Human Temperature Regulation Quiz

Temperature Regulation

• Temperature homeostasis is a balance between heat gain and heat loss.
• Heat balance: production must match loss.
• Heat conservation and generation are efficient for humans, but cooling capacity is limited.
• Heat loss is greater than heat production → body temperature decreases.
• Heat loss is less than heat production → body temperature increases.
• Long-term exercise poses a challenge to temperature homeostasis due to heat generated by contracting skeletal muscles.

Heat Production: Metabolic Heat

• Humans are 20-30% efficient, the remaining 70-80% appears as heat.
• During heavy exercise, this can result in a large heat load.
• Voluntary heat production: Exercise.
• Involuntary heat production:
  • Shivering.
  • Non-shivering thermogenesis.
  • Release of thyroxine and catecholamines increases whole body cellular metabolism.

Heat Loss: Four Processes

• Heat loss occurs through radiation, conduction, convection, and evaporation.

• Radiation: Transfer of heat through infrared rays (no contact necessary).

• At rest, ~60% of heat loss occurs via radiation.

• Skin temp > surrounding surface temps → heat loss.

• Surface temps > skin temp → heat gain.

• Conduction: Heat loss to a cooler object in physical contact with the skin.

• Plays a minor role in exercise heat loss.

• Cooler objects must be in direct contact with the skin for heat transfer.

• Convection: A form of conductive heat loss where heat is transferred to air or water molecules in contact with the body.

• Effectiveness depends on air flow.

• Use of a fan or moving air increases convective cooling.

• Water is ~25 times more effective than air at the same temperature.

• Evaporation: Heat from skin heats water (sweat) converting it to water vapor, removing heat from body.

• Accounts for ~25% of heat lost at rest.

• Most important for heat loss during exercise.

• Sweat is secreted onto the skin's surface, where it evaporates, cooling the skin.

• Rate of evaporative cooling depends on:

  • Air temperature and relative humidity.
  • Convective currents around the body.
  • Surface area of skin exposed to the environment.

• High temps and humidity reduce the ability of sweat to evaporate, reducing cooling effectiveness.

The Body's Thermostat (Preoptic-Anterior Hypothalamus (POAH))

• Thermoreceptors detect core temperature changes.
• Relaying information to the POAH induces mechanisms for heat loss or gain.
  • Increased core temps: sweating and increased skin blood flow.
  • Decreased core temps: shivering, increased norepinephrine and thyroxine release, and decreased skin blood flow.

Check Your Understanding

• Body heat loss: radiation, conduction, convection, evaporation.
• Evaporation rate depends on temperature, humidity, convective currents, and skin surface area exposed.
• During exercise in a hot environment, evaporation is the primary means of heat loss.

Exercise in the Heat

• Heat index considers temperature and relative humidity to determine perceived heat.
• High humidity reduces evaporative cooling.
• Heat storage and core temperature increase.
• Risk of hyperthermia and heat injury, increased sweating rates.
• Potential for dehydration, up to 4-5L/hr.
• Prolonged exercise in a hot environment results in gradual core temp increase until reaching steady state.
• Important to plan for heat loss during exercise at high temperatures.

Exercise in the Cold

• Cold exposure enhances the ability to lose heat, preventing heat injury.
• Risk of hypothermia is a main concern.
• Core temperature decreases rapidly during prolonged cold exposure.
• Acclimation to cold happens in 1 week: non-shivering thermogenesis & increased blood flow to extremities; improved sleep ability.

Heat Acclimation

• Short-term adaptations: Rapid adaptations over days to weeks in response to environmental changes.
• Long-term adaptations: Adaptations that occur over weeks or months in response to environmental changes.
• Exercise in a hot environment is needed to achieve adaptations.
• Elevated core temps promotes adaptations.
• Acclimation is lost within a few days of no exercise or heat exposure.

Physiological Adaptations During Heat Acclimation

• Increase in plasma volume (+10-12%).
• Earlier onset and higher sweat rate.
• Reduced sodium chloride loss in sweat →reduced risk of electrolyte disturbance.
• Reduced skin blood flow.
• Increased cellular heat shock proteins prevent cellular heat damage.

Impact of Heat Acclimation on Heart Rate and Core Temperature During Exercise

• Acclimation leads to smaller increases in both heart rate and core temperature during exercise in the heat.

Precooling the Body to Improve Exercise Performance

• Precooling reduces body temperature and enhances exercise performance in a hot environment.
• Methods include cold water immersion, cooling vests, cooling packs, and ingesting cold drinks.

Exercise-Related Heat Injury Prevention

• Exercise during the coolest parts of the day.
• Minimize intensity and duration of exercise.
• Expose maximal skin surface area.
• Frequent breaks.
• Hydration.
• Removal from direct sunlight, exposure to circulating cool air.
• Monitor weight to estimate fluid replacement needed.

Prevention of Dehydration During Exercise

• Dehydration of 1–2% body weight can impair performance.
• Hydrate before exercise.
• Consume fluids (400–800 ml) within 3 hours before exercise.
• Drink (~150-300 ml) every 15–20 minutes during exercise.
• Fluid consumption should be adjust based on environmental conditions.
• Consume an equivalent of 150% of weight loss.
• 1 kg lost = 1.5 Liters of fluid replacement.
• Monitor urine color (should be clear or light lemonade color).
• Sports drinks can be superior to water for rehydration.

Exercise Performance Impaired in a Hot Environment

• Accelerated muscle fatigue.
• Increased glycogen breakdown & depletion, lactic acid production and reduced pH.
• Increased Free Radical production, damage to muscle causing reduced contractile proteins.
• Afferent feedback from metaboreceptors in fatiguing muscles decreases central motor drive.
• Cardiovascular dysfunction; increased heart rate and reduced stroke volume; reduced muscle/blood flow competition.
• Hyperthermia →Impaired brain function
• Dehydration →Impaired brain function.
• Reduced central motor drive.

Test your knowledge on temperature homeostasis and the mechanisms of heat production and loss in humans. This quiz covers essential concepts such as metabolic heat efficiency and the processes involved in maintaining body temperature during various activities. Explore how long-term exercise affects temperature regulation as well.