Core body temperature maintenance

Core Temp - The Body's Thermostat

• Internal body temperature (thoracic/abdominal organs) tightly regulated at 36.5-37.5°C (97.7-99.5°F).
• The hypothalamus is the central thermoregulatory center.
  • Anterior Hypothalamus: Heat dissipation (cooling). Mediates sweating & vasodilation.
  • Posterior Hypothalamus: Heat conservation & production (heating). Mediates shivering & vasoconstriction.
• 📌 Mnemonic: Anterior Cools (A/C), Posterior Produces heat.

⭐ Rectal temperature is the most accurate clinical measure of core temperature; pulmonary artery catheter is the gold standard.

Heat Production - Firing Up the Furnace

• Basal Metabolic Rate (BMR): Heat as a byproduct of all essential metabolic functions.
• Thyroid Hormone (Thyroxine): ↑ cellular metabolism & O₂ consumption, a major long-term regulator.
• Sympathetic Stimulation: Epinephrine & norepinephrine cause a rapid, short-term ↑ in metabolic rate.
  • Activates chemical thermogenesis (non-shivering).
  • Prominent in brown adipose tissue via uncoupling protein-1 (UCP1).
• Muscle Activity:
  • Shivering: Involuntary rhythmic contractions; can ↑ heat production 5x.

⭐ Non-shivering thermogenesis in neonates relies heavily on brown fat metabolism, which is rich in mitochondria.

Heat Loss - The Body's Coolant System

• Primary Mechanisms: Heat dissipates from the skin to the environment via four key pathways:
  • Radiation (~60%): Infrared heat waves radiate from the body. The primary method of heat loss at rest in a neutral environment.
  • Evaporation (~22%): Heat loss through sweating (insensible) and respiration. The only mechanism when ambient temperature exceeds body temperature.
  • Convection: Heat transfer to air or water currents (e.g., wind).
  • Conduction: Direct heat transfer to an object in contact with the body (e.g., a cool chair).

⭐ In a thermoneutral environment, radiation accounts for the majority of heat loss. However, with increasing ambient temperature, evaporative cooling becomes progressively more critical.

Central Regulation - The Hypothalamic Hub

• Thermostat: The hypothalamus is the body's central thermostat, maintaining the core temperature set-point (normally 37°C).
• Anterior Hypothalamus: Mediates heat dissipation (cooling). 📌 Mnemonic: Anterior = A/C (Air Conditioning).
  • Senses ↑ temp → initiates sweating & cutaneous vasodilation.
• Posterior Hypothalamus: Mediates heat conservation & production (heating).
  • Senses ↓ temp → initiates shivering, cutaneous vasoconstriction, & non-shivering thermogenesis.

⭐ Fever Pathophysiology: Pyrogens (e.g., IL-1, TNF) ↑ prostaglandin E2 (PGE2) synthesis in the hypothalamus, which elevates the thermal set-point.

Fever Pitch - Resetting the Set-Point

• Pyrogens: Exogenous (e.g., bacterial LPS) or endogenous cytokines (IL-1, IL-6, TNF-α) trigger fever.
• Mechanism: They stimulate prostaglandin E₂ (PGE₂) synthesis via COX enzymes in the anterior hypothalamus.
• Set-Point Reset: PGE₂ elevates the hypothalamic thermoregulatory set-point.
• Body's Response: The body, now feeling "cold," initiates heat production (shivering) and conservation (vasoconstriction) to match the new, higher target temperature.

⭐ NSAIDs (Aspirin, Ibuprofen) are antipyretic because they inhibit COX enzymes, thus blocking PGE₂ synthesis.

High‑Yield Points - ⚡ Biggest Takeaways

• The hypothalamus is the central thermostat, balancing heat production and loss.
• Fever: a regulated ↑ in the hypothalamic set-point via pyrogens (e.g., IL-1, PGE₂).
• Hyperthermia: an uncontrolled temperature ↑ with a normal hypothalamic set-point.
• Malignant hyperthermia: a life-threatening reaction to succinylcholine or volatile anesthetics.
• Heat stroke: core temp >40°C, altered mental status (AMS), and often anhidrosis.
• Hypothermia: core temp <35°C; slows metabolism and cardiac conduction, risking arrhythmias.