Heat Production and Loss

Heat Production - Body's Furnace

• Primary Heat Sources:
  • Liver (major at rest), Brain
  • Skeletal muscle (↑↑ with activity)
  • Brown Adipose Tissue (BAT)
• Basal Metabolic Rate (BMR): Energy for vital functions at rest.
  • Key factors: Thyroxine (T4, T3), surface area, age.
• Hormonal Boost:
  • Thyroxine: ↑ long-term tissue metabolism.
  • Epinephrine/Norepinephrine: Rapid ↑ metabolic rate, glycogenolysis.
• Shivering Thermogenesis: Involuntary muscle contractions → rapid heat.
• Non-Shivering Thermogenesis (NST):
  • BAT: Mitochondria-rich, UCP-1.
  • UCP-1 (Thermogenin): Uncouples oxidative phosphorylation → heat (not ATP). Sympathetic activation.

⭐ BAT is vital for newborn heat; found in adult supraclavicular/paravertebral areas.

Heat Loss Mechanisms - Escape Routes

• Radiation (~60%): Infrared heat waves from body to cooler surroundings.
  • Main mechanism at rest in neutral environment.
• Conduction (~3%): Direct heat transfer to cooler objects/surfaces in contact.
  • E.g., sitting on a cold seat.
• Convection (~12-15%): Heat transfer by movement of air/water currents over body.
  • ↑ by wind or fan.
• Evaporation (~22-25%): Heat loss as water vaporizes from skin/lungs.
  • Insensible Perspiration: Continuous, from skin & respiratory tract.
  • Sweating (Sensible): Active secretion by sweat glands; primary cooling in heat.
    • Effectiveness ↓ in high humidity.

⭐ Evaporation of 1 g of water removes ~$0.58 \text{ kcal}$ of heat.

Thermoregulation - The Body's Thermostat

• Maintains core body temperature (CBT) around 37°C (98.6°F).
• Hypothalamus: Key control center.
  • Anterior Hypothalamus (Heat Loss Center): Responds to ↑ temp. 📌 A/C (Anterior Cooling).
    • Activates vasodilation, sweating.
  • Posterior Hypothalamus (Heat Gain Center): Responds to ↓ temp. 📌 Posterior Produces.
    • Activates vasoconstriction, shivering, ↑ metabolism.
• Thermoreceptors: Peripheral (skin: detect external temp) & Central (hypothalamus, spinal cord: detect core temp).
• Set-Point Theory: Hypothalamus compares receptor input to internal set-point (≈37°C); deviations trigger responses.
• Effector Mechanisms: Vasomotor (dilation/constriction), sudomotor (sweating), metabolic (shivering, Non-Shivering Thermogenesis via brown fat/thyroxine), behavioral (e.g., clothing, shelter seeking).

⭐ Fever: Pyrogens (e.g., IL-1 from macrophages) act on hypothalamus to ↑ set-point temperature by stimulating local prostaglandin (PGE2) release.

Applied Aspects - Temp Extremes

• Fever (Pyrexia)
  • Pathogenesis: Exogenous pyrogens (LPS) → Endogenous pyrogens (IL-1, IL-6, TNF-α) → ↑PGE2 in hypothalamus → ↑Set-point.
  • Antipyretics (Aspirin): Inhibit COX → ↓PGE2.
• Hyperthermia (Uncontrolled ↑Body Temp)
  • Heat Exhaustion:
    • Core temp < 40°C.
    • Cause: Water & electrolyte loss.
    • Symptoms: Weakness, profuse sweating, normal mentation.
    • Tx: Rest, cooling, fluids.
  • Heat Stroke:
    • Core temp > 40°C. Medical emergency.
    • Cause: Thermoregulatory failure.
    • Types: Classic (no sweat) vs. Exertional (sweat).
    • Symptoms: CNS dysfunction (delirium, coma), hot dry skin (classic).
    • Tx: Rapid cooling, supportive.

    ⭐ Heat Stroke: Core temp > 40°C + CNS dysfunction.

• Hypothermia (Core Temp < 35°C)
  • Changes: Initial shivering (stops <32°C); ↓Metabolic rate, ↓HR, ↓RR, CNS depression.
  • ECG: J wave (Osborn wave) at < 32°C.
  • Stages: Mild (32-35°C); Moderate (28-32°C, no shiver, arrhythmias); Severe (< 28°C, coma, V-fib).
  • Tx: Gradual rewarming.

High‑Yield Points - ⚡ Biggest Takeaways

• Hypothalamus (anterior: heat loss; posterior: heat production) is the primary thermoregulator.
• Brown Adipose Tissue (BAT) via UCP-1 (thermogenin) is crucial for neonatal non-shivering thermogenesis.
• Evaporation (sweating) is the most effective heat loss mechanism at high ambient temperatures.
• Shivering (involuntary muscle contraction) significantly boosts heat production.
• Fever: Hypothalamic set-point reset, often by prostaglandins (PGE2).
• Malignant hyperthermia: Anesthetic-triggered, uncontrolled Ca2+ release, massive heat production.
• Heat stroke: Core temperature > 40°C, CNS dysfunction, often anhidrosis (classic).