Mechanism of Thermoregulation in Human

1. Introduction

"A nude person can be exposed to temperatures as low as 55°F or as high as 130°F in dry air and still maintain an almost constant core temperature." — Guyton and Hall Textbook of Medical Physiology

2. Body Temperature Compartments

3. The Thermoregulatory System — Overview

1. Peripheral thermoreceptors — skin and visceral sensors
2. Thermal afferent pathways — carry signals to the CNS
3. Integration center — preoptic anterior hypothalamus (POAH)
4. Efferent pathways — autonomic and somatomotor outputs
5. Thermal effectors — mechanisms controlling heat production and heat loss

— Medical Physiology (Boron & Boulpaep)

4. Thermoreceptors (Sensors)

A. Peripheral (Skin) Thermoreceptors

• The skin contains both cold receptors and warmth receptors (free nerve endings).
• Cold receptors are 10× more numerous than warmth receptors — peripheral detection mainly senses cold.
• Warmth receptors: increase firing rate as skin temperature rises from ~32°C to ~45°C.
• Cold receptors: increase firing rate as temperature falls from ~40°C to ~26°C.
• Both show a phasic (dynamic) response to sudden change, followed by a tonic (static) response.
• The molecular basis involves TRP (transient receptor potential) channels on somatosensory neurons and epidermal cells.

• Cell bodies in the dorsal root ganglia → synapse in the dorsal horn → second-order neurons project rostrally to:
  • Thalamus → third-order neurons → cerebral cortex (conscious thermal perception)
  • Parabrachial nucleus (pons) → third-order neurons → preoptic hypothalamus (autonomic integration)

Skin thermoreceptors provide an anticipatory (feed-forward) signal — they relay information about ambient temperature changes to the hypothalamus before core temperature changes.

B. Central Thermoreceptors (Hypothalamic)

• Located in the preoptic area of the anterior hypothalamus (POAH)
• ~10% of POAH neurons are warmth-sensitive — discharge rate increases as local temperature rises
• ~One-third as many are cold-sensitive
• Heat-sensitive neurons increase firing 2–10 fold per 10°C rise in temperature
• These neurons use TRPV channels to detect local temperature change
• They are the primary sensors of core body temperature (Tcore)

— Guyton and Hall; Medical Physiology

5. The Hypothalamic Thermostat — Integration Center

1. Receives afferent signals from peripheral skin receptors (via parabrachial nucleus) and senses core temperature directly
2. Integrates thermal and non-thermal inputs (e.g., dehydration inhibits sweating efferents to conserve water; hypoxia inhibits thermogenesis)
3. Generates efferent commands via autonomic and somatomotor pathways to thermal effectors

The Set Point Concept

• At a critical core temperature of ~37.1°C (98.8°F), a balance between heat loss and heat production is achieved — this is the "set point."
• Above the set point → heat loss mechanisms activate
• Below the set point → heat production/conservation mechanisms activate
• Skin temperature can slightly shift the set point: warm skin lowers the threshold for sweating; cold skin raises it

6. Heat Production Mechanisms

Shivering Thermogenesis

• Initiated by POAH efferent signals when core temperature falls
• α-motor neurons activate skeletal muscle contractions (involuntary)
• Can increase heat production several-fold rapidly

Non-Shivering Thermogenesis (Chemical Thermogenesis)

• Mediated by brown adipose tissue (BAT) via uncoupling protein-1 (UCP-1/thermogenin)
• UCP-1 uncouples oxidative phosphorylation, releasing energy as heat
• Stimulated by sympathetic (noradrenergic) activation → β3-adrenergic receptors → lipolysis
• In adults: increases heat production by 10–15%; in neonates: up to 100% (important for newborn thermoregulation)

Long-Term Cold Adaptation — Thyroid Axis

• Cooling of POAH → hypothalamus releases TRH → anterior pituitary releases TSH → thyroid releases thyroxine
• Thyroxine activates uncoupling proteins and raises cellular metabolic rate throughout the body
• Requires weeks of cold exposure to become fully active

— Guyton and Hall

7. Heat Loss Mechanisms

A. Radiation (55–65% of total heat loss)

• Emission of infrared electromagnetic waves from the body surface to cooler surroundings
• Does not require a medium — effective in still air

B. Conduction (10–15%)

• Direct transfer of heat to objects in contact with the body (e.g., clothing, water)
• Greatly increased in cold water (water conducts heat 20× faster than dry cloth)

C. Convection

• Heat carried away from the skin surface by moving air currents
• Increased by wind (wind-chill effect)

D. Evaporation

• Sweating and insensible evaporation from the skin and respiratory mucosa
• The only effective mechanism when ambient temperature exceeds body temperature
• Each gram of water evaporated removes 0.58 kcal of heat

E. Respiration

• Expired air carries heat and water vapor
• Affected by ambient temperature and relative humidity

— Harrison's Principles of Internal Medicine, 22E

8. Cutaneous Blood Flow — The Heat Transfer Radiator

• Normal skin blood flow: 250–300 mL/min
• In extreme cold (vasoconstriction): near zero
• In extreme heat (vasodilation): up to 6–8 L/min, representing ~60% of cardiac output

— Fitzpatrick's Dermatology

9. Sweating Mechanism

• A deep secretory coil (produces primary secretion)
• A duct (modifies electrolyte content)

• Low sweating rates → nearly complete NaCl reabsorption → very dilute, hypo-osmotic sweat
• High sweating rates → less time for reabsorption → sweat approaches isotonic

• Maximum sweat rate increases from ~1 L/hr to 2–3 L/hr
• Aldosterone secretion increases (triggered by mild hyponatremia) → further NaCl conservation
• Salt loss decreases from 15–30 g/day (unacclimatized) to 3–5 g/day after acclimatization

— Guyton and Hall

10. Behavioral Thermoregulation

• Moving to cooler/warmer environments
• Changing clothing
• Adjusting posture or activity level

11. Efferent Pathways Summary

12. Fever — The Reset Set Point

1. Exogenous pyrogens (bacteria, viruses) → stimulate phagocytic cells
2. → Release of endogenous pyrogens: IL-1, IL-6, TNF-α
3. → Act on POAH → stimulate prostaglandin E₂ (PGE₂) synthesis via COX-2
4. → PGE₂ raises the set point temperature
5. → Normal core temperature is now perceived as "too low"
6. → Heat production mechanisms activate (shivering, vasoconstriction) to raise body temperature to the new set point

— Costanzo Physiology 7th Edition

13. Disturbances of Thermoregulation

14. Summary Diagram

Environmental change / Metabolic activity
         ↓
Peripheral thermoreceptors (skin TRP channels)
         ↓
Afferent pathway → Parabrachial nucleus (pons)
         ↓
Preoptic Anterior Hypothalamus (POAH) ← also senses Tcore directly
         ↓
Compare Tactual vs. Set Point (37.1°C)
    ↙                ↘
Tcore > Set Point      Tcore < Set Point
  ↓                       ↓
HEAT LOSS:             HEAT PRODUCTION:
• Sweating             • Shivering
• Skin vasodilation    • Skin vasoconstriction
• Behavioral           • Non-shivering thermogenesis
  responses            • ↑ Thyroxine (long-term)

References

• Guyton and Hall Textbook of Medical Physiology — Body Temperature Regulation and Fever (Ch. 74)
• Medical Physiology (Boron & Boulpaep) — Active Regulation of Body Temperature by the CNS (Ch. 59)
• Costanzo Physiology, 7th Edition — Regulation of Body Temperature
• Harrison's Principles of Internal Medicine, 22E — Thermoregulation
• Fitzpatrick's Dermatology — Thermoregulation (Ch. 9)