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Foundation Sciences · Physiology
Thermoregulation
Thermoregulation is the homeostatic process of maintaining a stable internal body temperature (core temperature), typically around 37°C. It is controlled by the hypothalamus, which acts as a biological thermostat. The body balances heat production (metabolism, shivering) and heat gain with heat loss (radiation, conduction, convection, evaporation) to ensure optimal conditions for enzymatic and cellular function.
📌 Learning Objectives
- Describe the physiological mechanisms involved in maintaining core body temperature.
- Explain the role of the hypothalamus in thermoregulation, including its afferent and efferent pathways.
- Identify the key heat production and heat loss mechanisms in the human body.
- Differentiate between fever and hyperthermia, outlining their underlying causes and physiological responses.
- Apply knowledge of thermoregulation to understand the clinical implications of hypothermia and hyperthermia.
📋 Overview
Core temperature is monitored by peripheral thermoreceptors in the skin and central thermoreceptors in the hypothalamus and spinal cord. The Preoptic Area (POA) of the hypothalamus integrates these signals. When core temperature rises, the hypothalamus triggers heat loss mechanisms: cutaneous vasodilation (bringing warm blood to the surface) and diaphoresis (sweating, which uses the latent heat of evaporation). Conversely, when temperature drops, heat conservation and production are activated: cutaneous vasoconstriction (minimizing heat loss), piloerection (limited in humans), and thermogenesis. Thermogenesis involves shivering (involuntary muscle contraction) and non-shivering thermogenesis (metabolism of brown adipose tissue, primarily in infants). A fever (pyrexia) differs from hyperthermia; in fever, pyrogens (like IL-1) reset the hypothalamic set-point higher, whereas hyperthermia is a failure of heat loss mechanisms. Maintaining this tight range is critical, as protein denaturation occurs at high temperatures and metabolic slowing/arrhythmias occur at low temperatures.
🔬 Basic Science
Radiation involves heat loss via infrared rays. Conduction is direct transfer to a cooler object (e.g., cold water). Convection is heat transfer to moving air/fluid. Evaporation is the only method effective when ambient temperature exceeds body temperature. In fever, exogenous pyrogens (bacteria/viruses) stimulate endogenous pyrogens (IL-1, IL-6, TNF-alpha), which act on the Organum Vasculosum of the Lamina Terminalis (OVLT). This leads to Prostaglandin E2 (PGE2) synthesis, which raises the hypothalamic set-point. This makes the normal body temperature feel 'cold,' triggering shivering and vasoconstriction (the 'chills'). Aspirin and Paracetamol work by inhibiting the COX enzymes responsible for PGE2 production. In the elderly, thermoregulation is less efficient due to reduced muscle mass (less shivering) and diminished sweat gland function. In neonates, heat loss is rapid due to a high surface area to volume ratio, making brown fat metabolism crucial.
🏥 Clinical Relevance
Hypothermia (<35°C) can cause J-waves (Osborn waves) on an ECG and progress to cardiac arrest; 'no one is dead until they are warm and dead.' Hyperthermia (Heat Stroke) is a medical emergency with core temp >40°C and CNS dysfunction. Malignant Hyperthermia is a rare pharmacogenetic reaction to volatile anesthetics (e.g., Halothane) or Suxamethonium, causing massive calcium release in muscles and life-threatening pyrexia. MLA focus: distinguishing fever from heat stroke and managing hypothermia.
🧪 Investigations
Core temperature measurement (tympanic, rectal, or esophageal probe). Blood tests: U&Es (dehydration), Clotting/FBC (DIC in heat stroke), CK (rhabdomyolysis). ECG for J-waves or arrhythmias in hypothermia.
💊 Management
Hypothermia: Passive rewarming (blankets), active external rewarming (forced air), or active internal rewarming (warm IV fluids). Fever: Antipyretics (Paracetamol/NSAIDs). Heat stroke: Rapid cooling (cold water immersion, evaporative cooling). Malignant Hyperthermia: Stop triggers and give Dantrolene.
Revision Resources – expand the sections below for high-yield notes, exam pearls, key facts and further reading.
MLA High-Yield Notes & Quick Revision ⌄
Osborn (J) waves in hypothermia are high yield. Know that fever is a 'resetting' of the thermostat, while hyperthermia is an 'overpowering' of it. Neonates lack the shivering reflex—they rely on brown fat.
Sepsis
Infection (e.g., Meningitis, Pneumonia)
Heatstroke/Heat Exhaustion
Hypothermia
Drug reactions (e.g., Neuroleptic Malignant Syndrome, Serotonin Syndrome)
Endocrine disorders (e.g., Hyperthyroidism)
- Thermoregulation maintains core body temperature around 37°C.
- Hypothalamus (Preoptic Area) is the primary control centre.
- Peripheral and central thermoreceptors provide temperature input.
- Heat loss mechanisms: vasodilation, sweating (evaporation).
- Heat gain/conservation mechanisms: vasoconstriction, shivering, non-shivering thermogenesis.
- Fever: elevated hypothalamic set-point due to pyrogens.
Exam Pearls ⌄
⭐ High Yield
Core body temperature is tightly regulated around 37°C by the hypothalamus.
The preoptic area (POA) of the hypothalamus acts as the primary thermoregulatory centre.
Heat loss occurs via radiation, conduction, convection, and evaporation (sweating).
Heat production involves metabolism, shivering, and non-shivering thermogenesis (brown fat).
Fever involves a raised hypothalamic set-point due to pyrogens (e.g., IL-1, TNF-alpha).
Hyperthermia is an uncontrolled rise in body temperature due to failed heat loss mechanisms.
Peripheral thermoreceptors (skin) and central thermoreceptors (hypothalamus, spinal cord) provide temperature input.
💡 Clinical Pearl
Fever (Pyrexia): A common clinical sign where the body's thermoregulatory set-point is elevated, often due to infection.
Heatstroke: A life-threatening condition resulting from the failure of thermoregulatory mechanisms to dissipate heat, leading to uncontrolled hyperthermia.
Hypothermia: A dangerous drop in core body temperature below 35°C, where the body's heat production cannot keep pace with heat loss.
Malignant Hyperthermia: A rare, inherited disorder triggered by certain anaesthetics, leading to a rapid and uncontrolled increase in body temperature due to excessive muscle contraction.
Thyrotoxicosis: Excess thyroid hormones increase metabolic rate, leading to increased heat production and often a feeling of being too warm.
⚠️ Exam Tip — Common Mistakes
Confusing fever with hyperthermia; fever is a regulated increase in set-point, hyperthermia is a loss of regulation.
Underestimating the role of evaporation (sweating) as the most effective heat loss mechanism in warm environments.
Forgetting that vasoconstriction is for heat conservation, not heat production.
Assuming shivering is the only form of thermogenesis; non-shivering thermogenesis is crucial, especially in neonates.
Not appreciating the critical role of the hypothalamus as the integrating centre for thermoregulation.
Key Facts ⌄
Normal core temperature: 36.5°C to 37.5°C.
Heat loss methods: Radiation (60%), Evaporation (22%), Convection (15%), Conduction (3%).
Hypothermia is defined as core temperature <35°C.
Brown adipose tissue contains 'thermogenin' (UCP1) for non-shivering thermogenesis.
The 'Shell' (skin/limbs) temperature varies widely, while the 'Core' is kept stable.
Sweating is mediated by sympathetic cholinergic fibers (using Acetylcholine).
Related Topics ⌄
References ⌄
- TeachMePhysiology - Thermoregulation
- NICE CKS: Fever in children
- State of NHS Guidelines: Hypothermia management
- GMC MLA Content Map - Physiology
Further Resources
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