W9 - Thermoregulation in Cold Environments Flashcards
1
Q
What type of variations do environmental temperature follow?
A
Circadian
Circannual
Geographic
2
Q
What temperature range is the body core kept at?
A
36-38 deg C
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
4
Q
Where does most of the heat gained by the body come from?
A
Metabolic heat production
5
Q
What external temperature causes body to gain heat from environment?
A
When ambient temperature exceeds skin temperature
6
Q
What processes cause heat loss from skin to air?
A
Conduction
Convection
Radiation
Evaporation
7
Q
What temperature range is the thermoneutral zone for a 70kg naked man?
A
27 - 31 deg C ambient temp
8
Q
What is TNZ?
A
Thermoneutral zone
9
Q
Describe thermoneutral zone.
A
Range of environmental temperatures over which it is relatively easy to maintain stable core temperature
10
Q
Describe metabolic heat production in TNZ.
A
Minimal metabolic heat prodn
Thermoregulation is maintained by vasomotor activity
11
Q
What is critical temperature?
A
Lowest temp within TNZ
12
Q
What happens below critical temperature?
A
Metabolic heat production has to be raised to offset heat losses
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
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
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
16
Q
Where are temperature sensitive receptors located?
A
Peripherally and centrally
17
Q
Where are peripheral temperature receptors found and what is density?
A
In skin, some mucous membranes
Lips = 18/cm2 Palm = 2/cm2
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
19
Q
Describe number of cold v warm receptors.
A
Cold receptors outnumber warm receptors by 10:1
20
Q
What skin temperature range do cold fibers send impulses frequently?
A
10 - 30 deg C
21
Q
What skin temperature range do warm fibers send impulses frequently?
A
Peak at 45 deg C
Range: 30 - 48 deg C
22
Q
Where are the central cold and warm receptors found?
A
Hypothalamus
Spinal cord
Around central great vessels
23
Q
Where are the warm receptors primarily found?
A
hypothalamus
24
Q
Where is information from thermoreceptors transmitted?
A
To hypothalamus and other integrating areas
25
Which brain regions mediate responses that result in heat loss?
Pre-optic region and anterior hypothalamus
26
Which brain region is involved in heat production and conservation?
Posterior hypothalamus
27
What is the effector organs from thermoreceptor information?
Sympathetic fibres innervate arteriolar smooth muscle in skin
Control degree of constriction or dilation of vessels
28
What happens in body when ambient temperature falls below TNZ?
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
29
What is thermoregulation below critical temperature reliant upon?
Metabolic heat prodn
30
What happens if body's responses to cold fail?
Fail to restore thermal balance
Core body temp falls
31
What is the clinical definition of hypothermia in man?
Core temperature <= 35 degC
32
What happens in body when above the TNZ?
Heat gain from environment/metabolic processes exceed heat loss
Incr core temperature
Evap of water from body surface = loss of heat
33
How much energy is required to transform 1L water into water vapour?
2500 kJ/L
34
What is clinical definition of hyperthermia in humans?
Core body temp >= 40 deg C
35
What are the 4 mechanisms of increasing heat production?
Increased activity
Non-shivering thermogenesis
Metabolic acclimitisation/adaptation
Increased feeding
36
What is an example of increasing activity to increase heat production?
Stamping of feet
Rubbing of hands
Shivering
37
Which muscles first show shivering?
Extensor muscles
Proximal muscles
Of upper limbs and trunk
38
What occurs in advanced shivering?
Jaw muscles shiver
| Teeth chatter
39
How much can shivering increase heat production?
5 x
40
What is downside of shivering?
Exhausting
Burden on energy reserves
Cannot be sustained over long time
41
What receptors are the main stimulus for shivering?
Skin receptors
42
Which receptors influence onset and extent of shivering?
Peripheral and central thermoreceptors
43
What is non-shivering thermogenesis?
Heat producing mechanism which liberates energy through processes not involving muscular contraction
44
What is BAT?
Brown adipose tissue
45
Where is BAT found?
Most newborn mammals
Persists into adulthood in species which hibernate
46
What tissues are involved in non-shivering thermogenesis if minimal BAT?
Skeletal muscle and liver
47
What is the thermogenic capacity of brown fat?
300W / kg
48
What is the thermogenic capacity of skeletal muscle when 20kJ/L O2 at STP?
2 W/ kg
| Large mass so can have significant contribution
49
What happens during thermogenesis of brown fat?
Mitochondrial phosphorylation is uncoupled from respiration so most energy ends up directly as heat
50
What stimulates thermogenesis of brown fat?
Noradrenaline
51
Which hormones have known calorigenic actions?
```
Thyroid hormones
ACTH
glucagon
insulin
corticosteroids
```
52
What is role of thyroid hormones in heat generation?
Incr numbers of mitochondria and activity of membrane bound Na+K+ ATPases
53
What is impact of repeated exposure to low environmental temperatures on homeotherms?
Significant increase in non shivering thermogenesis
54
What are 2 cold dwelling populations?
Eskimos
| Alakaluf Indians of Tierra del Fuego
55
Describe Alakaluf Indians sleep in cold temperatures.
Naked in simple shelters in 2-5 deg C
Do not shiver
56
What is Alakaluf Indians changes for survival in cold temperatures?
Have basal metabolic rate that is 30-40% higher than other populations
Developed heritable adaptation to cold
57
Do adult eskimos have brown fat?
No
58
What is impact of appetite being stimulated by cold environment?
Incr food intake
| Incr metabolic heat production
59
What causes increased metabolism in increased feeding?
Protein content of diet
60
What helps Eskimos offset heat losses in cold?
Traditional high protein diet of Eskimos contributes to elevated basal metabolism
61
What is secondary effect of increased metabolism after feeding?
Increased blood flow to peripheral regions of body
Incr heat losses to environment, reduces thermoregulatory contribution of feeding
62
What are the 4 mechanism of decreasing heat loss?
Behaviour
Insulation
Cutaneous Vasoconstriction
Vascular Acclimatisation / Adaptation
63
How can behaviour effect heat loss?
Reduce surface area available for heat exchange
64
What do most terrestrial mammals use to decrease heat loss?
Fur for insulation
65
What is role of fur in decreasing heat loss?
Fur insulates by trapping air next to skin
Reduces convection heat losses
Insulative value is directly related to length
66
What is the most noticeable result of horripilation?
Goose pimples
67
What is horripilation?
Low environmental temps cause contraction of piloerector muscles of the few hairs that are present on humans
68
What is humans main insulative protection against cold?
Clothing
| Traps air between fibres of fabric
69
What enhances the thermal insulation of clothing?
Several layers of clothing are worn under windproof outer garment
70
Describe the properties that outer garments should have?
Semi permeable to water vapour or insulation value of clothing would be decreased by condensation and possible freezing of evaporated sweat
71
Describe role of subcutaneous fat in decreasing heat loss.
Affords some protection against cold
72
Describe changes in body weight between seasons.
Incr weight (1-2kg) over winter and decreased in summer
May be reflection of incr food consumption, decr activity
73
Describe body shape and size of Eskimos, Alakalufs and Andeans.
Relatively lean
Shorter and more compact body shape
Smaller SA than tropical counterparts
74
What is the primary physiological response to reduced environmental temperature?
Cutaneous vasoconstriction
75
What is the result of prolonged cooling?
Paradoxical vasodilatation with consequent heat loss
76
Describe the Hunting Reaction.
Vasoconstrction alternates with periods of vasodilatation
77
Where does hunting reaction primarily occur?
Extremities of body where there is little metabolically active tissue (e.g. finger)
78
What is the role of hunting reaction?
Prevent tissue damage e.g. frost bite
79
What is the cause of vasodilatation of skin BV?
Results primarily from direct cold-induced paralysis of peripheral BV which lose their ability to respond to SNS at low temp
80
Describe vascular acclimatisation in low temperatures.
Fisherman, polar explorers have less severe initial vasoconstrictor response and more rapid subsequent vasodilatation
81
Describe peripheral blood flow in Eskimos.
Higher than normal rate of peripheral blood flow to extremities when exposed to cold
82
What is the purpose of vascular acclimatisation?
Prevent injury
| Permit greater manual dexterity in cold
83
Describe hiberation.
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
What is impact on body of hibernation?
Body temp falls
| HR, metabolic rate, others reduces
85
What governs hibernation?
Ambient temperature
Availability of food, water
Time of year (light, dark)
86
Describe arousal from hiberation.
```
Relatively quick (90 min)
Core temp raised by 30 deg C
```
87
What mechanisms does arousal from hibernation use?
Shivering and non-shivering thermogenic processes
Expensive metabolically
88
Which human race shows hypothermic response to lowered environmental temperature?
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
Describe ease of reducing core temp to below 35.5 deg C.
Difficult to reduce
| Due to effectiveness of behavioural responses and shivering
90
What is effect of core temp dropping below 35 deg C?
Muscular weakness
Decreases mobility
Diminished shivering
Core temp will decrease rapidly
91
What happens at core temps below 34 deg C?
Mental confusion and visual disturbances occur
92
What core body temp is consciousness lost?
30-32 deg C
93
What is the most important effect of hypothermia?
Reduced body temperature on heart
94
What is effect of core temp below 28 deg C on heart?
Cardiac arrythmias
w ventricular fibrillation
Death
Slows pacemaker and CO declines
95
Describe recovery from severe hypothermia.
Can be rapid and complete
96
Why is high level of physical fitness important for hypothermia survival?
High rate of energy expenditure (to offset heat losses) can be maintained for longer without fatigue
97
What environment accelerates hypothermia?
Immersion in cold water
| Thermal conductivity of water is greater than air
98
Which age group is at greater risk from accidental hypothermia?
Extremes of age
| Very young / old
99
What is the most serious form of cold injury?
Frostbite
100
What is frostbite?
Freezing of peripheral tissues
101
What occurs in mild cases of frostbite?
Skin freezes
102
What occurs in severe cases of frostbite?
Deeper tissues, muscle, bone, tendon may freeze
103
What is effect of frostbite on cells?
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
What is effect of frostbite on blood vessels?
Increased permeability
105
What happens in blood vessels during thawing?
Loss of fluid from circulation in tissue spaces
Incr packed cell volume causes reduced/stops blood flow
106
What causes gangrene after frostbite?
BV thaw incr packed cell volume
| Stops blood flow
107
What are chilblains?
Mild form of cold inury in fingers, toes, ears
Occur after prolonged cooling of extremities especially in people with poor circulation
108
What is the form of major heat loss in water?
Conduction
109
Describe survival time in cold water.
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
What type of people become hypothermic in water more rapidly?
Thin people become hypothermic more rapidly than fat people
111
What affects heat loss in water of subjects?
Degree of movement
