Hyperthermia - Extreme Physiology



When core body temperature gets too high

Heat balance = equal amount of heat loss and production

Can only lose heat by evaporation, you can't gain coldness so it makes it very difficult to stop overheating

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1. Caution - fatigue possible

2. Extreme caution - sunstroke, muscle cramps and/or heat exhaustion

3. Danger - sunstroke, muscle cramps and heat exhaustion likely

4. Extreme danger - heat stroke or sunstroke highly likely

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Critically high internal temperature hypothesis

No matter what the starting temperature, during exercise you will always end up at the same temperature

This makes it very difficult to stop

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Central Governor Model

The rate at which our temperature goes up gives us information about how we are heading for catastrophe

Muscle recruitment gets more difficult -  this is to slow rate at which temperature increases

It prevents critical internal temperature rising too high

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Maximum sweat rate is 2L/hr

Marathon sweat rate is 0.3-1.2L/hr

Critical dehydration = 5.5%

Medical emergency = 10%

If dehydration is combined with a hot environment, there is a much greater effect on VO2 max and endurance

Plasma volume decreases which decreases blood volume

This decreases stroke volume so heart rate increases to compensate cardiac output

Can only sustain 65-70% of VO2 max in a hot environment

The conflict between thermoregulation and metabolism means there is a reduced blood flow to skin, which decreases sweating so body temperature increases

Also increased risk of heat illness and exhaustion

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Heat synope

Breif fainting spells 

Without significant rise in body temperature

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Heat exhaustion

An inability to continue exercising

Results in:

Weak and rapid pulse, dizziness, headache, nausea, lethargy and general weakness


Stop exercising, remove heat source, lie down, control breathing, reduce panic, rehydrate

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Heat stroke

Failure of the thermoregulatory system - resulting in significantly elevated body temperature


Confusion, dry skin, circulatory instability, vomiting, coma


Artificial sweat, water immersion, deep body temperature monitered every 5 mins

Variable prognosis - can take up to one year to recover

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Enhancing and Mainting

1. Pre/post exercise cooling

2. Acclimatisation

3. Maintain hydration status

4. Maintain electrolyte balance

5. Behavioural regulations - train at coolest times of the day

6. Protective clothing

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