24.6 Energy and Heat Balance

The body's temperature control system is like a sophisticated thermostat. It keeps our internal temperature steady at around 37°C through various mechanisms. These include shivering to generate heat and sweating to cool down.

Heat exchange with the environment happens through radiation, conduction, convection, and evaporation. Our basal metabolic rate, influenced by factors like body size and age, plays a crucial role in heat production and overall energy balance.

Thermoregulation and Heat Balance

Body temperature maintenance

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• Hypothalamus acts as body's thermostat
  • Preoptic area of anterior hypothalamus contains temperature-sensitive neurons detect changes in blood temperature
  • Posterior hypothalamus integrates temperature information from various body regions initiates appropriate thermoregulatory responses
• Mechanisms of thermoregulation maintain stable internal body temperature around 37°C (98.6°F)
  • Heat production (thermogenesis) increases body temperature
    • Shivering thermogenesis involves involuntary muscle contractions that generate heat (teeth chattering, goosebumps)
    • Non-shivering thermogenesis occurs in brown adipose tissue through metabolism generates heat (infants, cold-adapted individuals)
  • Heat loss mechanisms decrease body temperature
    • Vasodilation of cutaneous blood vessels increases blood flow to skin surfaces promotes heat loss through radiation and convection (flushed skin)
    • Sweating increases heat loss through evaporation of sweat from skin surface (perspiration, cooling sensation)
  • Heat conservation mechanisms reduce heat loss
    • Vasoconstriction of cutaneous blood vessels decreases blood flow to skin surfaces minimizes heat loss (pale skin, cold extremities)
    • Behavioral responses, such as seeking warm environments (indoor heating) or adding clothing layers (sweaters, blankets), conserve heat

Heat exchange mechanisms

• Radiation transfers heat through electromagnetic waves without direct contact
  • Body exchanges heat with surrounding objects (walls, furniture) proportional to temperature difference
• Conduction transfers heat directly between objects in physical contact
  • Heat flows from warmer to cooler objects (bare feet on cold floor, holding ice cube)
• Convection transfers heat through movement of fluids or gases
  • Air or water currents carry heat away from body (wind chill, swimming in cool water)
• Evaporation converts liquid (sweat) to vapor, absorbing heat from body
  • Most effective heat loss mechanism during exercise or in hot environments (sweating during workouts, hot summer days)

Thermoregulation and Homeostasis

• Thermoregulation is a key aspect of maintaining homeostasis in the body
• Endothermy allows mammals to maintain a constant internal temperature independent of the environment
• Metabolism plays a crucial role in heat production and energy balance
• Heat stress can occur when the body is unable to effectively dissipate excess heat
• Hypothermia results from excessive heat loss or inadequate heat production
• Acclimatization involves physiological adaptations to new environmental conditions, such as changes in temperature or altitude

Basal metabolic rate factors

• Basal metabolic rate (BMR) represents minimum energy required to maintain vital functions at rest
  • Measured under standard conditions: 12-hour fast, at complete rest, in thermoneutral environment (68-77°F or 20-25°C)
• Body size and composition influence BMR
  • Larger individuals generally have higher BMR due to greater total body mass (tall, muscular people)
  • Lean body mass (muscle) has higher metabolic activity than fat tissue (athletes, bodybuilders)
• Age affects BMR
  • BMR decreases with age due to changes in body composition (muscle loss, fat gain) and hormonal factors (lower testosterone, estrogen)
• Sex differences in BMR
  • Males generally have 5-10% higher BMR than females due to greater muscle mass and size (average man vs. woman of same height)
• Thyroid hormones regulate BMR
  • Thyroxine (T4) and triiodothyronine (T3) increase BMR by stimulating cellular metabolism (hyperthyroidism increases BMR, hypothyroidism decreases BMR)
• Harris-Benedict equations calculate BMR based on sex, weight, height, and age:
  • Males: $BMR = 88.362 + (13.397 \times weight_{kg}) + (4.799 \times height_{cm}) - (5.677 \times age_{years})$
  • Females: $BMR = 447.593 + (9.247 \times weight_{kg}) + (3.098 \times height_{cm}) - (4.330 \times age_{years})$
• Interpreting BMR values
  • Comparing individual's BMR to population norms assesses relative metabolic rate (high, low, average)
  • Deviations from expected BMR may indicate underlying health conditions (thyroid disorders, malnutrition) or metabolic adaptations (athletes, dieters)