Summary Physiology of Performance in Extreme Environments (B_PPEE)

Physiology of performance in extreme environments

Lecture 1 – Introduction
Humans > Homeotherm

Mechanisms to cope with thermal extremes
Thermal neutral zone = vasodilation & vasoconstriction
Heat
- Vasodilation
- Sweat loss (max. 3 L/hr = 2458 W)
o Sweat dripping of is not efficient
o Evaporation of sweat has a cooling effect
o Cotton sucks up a lot of sweat → sweat cannot evaporate
▪ Loose fitting shirt is even worse
o Sweat systems
▪ Hormonal control → emotional sweat
▪ Nerve control (sympathetic system) → thermal sweat
o Unacclimatised: more salt in the sweat
- Powerful acclimatization

Cold
- Vasoconstriction
- Shivering: max. 400 W/hr
o Well trained: more shivering
- Fur coat
- Brown fat
➔ Poor acclimatization

Rest: Brains and liver produce most heat (90-100 W/hr)
- Main dry heat loss because of thermal gradient/convection between skin (30°) and air (20°)

Sympathetic nerve innervation
Sympathetic action:
- Alpha 2 receptors: skin vasoconstriction
- Beta 2 receptors: dilation of blood vessels in muscles

Evolution
- Left Africa 40,000 years ago
- Clothing allows extending the habitat behind physiological limits
- Innuits have shorter fingers
- Longer fingers by people who live in a warm climate

Heat balance: M ± R ± C – E = S
M: Metabolism
➢ Total production of all energy
➢ Oxygen measurement in expirated air
➢ Watt = Joule/sec
R: Radiation
➢ Measurement: Temp. difference between skin and environment
C: Convection/Conduction

, ➢ Convection = stroming (wind/water)
➢ Conduction = heat transfer from material to material
➢ Measurement: Temp. difference between skin and environment
E: Evaporation
➢ Wet heat loss
➢ Measurement: weight people before & after exercise
o Correct for clothing and drinking
S: Storage
➢ Difference between heat production & heat loss
➢ Measurement: measuring body core and body skin temperature
o Cold: core 60%, skin 40%
o Warm: core 80%, skin 20%




Heat balance in;
Rest:
- Equilibrium between heat production and heat loss
- Diet induced thermogenesis
Exercise:
- Heat Production exceeds heat loss
- Core temperature increase
- Performance decrement

➢ Example 8 km run thermal balance
o Lower ambient temperature →
lower sweat loss

,Wet and dry heat loss




Thermal balance calculation
Heat balance climb 1500 m
Mass: 82 + 14 kg
Energy: m*g*h =96*9,81*1500 = 1440 kJ needed to climb the mont ventoux
Time: 2 hr (7200 s)
1440000/7200 = 200 Watt of power needed in bike
Power: 200 W
Efficiency: 20%
Total: 80% over = 800 Watt and 20% in bike = 200 Watt
Heat loss: 800 W
Ambient temperature: 35°C
Heat loss has to be cooled: 800/650 = 1,23 Liter/hr evaporation
Ereq: 2.4 l (1 L in 1 hour ~ 650 W)
➢ If you cannot evaporate, than the body core temperature will go up

Heat balance factors
• Climate:
- Wind
- Temperature
- Humidity
• Clothing:
- Isolation value
- Water vapor probability: Barrier against sweat evaporation
• Metabolism
• Individual factors
- Body shape and fat storage
- Heat acclimation status

, Required clothing for thermal neutrality
- Used in the mountains and military




Mortality and heat balance




Weather related mortality
23 year database of daily mortality
Cold-related mortality exceeds heat-related mortality!