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HUMB3001 > W9 - Thermoregulation in Cold Environments > Flashcards

W9 - Thermoregulation in Cold Environments Flashcards

1
Q

What type of variations do environmental temperature follow?

A

Circadian
Circannual
Geographic

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2
Q

What temperature range is the body core kept at?

A

36-38 deg C

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3
Q

What must be balanced in order to maintain stable core temperature?

A

Balance of heat gained by body with heat lost to environment

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4
Q

Where does most of the heat gained by the body come from?

A

Metabolic heat production

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5
Q

What external temperature causes body to gain heat from environment?

A

When ambient temperature exceeds skin temperature

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6
Q

What processes cause heat loss from skin to air?

A

Conduction
Convection
Radiation
Evaporation

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7
Q

What temperature range is the thermoneutral zone for a 70kg naked man?

A

27 - 31 deg C ambient temp

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8
Q

What is TNZ?

A

Thermoneutral zone

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9
Q

Describe thermoneutral zone.

A

Range of environmental temperatures over which it is relatively easy to maintain stable core temperature

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10
Q

Describe metabolic heat production in TNZ.

A

Minimal metabolic heat prodn

Thermoregulation is maintained by vasomotor activity

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11
Q

What is critical temperature?

A

Lowest temp within TNZ

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12
Q

What happens below critical temperature?

A

Metabolic heat production has to be raised to offset heat losses

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13
Q

Why is the person naked for TNZ?

A

Critical temp of clothed individual would be much lower than 27, would vary according to amount and type of clothing worn

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14
Q

What is effect of peripheral vasoconstriction on temperature?

A

Reduces amount of blood moving from deep/warm areas of body to skin

Less heat lost to environment

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15
Q

What is effect of peripheral vasodilatation on temperature?

A

Incr blood moving from deep/warm areas of body to skin

Incr heat loss to environment

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16
Q

Where are temperature sensitive receptors located?

A

Peripherally and centrally

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17
Q

Where are peripheral temperature receptors found and what is density?

A

In skin, some mucous membranes

Lips = 18/cm2
Palm = 2/cm2
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18
Q

What are the two types of peripheral receptors?

A

Ad fibres = cold receptors = stimulated by lower range of temperatures

C fibres = warm receptors = stimulated by higher range of temperatures

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19
Q

Describe number of cold v warm receptors.

A

Cold receptors outnumber warm receptors by 10:1

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20
Q

What skin temperature range do cold fibers send impulses frequently?

A

10 - 30 deg C

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21
Q

What skin temperature range do warm fibers send impulses frequently?

A

Peak at 45 deg C

Range: 30 - 48 deg C

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22
Q

Where are the central cold and warm receptors found?

A

Hypothalamus
Spinal cord
Around central great vessels

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23
Q

Where are the warm receptors primarily found?

A

hypothalamus

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24
Q

Where is information from thermoreceptors transmitted?

A

To hypothalamus and other integrating areas

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25
Q

Which brain regions mediate responses that result in heat loss?

A

Pre-optic region and anterior hypothalamus

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26
Q

Which brain region is involved in heat production and conservation?

A

Posterior hypothalamus

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27
Q

What is the effector organs from thermoreceptor information?

A

Sympathetic fibres innervate arteriolar smooth muscle in skin

Control degree of constriction or dilation of vessels

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28
Q

What happens in body when ambient temperature falls below TNZ?

A

Heat losses from body increase
Exceeds heat production
Thermal balance lost
Core temp will fall

Rapid homeostatic response act to return body temp to normal

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29
Q

What is thermoregulation below critical temperature reliant upon?

A

Metabolic heat prodn

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30
Q

What happens if body’s responses to cold fail?

A

Fail to restore thermal balance

Core body temp falls

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31
Q

What is the clinical definition of hypothermia in man?

A

Core temperature <= 35 degC

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32
Q

What happens in body when above the TNZ?

A

Heat gain from environment/metabolic processes exceed heat loss

Incr core temperature

Evap of water from body surface = loss of heat

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33
Q

How much energy is required to transform 1L water into water vapour?

A

2500 kJ/L

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34
Q

What is clinical definition of hyperthermia in humans?

A

Core body temp >= 40 deg C

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35
Q

What are the 4 mechanisms of increasing heat production?

A

Increased activity
Non-shivering thermogenesis
Metabolic acclimitisation/adaptation
Increased feeding

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36
Q

What is an example of increasing activity to increase heat production?

A

Stamping of feet
Rubbing of hands
Shivering

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37
Q

Which muscles first show shivering?

A

Extensor muscles
Proximal muscles
Of upper limbs and trunk

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38
Q

What occurs in advanced shivering?

A

Jaw muscles shiver

Teeth chatter

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39
Q

How much can shivering increase heat production?

A

5 x

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40
Q

What is downside of shivering?

A

Exhausting
Burden on energy reserves
Cannot be sustained over long time

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41
Q

What receptors are the main stimulus for shivering?

A

Skin receptors

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42
Q

Which receptors influence onset and extent of shivering?

A

Peripheral and central thermoreceptors

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43
Q

What is non-shivering thermogenesis?

A

Heat producing mechanism which liberates energy through processes not involving muscular contraction

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44
Q

What is BAT?

A

Brown adipose tissue

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45
Q

Where is BAT found?

A

Most newborn mammals

Persists into adulthood in species which hibernate

46
Q

What tissues are involved in non-shivering thermogenesis if minimal BAT?

A

Skeletal muscle and liver

47
Q

What is the thermogenic capacity of brown fat?

A

300W / kg

48
Q

What is the thermogenic capacity of skeletal muscle when 20kJ/L O2 at STP?

A

2 W/ kg

Large mass so can have significant contribution

49
Q

What happens during thermogenesis of brown fat?

A

Mitochondrial phosphorylation is uncoupled from respiration so most energy ends up directly as heat

50
Q

What stimulates thermogenesis of brown fat?

A

Noradrenaline

51
Q

Which hormones have known calorigenic actions?

A 
Thyroid hormones
ACTH
glucagon
insulin
corticosteroids
52
Q

What is role of thyroid hormones in heat generation?

A

Incr numbers of mitochondria and activity of membrane bound Na+K+ ATPases

53
Q

What is impact of repeated exposure to low environmental temperatures on homeotherms?

A

Significant increase in non shivering thermogenesis

54
Q

What are 2 cold dwelling populations?

A

Eskimos

Alakaluf Indians of Tierra del Fuego

55
Q

Describe Alakaluf Indians sleep in cold temperatures.

A

Naked in simple shelters in 2-5 deg C

Do not shiver

56
Q

What is Alakaluf Indians changes for survival in cold temperatures?

A

Have basal metabolic rate that is 30-40% higher than other populations

Developed heritable adaptation to cold

57
Q

Do adult eskimos have brown fat?

A

No

58
Q

What is impact of appetite being stimulated by cold environment?

A

Incr food intake

Incr metabolic heat production

59
Q

What causes increased metabolism in increased feeding?

A

Protein content of diet

60
Q

What helps Eskimos offset heat losses in cold?

A

Traditional high protein diet of Eskimos contributes to elevated basal metabolism

61
Q

What is secondary effect of increased metabolism after feeding?

A

Increased blood flow to peripheral regions of body

Incr heat losses to environment, reduces thermoregulatory contribution of feeding

62
Q

What are the 4 mechanism of decreasing heat loss?

A

Behaviour
Insulation
Cutaneous Vasoconstriction
Vascular Acclimatisation / Adaptation

63
Q

How can behaviour effect heat loss?

A

Reduce surface area available for heat exchange

64
Q

What do most terrestrial mammals use to decrease heat loss?

A

Fur for insulation

65
Q

What is role of fur in decreasing heat loss?

A

Fur insulates by trapping air next to skin
Reduces convection heat losses

Insulative value is directly related to length

66
Q

What is the most noticeable result of horripilation?

A

Goose pimples

67
Q

What is horripilation?

A

Low environmental temps cause contraction of piloerector muscles of the few hairs that are present on humans

68
Q

What is humans main insulative protection against cold?

A

Clothing

Traps air between fibres of fabric

69
Q

What enhances the thermal insulation of clothing?

A

Several layers of clothing are worn under windproof outer garment

70
Q

Describe the properties that outer garments should have?

A

Semi permeable to water vapour or insulation value of clothing would be decreased by condensation and possible freezing of evaporated sweat

71
Q

Describe role of subcutaneous fat in decreasing heat loss.

A

Affords some protection against cold

72
Q

Describe changes in body weight between seasons.

A

Incr weight (1-2kg) over winter and decreased in summer

May be reflection of incr food consumption, decr activity

73
Q

Describe body shape and size of Eskimos, Alakalufs and Andeans.

A

Relatively lean
Shorter and more compact body shape
Smaller SA than tropical counterparts

74
Q

What is the primary physiological response to reduced environmental temperature?

A

Cutaneous vasoconstriction

75
Q

What is the result of prolonged cooling?

A

Paradoxical vasodilatation with consequent heat loss

76
Q

Describe the Hunting Reaction.

A

Vasoconstrction alternates with periods of vasodilatation

77
Q

Where does hunting reaction primarily occur?

A

Extremities of body where there is little metabolically active tissue (e.g. finger)

78
Q

What is the role of hunting reaction?

A

Prevent tissue damage e.g. frost bite

79
Q

What is the cause of vasodilatation of skin BV?

A

Results primarily from direct cold-induced paralysis of peripheral BV which lose their ability to respond to SNS at low temp

80
Q

Describe vascular acclimatisation in low temperatures.

A

Fisherman, polar explorers have less severe initial vasoconstrictor response and more rapid subsequent vasodilatation

81
Q

Describe peripheral blood flow in Eskimos.

A

Higher than normal rate of peripheral blood flow to extremities when exposed to cold

82
Q

What is the purpose of vascular acclimatisation?

A

Prevent injury

Permit greater manual dexterity in cold

83
Q

Describe hiberation.

A

Mammals opt out of homeothermy and allow core temperature to fall to level approaching that of environment

Eliminates increased cost of keeping warm, cold tissues use less fuel, energy reserves last longer

Animal retains ability to rewarm itself to original temperature without heat from environment

84
Q

What is impact on body of hibernation?

A

Body temp falls

HR, metabolic rate, others reduces

85
Q

What governs hibernation?

A

Ambient temperature
Availability of food, water
Time of year (light, dark)

86
Q

Describe arousal from hiberation.

A 
Relatively quick (90 min)
Core temp raised by 30 deg C
87
Q

What mechanisms does arousal from hibernation use?

A

Shivering and non-shivering thermogenic processes

Expensive metabolically

88
Q

Which human race shows hypothermic response to lowered environmental temperature?

A

Aboriginals (Australian)
Core temp falls below 35 deg C

Can afford to cool down during night as they will quickly warm up in morning sun

89
Q

Describe ease of reducing core temp to below 35.5 deg C.

A

Difficult to reduce

Due to effectiveness of behavioural responses and shivering

90
Q

What is effect of core temp dropping below 35 deg C?

A

Muscular weakness
Decreases mobility
Diminished shivering
Core temp will decrease rapidly

91
Q

What happens at core temps below 34 deg C?

A

Mental confusion and visual disturbances occur

92
Q

What core body temp is consciousness lost?

A

30-32 deg C

93
Q

What is the most important effect of hypothermia?

A

Reduced body temperature on heart

94
Q

What is effect of core temp below 28 deg C on heart?

A

Cardiac arrythmias
w ventricular fibrillation
Death

Slows pacemaker and CO declines

95
Q

Describe recovery from severe hypothermia.

A

Can be rapid and complete

96
Q

Why is high level of physical fitness important for hypothermia survival?

A

High rate of energy expenditure (to offset heat losses) can be maintained for longer without fatigue

97
Q

What environment accelerates hypothermia?

A

Immersion in cold water

Thermal conductivity of water is greater than air

98
Q

Which age group is at greater risk from accidental hypothermia?

A

Extremes of age

Very young / old

99
Q

What is the most serious form of cold injury?

A

Frostbite

100
Q

What is frostbite?

A

Freezing of peripheral tissues

101
Q

What occurs in mild cases of frostbite?

A

Skin freezes

102
Q

What occurs in severe cases of frostbite?

A

Deeper tissues, muscle, bone, tendon may freeze

103
Q

What is effect of frostbite on cells?

A

Damage to affected cells results from mechanical action of ice crystals and cell dehydration

Ice forming within cells reduces water content and incr osmolality

104
Q

What is effect of frostbite on blood vessels?

A

Increased permeability

105
Q

What happens in blood vessels during thawing?

A

Loss of fluid from circulation in tissue spaces

Incr packed cell volume causes reduced/stops blood flow

106
Q

What causes gangrene after frostbite?

A

BV thaw incr packed cell volume

Stops blood flow

107
Q

What are chilblains?

A

Mild form of cold inury in fingers, toes, ears

Occur after prolonged cooling of extremities especially in people with poor circulation

108
Q

What is the form of major heat loss in water?

A

Conduction

109
Q

Describe survival time in cold water.

A

Varies directly with water temp

Naked person hypothermic after 20-30min in water at 5 deg C

Naked person hypothermic after 1.5-2h in 15 deg C water

110
Q

What type of people become hypothermic in water more rapidly?

A

Thin people become hypothermic more rapidly than fat people

111
Q

What affects heat loss in water of subjects?

A

Degree of movement

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