens when sweating and drinking water is restricted?

A

Dehydration occurs

43
Q

What level of fluid loss triggers thirst mechanisms?

A

Fluid loss of 2% body weight

44
Q

What is impact of excessive sweating?

A

May lead to negative salt balance unless salt intake increased

45
Q

How does body counteract salt losses in sweating?

A

Increasing secretion of aldosterone to conserve sodium

46
Q

How long does it take humans to acclimatise to desert conditions?

A

1-2 weeks

47
Q

What change occurs in desert acclimatisation?

A
  • 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
Q

What are symptoms of salt deficiency heat exhaustion?

A

Muscular cramps

49
Q

What mechanism do mammals use to increase heat loss?

A

Cool primarily by panting

50
Q

Describe panting.

A

Increase evaporation from upper respiratory tract

51
Q

What are the advantages of panting over sweating?

A

Provide own air currents to facilitate evaporation

Does not result in loss of Na

52
Q

What is disadvantages of panting?

A

Increased ventilation
Increases work of breathing
Causes alkalosis

53
Q

What CVS response occurs as ambient temperature rises?

A

Rapid cutaneous vasodilatation

54
Q

What is purpose of cutaneous vasodilatation in heat loss?

A

Promotes transfer of heat from deep to superficial body tissues to environment

55
Q

What is impact of increased core temperature on CVS?

A

Cardiac output increases with temperature = direct effect on sinoatrial node = increase HR

56
Q

What happens to CVS when heat stress is prolonged?

A

Increased stroke volume

57
Q

What happens to CVS during acclimatisation to heat?

A

Some CVS changes are reversed

HR will fall if core temperature is normalised

58
Q

Describe tolerance of camels to hot environments.

A

No attempt to thermoregulate until core temp > 41 deg C

59
Q

Describe tolerance of African gazelles to hot environments?

A

Can tolerate core temp of 46 deg for several hours

60
Q

How can camels and gazelles tolerate higher temperature environments?

A

Brain temperature is kept at 40 deg C by special heat exchange network at base of brain

61
Q

What is the significance of camel not thermoregulating until core temp > 41 ?

A

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
Q

What are 3 conditions that happen when failure to thermoregulate in extreme heat occurs?

A

Heat stroke
Heat exhaustion
Heat collapse

63
Q

What is heat stroke and how does it result?

A

Most serious consequence of heat stress

Resulting from complete loss of thermoregulatory control - typically failure of sweating mechanism

64
Q

What are characteristics of heat stroke?

A 
Loss of energy
Irritability
Serious neurological and mental disturbances
Sweat diminishes/ceases
Subject becomes comatosed
Death
65
Q

At what core temperature in humans does heat stroke cause coma?

A

42 degrees or above

66
Q

What is cause of water deficiency heat exhaustion?

A

Insufficient water replacement of fluid losses

67
Q

Describe tolerance of humans to water deficit.

A

Tolerate: 3-4% water deficit

68
Q

What happens with water loss of 5-8%?

A

Fatigue and dizziness

69
Q

What happens with water loss of >10%?

A

Physical and mental deterioration

70
Q

What is the lethal limit of fluid loss for humans?

A

15-25% of body weight

71
Q

What is impact of water deficiency on human body?

A

Affects ECF volume then intracellular dehydration

Gradual decr in plasma volume (resp for initial symptoms)

72
Q

What are the characteristics of heat collapse?

A

Fatigue, dizziness, temporary loss of consciousness

Characteristic of unacclimatised indivudal in hot climate

73
Q

What is heat syncope?

A

Heat collapse

74
Q

What causes heat syncope?

A

Pooling of blood in dilated BV of skeletal muscles + skin in lower limbs

Cerebral circulation becomes compromised, consciousness fades

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