FLUIDS HEAT AND METABOLISM
LECTURE 5:
SENSIBLE HEAT LOSS AND
SHIVERING (SWEATING)
Eamonn O’Connor
Trinity College Dublin
Mechanical Power, Efficiency & Heat Production
1
1000
Energy lost as heat
200
Metabolic energy transduced to
effective, mechanical work
Fluids Heat & Metabolism 11-12
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Measuring efficiency during exercise at 200W
2
3.0
2.85
2.5
2.0
Energy lost as heat
1.5
1.0
0.5
Metabolic energy transduced to
effective, mechanical work
0
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Fluids Heat & Metabolism 11-12
Heat Balance During Exercise
Predict heat production and core temperature in the absence of heat loss
during exercise for a 70kg person cycling at 200W for 30min
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2
4
Fluids Heat & Metabolism 11-12
Core temperature response to exercise at 23°C
5
Fluids Heat & Metabolism 11-12
3
6
Evaporative heat loss in Humans:
Non-sweating vs Sweating
0
10
20
30
40
Fluids Heat & Metabolism 11-12
Evaporative Heat Loss
7
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4
Problem
8
Fluids Heat & Metabolism 11-12
Heat loss is controlled to regulate body temperature
9
Fluids Heat & Metabolism 11-12
5
Effect of Exercise on Sweating
10
1260
1080
900
720
540
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Problem
11
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 
 
Estimate average heat loss due to sweating during
exercise at 200 W over 60 min
Assume that all sweat is evaporated
Then calculate its relative impact on damping the
potential rise in core temperature over 60 min in the
absence of heat loss mechanisms
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Summary
12
 
 
 
 
 
 
Controlling (or changing) sensible heat loss depends on controlling
(or changing) skin blood flow, and therefore heat lost from the blood
through the skin to the surroundings
Skin blood flow is controlled primarily by the sympathetic NS
Effective thermoregulation during heat stress depends on
evaporative heat loss through sweating
Exercise creates an immediate large increase in heat production
through metabolism
A delay in the rise of heat loss leads to an increase in body heat
content and temperature
When heat loss through sweating and sensible routes (skin blood
flow) equals heat production, core T stabilises at a new, higher level
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