Test 3: Chapter 12 Flashcards
Voluntary Heat Production
Exercise
- 70–80% energy expenditure appears as heat
Involuntary Heat Production
Shivering
- Increases heat production by ~5x
Action of hormones
- Thyroxine
- Catecholamines
- Called “non-shivering thermogenesis”
Heat Loss
Radiation
– Transfer of heat via infrared rays
– 60% heat loss at rest
– Can be a method of heat gain
Conduction
– Heat loss due to contact with another surface
Convection
– Heat transferred to air or water
– Example: a fan pushing air past skin
Evaporation
– Heat from skin converts water (sweat) to water vapor
Heat Storage in the Body During Exercise
Heat produced that is not lost is stored in body tissues
– Will raise body temperature
Body heat gain during exercise = (heat produced - heat loss)
Amount of heat required to raise body temperature
– Specific heat of human body is 0.83 kcal/kg
Heat required to raise body temp 1°C =
(specific heat x body mass)
The body’s thermostat preoptic anterior hypothalamus (POAH)
POAH
- Operates like a thermostat in an attempt to maintain constant core temp.
Response to increase core temp
- stimulation of sweat glands
- cutaneous vasodilation, allowing increased heat loss
Response to decrease core temp
- shivering and increased norepinephrine release
- decreased skin blood flow
Heat Index
Measure of body’s perception of how hot it feels
– Relative humidity added to air temperature
Example:
– Air temperature = 82°F, relative humidity = 80%
– Heat index = 89°F
High relative humidity reduces evaporative heat loss
– Lowers heat loss
– Increases body temperature
Prevention of Dehydration during Exercise
Dehydration of 1–2% body weight can impair performance
Guidelines
– Hydrate prior to performance
– Consume 150–300 ml fluid every 15–20 min
– Monitor urine color
Many sports drinks are superior to water for rehydration
Exercise Performance is Impaired in a Hot Environment
CNS dysfunction
- Decreased motivation
- Reduced voluntary activation of motor units
Cardiovascular dysfunction
- Reduced stroke volume
- Decreased CO during high intensity exercise
- Decreased muscle blood flow
Accelerated muscle fatigue
- Increased free radical production
- Decreased muscle pH
- Muscle glycogen depletion
Gender/Sex and Age Differences in Thermoregulation
Gender differences in heat tolerance are small
– When matched for level of acclimation and body
composition
Age itself does not limit ability to thermoregulate
– Decreased thermotolerance with age due to:
Deconditioning with age
Lack of heat acclimatization
Precooling the Body Improves Exercise Performance in the Heat
Precooling body before exercise in the heat lowers
body temperature and has been shown to improve
exercise performance in a hot environment
Various techniques exist
– Cold water immersion
– Cooling Ice Vest
– Cooling packs
– Ingestion of cold drinks
A combination of cooling techniques (e.g., combining
cold water immersion and cooling packs) improves
performance more than a single technique
Acclimation
Rapid adaptation (days to weeks) to environmental
change
Acclimation lost within a few days of inactivity
or no heat exposure
– Significant decline in 7 days
– Complete loss in 28 days
Acclimatization
Adaptation over a long time period (weeks to months)
Physiological Adaptations During Heat Acclimation
10–12% increased plasma volume
– Maintains blood volume, stroke volume, and sweating capacity
Earlier onset of sweating and higher sweat rate
– Less heat storage, maintain lower body temperature
Reduced sodium chloride loss in sweat
– Reduced risk of electrolyte disturbance
Reduced skin blood flow
Heat Acclimation and Heat Shock Proteins
Heat acclimation reduces the risk of heat injury
– In response to exposure of heat stress
Protection likely related to synthesis of heat shock proteins in cells
– Protect cells from thermal injury
– Stabilizing and refolding damaged proteins
