Thermoregulation in the cold
Effect on exercise
Fluids Heat and Metabolism 08-09
Mikel Egaña
Trinity College Dublin
Lecture Outline
I.
Factors affecting body heat loss
II.
Physiological Responses to Exercise in the cold
III. Cold injuries
IV. Acclimatization / habituation to Cold
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
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How Does the Body Conserve Heat?
Cold stress: environmental condition that causes a loss
of body heat that threatens homeostasis
•
Shivering—rapid involuntary cycle of contraction and
relaxation of muscles
•
Nonshivering thermogenesis — stimulation of
metabolism
•
Peripheral vasoconstriction—reduces blood flow
(and metabolic rate) to the skin and prevents
unnecessary heat loss
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
BODY HEAT GAINED < BODY HEAT LOST
In a cold environment, Conduction. Convection, Radiation and
Evaporation can dissipate heat faster than the body produces it
Risk of hypothermia (low body temperature)
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
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Factors That Affect Body Heat Loss
Body size and composition
•
Subcutaneous fat content: indicator of
tolerance for cold exposure due to low
thermal conductivity of fat
•
Women versus men:
•
•
Women higher tolerance than
men due to higher fat content
(no true sex differences)
Body surface area:
•
children larger area to mass
ratio compared to adults.
•
More difficult to maintain body
temperature in the cold
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
Factors That Affect Body Heat Loss
Air temperature: The larger the difference between the temperature of
skin and cold environment, the greater the heat loss
Windchill: Chill factor created by the wind (via convection and
conduction). The more humid the greater the physiological stress
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
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HEAT LOSS IN COLD WATER
• In air: radiation and evaporation main mechanisms for heat
loss
• In water: conduction allows greater heat transfer.
• Water larger thermal conductivity (x26) than air.
• Considering all heat transfer mechanisms heat loss 4
times faster in water than in air
• Drop in temperature (hypothermia) proportional to
duration of exposure or the thermal gradient
• Immersion at 15°C: drops rectal temp 2.1°C/hour
• Immersion at 4°C: drops rectal temp 3.2°C/hour
• Heat loss accelerated if cold water moving
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
HEAT LOSS IN COLD WATER
Swimming in cold water:
- High body fat content helps maintain rectal temp (Pugh & Edholm 1995)
- Lean swimmers can maintain rectal T° at low water T° (17.4°C)
Trappe et al. MSSE 1995
For training/competition appropriate water temp: 23-28 °C
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
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Responses to Exercise in the Cold
•
•
Oxygen consumption (VO2)
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VO2 max:
•
Submaximal exercise VO2:
•
Exercising with wet clothing:
Ventilation
•
Sudden exposure to cold:
•
Submaximal exercise Ve:
•
As exercise intensity increases:
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
Responses to Exercise in the Cold
•
•
Muscle function
•
Muscles weaken
•
Fiber recruitment patterns and muscle efficiency altered.
•
Shortening velocity and power reduced (higher EE)
•
Thus, fatigue occurs more rapidly
•
If proper clothing: exercise performance may be
unimpaired (but only until fatigue sets)
Metabolism
•
Fat metabolism:
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
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Responses to Exercise in the Cold
•
•
Glucose metabolism
•
Blood glucose
•
Muscle glycogen
•
Lactate:
Protein metabolism
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
Responses to Exercise in the Cold
Table 22-1 Brooks et a 3rd Edl
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
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Cold injury : Hypothermia
Inability to regulate body temperature begins when Tbody drops <34.5 °C.
Completely lost when Tbody drops below 29.5 °C
Hypothermia causes:
- Slowing metabolic reactions to one half their normal rates
- Reduction in NS: inability to shiver, sleepness and coma
- Heart rate to drop, which reduces cardiac output.
- Can lead to cardiac arrest (respiration might be functional)
Causes of hypothermia:
- Exposure to cold water
- High wind chill
- Use of alcohol of drugs in the cold
- Endurance exercise competitions in the cold: glycogen depletion
(hypoglycemia) increases risk of hypothermia and thus, reduced CNS
function
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
Cold injury : Hypothermia
- Effect on respiratory track?
Figure 11.15 W&C 4th Ed.
Treatment:
- Mild hypothermia: dry clothing and warm beverages
- Severe hypothermia: medical treatment (hospital)
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
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Cold injury : Frostbite
•
•
•
•
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Ice crystal formation within tissues
Vasoconstriction in the skin reduces blood flow to skin,
eventually causing frostbite.
Risk increase when environmental air temp: <-6 °C
Mainly fingers, nose, earlobes and toes
If not treated may cause gangrene
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
Acclimatization / Habituation to cold
1. Shivering threshold
•
•
Chronic daily exposure to cold increase secretion of thyroid
hormones: tissues become more sensitive to norepinephrine.
Also it may increase subcutaneous body fat
2. Hand and cold temperature
•
Chronic exposure of areas of the skin (hands) may provide
greater cold tolerance due to improved intermittent peripheral
vasodilation
3. Capacity to sleep
•
Higher capacity to vasoconstrict peripheral blood vessels
allowing the skin temp to decrease without shivering
Fluids Heat and Metabolism 08-09. Thermoregulation in the cold.
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