Temperature regulation in physiology uc exam 1 st year answer

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Temperature Regulation in Physiology - 1st Year UC Exam Answer

1. Introduction - Homeothermy

Humans are homeothermic (warm-blooded) animals who maintain a relatively constant internal body temperature despite wide fluctuations in environmental temperature. This stability is essential because enzyme systems of the body have narrow temperature ranges for optimal function. The regulation is primarily integrated in the hypothalamus.

Guyton & Hall, Medical Physiology

2. Normal Body Temperature

Site	Normal Value
Oral (standard)	37°C (98.6°F)
Rectal (core)	~37.5°C (0.5°C above oral)
Axillary	~36.5°C (0.5°C below oral)
Skin	Varies with environment

Core temperature is the temperature of deep tissues (thorax, abdomen, brain). It remains constant within ±0.6°C. Skin temperature fluctuates with environment and is what matters for heat loss to surroundings.

Circadian variation: Core temperature is lowest around 6 AM and highest in the evenings, showing a daily fluctuation of 0.5-0.7°C.

In women: An additional monthly cycle exists - basal temperature rises ~0.5°C at the time of ovulation (useful for detecting ovulation).

Ganong's Review of Medical Physiology, 26th Ed.

3. Heat Balance: Production vs. Loss

Body temperature = Balance between heat production and heat loss.

If Heat Production > Heat Loss → Body temperature rises
If Heat Loss > Heat Production → Body temperature falls

4. Heat Production

Sources of heat in the body:

Basal metabolic rate (BMR) - all cells continuously produce heat
Muscular activity - exercise is the greatest source; shivering is the most potent cold-response mechanism
Thyroid hormones (thyroxine T3/T4) - stimulate Na⁺-K⁺ ATPase, increase O₂ consumption, increase metabolic rate → thermogenic
Sympathetic nervous system / catecholamines (epinephrine, norepinephrine) - stimulate β receptors in brown adipose tissue → non-shivering thermogenesis
Specific dynamic action (SDA) of food / thermogenic effect - digestion, absorption and storage of food produces heat
Chemical reactions - cell temperature increase accelerates reactions (Q₁₀ effect)

Guyton & Hall, Medical Physiology

5. Heat Loss Mechanisms

Heat is lost from the body surface by four physical methods:

A. Radiation (~60% at rest, room temperature)

Transfer of heat as infrared electromagnetic waves from body to surroundings (and vice versa)
Does not require a medium
Most important mechanism at normal room temperature
Depends on temperature gradient between body surface and surroundings

B. Conduction (~15%)

Direct transfer of heat by physical contact with cooler objects
Water conducts heat ~20 times better than air (explains rapid heat loss in cold water)
Convection is a special form: air/water currents carry away conducted heat

C. Evaporation (~25%)

Two types:

Insensible perspiration (~600 ml/day, 12-16 kcal/hour): continuous water evaporation from skin and respiratory tract, cannot be controlled
Sensible (active) sweating: controlled by the hypothalamus via sympathetic cholinergic fibers to sweat glands; major heat loss mechanism during exercise or high environmental temperature
Each gram of water evaporated removes ~0.58 kcal (580 cal) of heat

When environmental temperature > skin temperature, evaporation becomes the ONLY effective heat loss mechanism.

D. Radiation from Expired Air (minor)

Warming of inspired air + humidification accounts for ~10% of heat loss at rest

Summary table:

Method	% at Rest	Key Feature
Radiation	~60%	Infrared waves, no contact needed
Conduction + Convection	~15%	Direct contact; wind enhances
Evaporation	~25%	Only method when temp ≥ skin temp

Guyton & Hall, Medical Physiology; Costanzo Physiology, 7th Ed.

6. Role of the Hypothalamus (The Thermostat)

The hypothalamus is the central integrator of temperature regulation.

Anterior Hypothalamus (Pre-optic area) - "Heat Loss Center"

Contains heat-sensitive neurons that respond to rising blood temperature
Coordinates heat dissipation: vasodilation of skin vessels, sweating
Lesion → inability to lose heat → hyperthermia

Posterior Hypothalamus - "Heat Conservation/Production Center"

Activated by cold
Coordinates heat conservation: vasoconstriction, shivering, sympathetic activation
Lesion → inability to generate/conserve heat → hypothermia

Set-point concept:

The hypothalamus acts like a thermostat set at ~37°C
Temperature sensors (peripheral in skin + central in hypothalamus) feed information to hypothalamus
If actual temperature deviates from set point, corrective responses activate

7. Responses to Cold (Mechanisms for Generating/Conserving Heat)

Mechanism	Mediator	Effect
Cutaneous vasoconstriction	Sympathetic α₁ activation	Reduces skin blood flow → reduces heat loss
Shivering	Posterior hypothalamus → α & γ motor neurons	Rhythmic skeletal muscle contraction → most potent heat producer
Non-shivering thermogenesis	Sympathetic β activation in brown fat	Uncoupled oxidative phosphorylation → heat without work
Thyroid hormone release	TRH → TSH → T3/T4	Increased Na⁺-K⁺ ATPase, O₂ consumption, metabolic rate
Behavioral responses	Conscious cortex	Curling up, adding clothing, seeking warmth
Piloerection (goosebumps)	Sympathetic → arrector pili	Traps air layer - minor in humans

Costanzo Physiology, 7th Ed.

8. Responses to Heat (Mechanisms for Dissipating Heat)

Mechanism	Mediator	Effect
Cutaneous vasodilation	Decreased sympathetic tone	Up to 30% of cardiac output to skin; conducts heat to surface
Sweating	Sympathetic cholinergic fibers	Evaporation cools skin; up to 2-3 L/hour when acclimatized
Behavioral responses	Cortex	Seeking shade, removing clothing, drinking cold water
Decreased muscle tone	CNS inhibition	Reduces heat production

Sweat glands:

Secretion controlled by sympathetic cholinergic fibers (exception to usual adrenergic sympathetic rule)
Blocked by atropine (anticholinergic) → dangerous hyperthermia
Guyton & Hall, Medical Physiology

9. Heat Acclimatization

With repeated heat exposure over 1-6 weeks:

Sweat production increases: up to 2-3 L/hour (vs. 1 L/hour unacclimatized)
Sweat sodium concentration decreases (aldosterone-mediated salt conservation)
Earlier onset of sweating
Salt loss decreases from 15-30 g/day to 3-5 g/day
Acclimatization disappears within weeks if not maintained

10. Fever

Definition: Body temperature above the normal range due to upward resetting of the hypothalamic set point.

Pathophysiology:

Infection/injury → macrophages/leukocytes phagocytize pathogens
Release of cytokines (especially IL-1/Interleukin-1, also IL-6, TNF-α) - "endogenous pyrogens"
IL-1 acts on OVLT (organum vasculosum of the lamina terminalis) of hypothalamus
Triggers prostaglandin E₂ (PGE₂) synthesis via cyclooxygenase (COX)
PGE₂ raises the hypothalamic set point
Body now perceives normal temperature as "too cold" → activates heat conservation mechanisms (shivering, vasoconstriction) = chills/rigor (fever rising phase)
When infection resolves → set point drops → body sweats profusely to lose heat = defervescence

Exogenous pyrogens: LPS (lipopolysaccharide) from gram-negative bacteria, bacterial toxins

Antipyretics (e.g., aspirin, paracetamol): Inhibit COX → block PGE₂ synthesis → lower set point → sweating → fever reduction

Beneficial effects of fever:

Impairs bacterial/viral replication
Enhances immune cell activity
Increases antibody production

11. Abnormalities of Temperature Regulation

Hyperthermia (Heat Illnesses)

Condition	Description
Heat cramps	Painful muscle spasms; NaCl depletion from heavy sweating
Heat exhaustion	Volume depletion + electrolyte loss; weakness, dizziness, tachycardia; temp < 40°C
Heat stroke	Core temp > 40°C + CNS dysfunction (confusion, loss of consciousness); failure of thermoregulation; medical emergency
Malignant hyperthermia	Genetic defect in RYR1/CACNA1A; triggered by volatile anesthetics; uncontrolled Ca²⁺ release from SR → massive muscle contraction → extreme heat; treat with dantrolene
Neuroleptic malignant syndrome	Dopamine D2 blockade (antipsychotics); hyperthermia + rigidity + autonomic instability

Hypothermia

Core temp < 35°C
At ~28°C: loss of ability to auto-rewarm; loss of consciousness
Metabolic/physiologic processes slow down
Humans can tolerate 21-24°C with careful management
Useful clinically (cardiac surgery, brain protection)

12. Peripheral Temperature Receptors

Warm receptors: Free nerve endings; respond to 30-43°C; C-fibers
Cold receptors: Free nerve endings; respond to 10-40°C (peak 25°C); Aδ fibers (fast); C-fibers (slow)
Paradoxical cold sensation: Extreme heat (>45°C) can stimulate cold receptors → "burning cold"
Signals travel via spinothalamic tract → thalamus → somatosensory cortex

13. Key Points Summary (Exam Mnemonics)

Heat loss = RCES (Radiation, Conduction/Convection, Evaporation, Sweating)

Hypothalamus roles:

Anterior = Loss of heat (A for Anterior = cAring about cooling)
Posterior = Production/conservation of heat

Fever sequence: Pathogen → Macrophage → IL-1 → Hypothalamus → PGE₂ → ↑Set point → Shiver → Fever → Sweat → Defervescence

Sweat glands = Sympathetic but CHOLINERGIC (remember this exception!)

Sources: Guyton & Hall Textbook of Medical Physiology; Costanzo Physiology, 7th Ed.; Ganong's Review of Medical Physiology, 26th Ed.

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