Human performance at environmental extremes Flashcards
What are the components of thermoregulation
Behaviour!
Cold Shivering Non shivering thermo BAT (increased metab) Pilo erection (minor) Vasoconstriction
Hot
Vasodilation
sweat
Metabolic rate can decrease here
What is meant by the ‘environment driven zone’ and the ‘prescriptive zone’ and what factor can change this?
EDZ = Temperatures at which the core temperature is raised after exercising above the typical amount for lower temperatures e.g. normally 10-25 deg it will always be raised to x and above 25 it will be >x.
Prescriptive zone = ambient temperatures at which the body raises the core temperatures to a consistent set level.
Potential effects of cold temperature on HR?
Shivering so increased HR
Gold standard of temperature measuring?
Oesophageal
Describe what happens if core temperature rises
39 normal in exercise (possibly 40)
41 ok - used in fever therapy
44 - heat stroke, death, brain damage
What happens if core temperature falls?
35 - reduced consciousness
31-33 VF
31 Death
Depending on speed people have survived as low as 14 degree core temp.
Describe the unified controller model of thermoregulation?
Sesory detect temperatures in core and shell - feel into single controller with a temperature set point. Mismatch in sensor temperature and set point causes effector stimulation.
Inibitory connectsions between ant and post so that there is only hot or cold response.
Describe the unified controller with individual effector loops model and describe the effector responses.
Set point in the brain individually triggers effectors/ These systems then cause a rise or fall in temperatre which is sensed and feedsback into the effector activity. Work towards same set point but effectors are activated in different amount.
Proportionality differs for each response based on rise/ fall in temperature and difference between shell and core temperature. Larger =sweating/shivering. smaller = vasocon/dilation.
Roles of anterior and posterior hypothalamus.
Anterior - vasodilation and sweating
Posterior - vasoconstriction and shivering
Describe the independent control loops for different effectors model.
The activity of each effector is dependent on an independent threshold value.
For example shivering and sweating need a start when the temperature deviates from the
Where is the central control point in the brain?
nucleus pre opticus - anterior hypothalamus
Describe the perception of temperature and factors that effect this.
Perception depends on sensation, sensitisation and rate of temperature change.
Sensors consist of free nerve endings.
Sense via TRP receptors which are independently triggered by different temperature range (peak activity varies for each one)
Extreme temperature TRP channels cause pain.
Can be triggered by menthol/ capacian.
Activity increased with sudden temperature change and decrease by prolonged stimulation (sensitisation)
How is metabolic rate at certain temperatures linked to age?
As a baby - higher SA:body so need more heat production as proportionally more heat loss. Metabolism increases faster as ambient lowers and starts increasing at higher temperatures
How significant is the role of BAT and where is it found
Neck and shoulders (perirenal in babies).
Only produces 8/10Ws
Describe apocrine sweat
Found on hair follicles Mixes with sebaceous glands to produce odour (proteins broken down). Located circum oral, eyelids, nipples, axillae, outer ear.
Not much role in temp reg
Describe eccrine sweat glands
Located all over body.
Can only become active in first 2 years of life (hot countries have more) however have capacity to grow. More dense (more sweat) on back than trunc, around spine, forehead, upper leg.
What is sweat capacity effected by?
Fitness, training, age, sex (males sweat more and have a higher capacity?), acclimatisation
How is blood follow affected by temperature? (distribution) effect of exericse?
Vasoconstriction and dilation.
Keeps blood to core structure to prevent heat loss.
Arteriovenous anastomoses prevent blood travelling through more superficial capillaries (preventing heat loss) and also shunts blood to deeper veins (preventing them going to superficial ones) means blood can get warmed by accompanying artery.
25-30c variation in arm blood flow.
In heat everything wants blood, vasdilation often is more important than maintaining BP in extremes of temp.
How is blood follow affected by temperature? (distribution) effect of exericse?
Vasoconstriction and dilation.
Keeps blood to core structure to prevent heat loss -blood to superficial veins closed off so has to go deep. shunts blood to deeper veins (preventing them going to superficial ones) means blood can get warmed by accompanying artery. (countercurrent heat exchange via venae comitans).
If too hot.
Arteriovenous anastomoses prevent blood travelling through capillaries meaning resistance is reduced and more blood travels to arm. Causes arm to get warmer, more blood in superficial veins and more heat loss.
25-30c variation in arm blood flow.
In heat everything wants blood, vasdilation often is more important than maintaining BP in extremes of temp.
Describe the various ways to measure temperature
Thermistors - adds resistance with heat
Thermocouples - 2 wires of different material that produce volage dependent on temperature difference
Radio pill - expensive, swallow with transmittor
Infrared thermometer e.g. tympanic membrane
heat flux disc - transducer than gives electric signal based on heat rate applied.
Many areas e.g. orifice, intra abdominal, su lingual, axillary.
Aural-Hearing defenders
Urine temp
Rectal good but slow to pick up changes.
Describe the various ways to measure temperature
Thermistors - adds resistance with heat
Thermocouples - 2 wires of different material that produce volage dependent on temperature difference
Radio pill - expensive, swallow with transmittor
Infrared thermometer e.g. tympanic membrane
heat flux disc - transducer than gives electric signal based on heat rate applied.
Many areas e.g. orifice, intra abdominal, su lingual, axillary.
Aural-Hearing defenders
Urine temp
Rectal good but slow to pick up changes.
What is a homeotherm?
An organism that maintains its temperature at a constant level via its metabolic activity
What is thermal conductance? How is relevant to the body?
Degree to which heat can be transferred to or from. different body parts have different conductances- a lot sue to blood supply. Conduction is due to gradients.
Also convection = ECF
Describe where thermoregulation fits in in the hierarchy of systems
Ranks highly
in heat - BP can drop (heatsyncope) and dehydration can occur
Location of heat sensors
Centrally - hypo, medulla oblongata, spinal cord
Peripherally free nerve endings via adelta or C fibres and spinothalamic tract.
Describe briefly the types of effectors of thermoregulation and their control
Shivering
Skeletal nervous supply
Sweating
Non medullated cholinergic symptathetic
Vasconstriction
Adrenergic Non medullated sympathetics
Vasodilation
Symp nerve or artine vasodilator nerves
Non shivering thermogenesis
Sympathetic adrenomedullary system
Catechoamine release (higher CO, vasoconstric, HR, non shvering thermo, glucose) Symp to adrenal medulla.
Endocrine control relating to seasons
How is the brain cooled? (possibly)
Cooler blood from mouth and nasal area is shunted towards brain known as selective brain cooling (SBC). Maybe only in animals/ dogs
Role of piloerection?
trap a layer of air - more sig with clothing?
What is hidromeiosis? Why has it developed?
Local phenomenon whereby sweat production is decreased by moisture on skin (only an effective heat loss mechanism if sweat actually evaporates).
Swelling of epidermis to block sweat glands
Reasons for reduction in sweating (x3)
Hydromeiosis, decrease in temperature (exponential decline)
Sweat gland fatigue (?)
Describe unconscious behavioural thermoreglation and technical regulation
Body postioning - towards fire, babies spread out, lizard off floor, clenched fists.
Technical = building shelter ect.
Also psychological changes e.g. mood and warmth/ cold.
How is mean body temperature calculated
Weighed averages
5 ways the body can gain or lose heat
Radiation Conduction Convection Respiration Evaporation
Describe conduction
Heat lost down temperature gradient from physical contact with solid materials in the environment
Minimal effect although more significant in extremes e.g. frost bite.
Depends on surface’s conductivity and specific heat capcity
Describe convection
Physical movement of air/water past the body which serves to carry heat.
Heat of body warms surrounding air that can be displaced by cold air due to a draught (forced) or natural buoyancy of warmer air.
70-80% of heat loss in cold
100% in water
Can cause heat gain if hot
Describe radiation
Can gain from envioronment too e.g infrared from a hot desert floor or working with fires ect.
Includes direct sun, reflected from sun and infrared (two way Infrared)
Describe the efficiency of the bodies metabolism to do work
Very inefficient. Not much energy actually lost in the form of mechanical work e.g. 0 in walking
What is the conceptual heat banalane equation?
Used to calculate rate of energy storage (M-W) = (K+C+R+Esk) + S M = metab produc W =Mechanical work K = Conduc loss C = Convec R = Resp Esk = Evap S = rate of energy storage
What parameters affect heat storage?
Energy production Energy loss Climate -Temp -Humidity -Air speed -Radiation temp - sunny or hot ground -Surface temp - determines conductive heat exchange
Personal
- clothing
- Metab rate
General formula to calculate heat loss from a speciefic mechanism?
Heat transfer = (SA x gradient difference)/resistance
How to calculate dry heat loss?
(temp of skin- ambient temp)/ Insulation from clothing and air layers
none at 35deg
Body maintains temp well in dry heat but still a relationship between skin temp and air temp
Describe evaporation
Heat transfer due to sweating
Liquid- vapour change requires heat (must evaporate so dripping doesnt count)
Describe respiration
Not much effect but large in dogs
How to calculate evaporative heat loss and trends with temp
(PH20 skin - PH20 ambient)/ (vapour resistance of clothing and air layers)
Depends on humidity
100% at 35 deg then no heat loss. and condensation/ heat gain above this.
At 50% (normal) can lose heat up to 50 deg
Describe heat balance in 100% humidity whilst exercising. What could limit maximal heat loss here?
No heat loss or gain at 35deg.
Increases/ decreases as you go up or donw in temp.
Resp heat loss max + dry heat loss max + wet heat loss max = limit to heat loss.
So if exercising at 1300 Ws to maintain heat balance the temp needs to be 25deg or lower and this is with maximal heat loss from all methods.
Often limited by sweat production.
Heat gained from radiation all affects this capcity to lsoe heat
Normal sweat production limit?
2/3l an hour max
Limited by dehydration
Describe repsiratory heat loss
Similar to evaporative.
More positive at lower and more neg at higher
Can lose up to 10% of total heat production
What does the comfort of clothing depend on?
Pressure sensors and temperature sensors (not wet sensors)
33dg skin temp = comfort
Skin wetness = % of skin considered to be fully wetted
How does clothing limit heat transfer?
Resistance in the form of thermal insulation from clothing layers and trapped air layers.
Alows effectors to stay in their utility range function
resistance against vapour (sweat transfer)
Why are humans considered tropic animals in terms of temperature regulation?
Maintain their temperature only above 20 deg when naked
How can cultural aspects to clothing affect temp reg?
Jobs e.g. chemical protection but overheating
Wearing clothes for fashion
How does core temp in humans vary? Fever vs exercise?
Increase to 39/40 possible in exercise
FEver = raised set point
Exercises = raised due to metabolim but body actively tries to decrease
What is postivie and negative error in terms of sensation?
Positive error = large positive difference in ambient - afferent ignals are above set point
Vice versa
What is energy efficiency
Energy consumed: External work performed
Describe the temperature ranges of comfort and how they are effected
33 degree skin temp in a 3.5 deg rang
0.2 clo increases 1 deg (jeans and tee is .6)
20W exercise decreases 1 deg
Increased air speed increases
At what skin temperatures cause skin wetness? Effects of local skin time?
34 = 6% wetness 35 = 20% wetness (slightly uncomfortable) ect. Burning above 45 Reduced dext under 20 Pain under 15 Numb under 7
How does clothing create a large still layer
Trapped air layers either side of clothing (6mm)
What type of heat transfer does fibre type primarily effect?
Primarily radiation e.g. reflective, re emit or absorptive clothing. (not to do with thickness)
What significance does clothing thickness have?
Increases vapour resistance - increases diffusion pathway - less on conductance/ convection
Weave of fibres also effects diffusion.
Also effect air permeability which is more significant in high winds.
Effect of clothing ensembles?
More layers of trapped air so incresed diffusion pathways. Each up to 6mm
How many units of equivalent still air thickeness does a 2mm clothing give? what does this depend on?
6 on body and also from layer + 2mm (size) +still layer of outside clothing (7mm) = 21
Loose fit- ight is less, location e.g. tight at shoulder and body shape.
How does air movement effect insulation?
Disturb air layers (typically outer layers), replace warm air with cold
How does garment movement effect insulation? effect of this?
Disturbed inner air layers. walking at 4m/h decreases vapour resis 80% and air resistance 60%
Other ways in which clothing effects heat balance besides providing insulation?
Weight and stiffness of the clothing require more energy to move.
Physiology - more fat in places with less insulation
Describe clothing comfort in more detail? How can it be improved through clothing qualities?
ALso depends on physical, physiological and psychological factors.
Temp sensation also depends on rate of change.
Increases feel of wetness as adds friction.
Looseness is also less comfortable particularly if wet as between toughing skin the clothing cools and then feels cold. If absorption also causes the skin to cool quicker as it gets wet.
Fibre weave and size affect the amount of direct contact with skin.
Why were nylon shirts uncomfortable? What would be a good design
Non permeable, flat weave structure that sticks to the skin.
2 layers, absorptive outside and permeable but low absorption inside (moisture condutive)
Why is buffering capacity to heat and moisture important for clothing? Gold standard?
Prevents clothing becoming wet and resulting in sharp decreases in temp after exercise (afterchill). Best in varying activity levels.
Phase change aterials where structure changes at certain temps
Describe evidence for acclimatization to cold
Not much - military for 4 weeks at - 20. only difference was they went to the toilet in the night less due to adaptations of kidney preventing hypertension.
Describe potential mechanisms for acclimatisation and differences in fitness
Hypothermic - allowing body temperature to fall to preserve energy
Insulative - Enhanced insulation hence preventing cooling **eskimos have lower sub-cut fat
metabolic - Increased production by non-shivering thermogenesis.
Fitness - tend to be metabolic
Low fitness - tend to be insulative (chicken vs egg)
Explain how champion channel swimmers deal with the cold
Used to be fatter
Now leaner but higher BMR due to nutrition and fitness
Relationship between skin fold and critical temperature
Higher skin fold then higher critical temperature (temp at which O2 uptake and metabolic rate start rising
How do eskimos adapt to cold climates?
behaviour - walrus skin
Higher hand blood flow at any temperature to maintain dexterity and prevent frostbite
Possible reasons for naked divers in Korea shivering the least?
Selection bias (people that have most fat are chosen to dive) vs developmental? (not genetic as now use wetsuits and effects have dissapeared)
Describe the balacne between function and safety of helicopter immenrsion suits and the types of clothing used
immersion suits can be wet or dry (water tight).
If impermeable then the wearer is subject to heat stress.
If permeable then cant survive in water as long if stranded
How does metabolic rate change with temperature?
Increases linearly
At what temperature will you see no ECG or not EEG
31 - but only pronounce dead when warm
What can survival data of shipwrecked people be used for?
Predicting chances of survival after abandonment in cold water. Planning rescue operations. Time decreases with age
Why do people die from the cold at different times?
stage 1 - Cold shock 3-5 minutes 2 - Swimming failure <30 minutes 3- Hypothermia >30 minutes 4- Post rescue collapse 50% stage 1 ad 2
What is cold shock?
Involuntary inhalation and hyperventilation - drowning.
Vasoconstriction - raised BP and more straign on heart - CVS event, Cerebrovasc event, inability to self rescue.
Reduces with repeated exposure
How is cognition effected by cold?
Under 33 reduced consciousness
10-15 fine
75 stunned denial and low reasoning
10-15 inappropriate behaviour
Describe hypothermia types and treatments
Acute (immersion) - severe cold stress, rapid body cooling before energy reserve is used. Rewarm spontaneously when removed from the cold.
Subacute (exhaustion) hypothermia - less sebere cold stress, energy reserves depleted. Spontaneous rewarming is less certain.
subchronic (urban) - moderate cold of rlong Fluid balance changes due to changes in BP - needs gentle warming
Submersion - >60 minutes under water - need rewarming before pronounce dead
Different treatments and outcomes
Why can children survive very cold temperatures underwater? Risks?
Larger SA:weight ratio so faster cooling of brain- reduces O2 demand. May be brain damage
Describe different methods of rewarming
Passive external - blankeys, bubblewrap, give energy, avoid water evap (change clothing)
Active external - forced air warming, chemical packs (50) chemical blankets (TNO blanket) (40), water bath
Active internal - Inhalation warming (pretty crap), Drinks, IV fluids (40-42), rewarming circuit - cardiopulmonary bypass - fem to fem
What is afterdrop? How can it be prevented (best method)
Physical afterdrop - core temp continues to decrease as warming outside takes time to effect the core.
Physiological afterdrop - vasodilation due to external warming so warm blood from core is pumped outwarms and is replaced by cold blood.
Best is total submersion (not rewarming hands first ect.
Also reduce… no exercise? keep extremities cold?
When does lethal aspiration amount occur?
22ml/kg sea water and 44 fresh
Red cross swimming advice?
Dont but can be helpful
Describe signs and symptoms of mild, moderate and severe hypothermia
Mild - symp - shivering BP, increased urine, mental confusion, hepatuc dysfunction? hypoglycaemia?
Mod - violent shiv, muscle miscoordination, stimbling, mild confusion, pale, blue extremities
Sev - HR, RR, BP decrease - HR in 30s, difficulty speaking, amnesia, inability to use hands, legs, blue and pufy skin below 30, stupor, irrational behaviour, AF, VT, organ failure
Paradoxical undressing - occur in mod-sev.
Terminal burrowing - hide and die syndrome - assocaiated with paradoxical undressing. If temp drops slowly, find a hole - primative self-protective behaviour?
Why is cooling bad?
Cellular structures, enzyme systems and temp dependent chemical reactions are disrupted
What is physiological amputation?
AVAs close, vasoconstriction occurs. Reduced blood flow to arm drastically
What are the possible reasons for decreased dexiterity in the hands? What is each caused by physiologically?
Decreased sensibility (change in receptors) Descreased muscle function Increased reaction time (change in receptors, nerves, effectors) Decreased joint function Decrease in cognitive functions/ motivation (central effects)
How can risk of skin freezing and dexterity problems be describe in terms of risks posed by conduction
Can be described in terms of contact coefficient (specific heat capacity, density and conductance) and contact time
How is risk of frostbite modulated and what is a potential negative effect of this?
Increase in metabolism
Wearing gloves - decreases dexterity in itself
