Physiology of Thermal Regulation

Dr. Bashair M. Mussa Contact Information

  • Position: Associate Professor of Neurophysiology

  • Email: bmussa@sharjah.ac.ae

  • Office Location: 147A Level 1 – M27 Basic Medical Science Department, College of Medicine

  • Office Hours: Mondays & Tuesdays (3-5 pm)

Multisystem Unit Overview

  • Topics covered include:

    • Reproductive System

    • Neuroendocrine System

    • Thermoregulatory System

    • Skin

    • Pancreas

    • Breast

Physiology of Thermal Regulation

  • Objectives:

    • Describe fundamental thermoregulatory systems.

    • Explain the processes of radiation, conduction, convection, and evaporation.

    • Describe short-term mechanisms in response to low temperature.

    • Explain heat transmission from core structures to skin and surroundings.

    • Define hypothermia, including its consequences, types, and stages.

    • Describe heat production and loss processes.

    • Differentiate between apical and non-apical skin in terms of conduction.

    • Describe long-term mechanisms in response to low temperature.

1. Homeothermy

  • Definition:

    • Homeothermy is the maintenance of stable internal body temperature regardless of external influences.

2. Fundamental Thermoregulatory Systems

  • Key Components:

    • Thermal Sensors: Detect changes in temperature.

    • Thermo-sensory Afferent Pathway: Carries sensory information to the central nervous system (CNS).

    • Integration CNS Center: Processes sensory information and integrates responses.

    • Efferent Pathways: Relay signals from the CNS to effectors.

    • Thermal Effectors:

    • Generate heat via brown adipose tissue and skeletal muscles.

    • Transfer and dissipate heat through circulation in the skin and sweat glands for heat loss.

Central and Peripheral Thermosensors

  • Central Thermosensors: Located in the hypothalamus and integrated with the sympathetic nervous system.

  • Peripheral Thermosensors: Found in the skin, transmitting temperature information to the central system.

3. Heat Production

  • Heat production sources:

    • By-products of metabolic processes.

  • Factors determining heat production rates:

    • Basal Metabolic Rate: Rate of energy expenditure at rest by all body cells.

    • Muscular Activities: Increase metabolism and heat production.

    • Hormonal Influence: E.g., thyroxin, growth factors affecting metabolism.

    • Neurotransmitter Effects: Influence on metabolic rates.

    • Metabolism during Digestion: Energy needed for digestion, absorption, and storage of food.

4. Heat Loss

  • Sources of heat loss:

    • Produced mainly in organs such as the liver, brain, heart, and muscles.

  • Heat transfer mechanism:

    • From deeper organs and tissues → skin → surrounding environment.

  • Factors influencing heat loss:

    • Speed of Conduction: Heat movement from body core to skin and from skin to surroundings.

    • Necessity of balancing heat production and loss for homeostasis.

5. Heat Transfer from Body Core to Skin

  • Insulating Role of Fat:

    • Fat is a primary insulator, conducting heat only 1/3 as readily as other tissues.

  • Subcutaneous tissues act as thermal insulators, maintaining normal internal core temperature.

Blood Flow and Heat Transfer

  • Abundant blood vessels located just beneath the skin facilitate heat transfer.

  • Blood flow from internal organs to the skin is a main method of heat transfer.

  • Continuous venous plexus receives blood flow from skin capillaries and is supplemented in exposed areas by arteriovenous (a-v) anastomosis.

6. Effective Heat Transfer Methods

  • Up to 30% of total cardiac output may flow into the skin for heat transfer.

  • Rate of blood flow to the skin directly influences heat conduction efficiency.

7. Apical vs Non-Apical Skin

  • Apical skin characteristics:

    • Found in hairless areas such as the nose, lips, ears, hands, and feet.

    • High surface-to-volume ratio facilitates heat loss.

    • Contains arteriovenous anastomoses (glomus bodies).

  • Non-Apical Skin:

    • Covers the rest of the body, lacks a-v anastomoses.

    • Innervation from sympathetic neurons releasing norepinephrine and acetylcholine.

Control of the Sympathetic Nervous System

  • Blood heat conduction to the skin is regulated by arterioles and a-v anastomoses.

  • Vasoconstriction is largely sympathetic-driven, responding to core temperature and environmental changes.

  • Vasodilation is inhibited by atropine but not entirely, as other factors like vasoactive intestinal peptide and nitric oxide are involved.

8. Heat Transfer Mechanisms

  • Heat can be lost from the body through the following means:

    • Radiation:

    • Accounts for 60% of total heat loss.

    • Infrared rays emitted from all bodies above absolute zero.

    • Wavelength range of 5 to 20 micrometers; significantly longer than visible light.

    • Direction of heat radiation depends on temperature differences between bodies.

    • Conduction:

    • Direct heat loss to solid objects (3%) and air (15%).

    • Skin molecules are in constant motion, contributing to kinetic energy and temperature response.

    • Process ceases when skin temperature equals surrounding temperature unless new, cooler air is introduced.

    • Convection:

    • Process of heat loss following conduction; air heated next to the skin rises, allowing unheated air to come into contact with the skin, leading to increased cooling.

    • Evaporation:

    • Key for heat regulation, especially when surroundings are hotter than skin; vital for dissipating internal heat.

    • Lack of evaporation can lead to dangerously high internal temperatures; anhidrosis (absence of sweat glands) can result in heatstroke and even death.

9. Physiological Consequences of Hypothermia

  • Endocrine and Metabolic Effects:

    • Decrease in metabolism, oxygen consumption, carbohydrate metabolism, leading to hyperglycemia; reduced drug metabolism and clearance.

  • Hematological Effects:

    • Increased hematocrit, blood viscosity, neutropenia, thrombocytopenia, coagulopathy, and platelet dysfunction.

  • Respiratory Effects:

    • Decrease in respiratory rate, medullary sensitivity to CO2, leading to alkalosis and hypocapnia (rising pH and falling PCO2 with decreasing body temperature).

  • Cardiovascular Effects:

    • Decreased cardiac output, bradycardia, prolonged QT interval, J waves, susceptibility to arrhythmias (e.g., atrial fibrillation and ventricular fibrillation), resistance to defibrillation, and vasoconstriction.

  • Renal Effects:

    • Cold diuresis due to decreased vasopressin synthesis.

  • Central Nervous System Effects:

    • Confusion, reduced consciousness, shivering, increased seizure threshold.

  • Immunological Effects:

    • Reduced granulocyte and monocyte activity.

10. Hypothermia: Types and Stages

  • Types:

    • prim Hypothermia: Caused by impaired thermoregulation.

    • sec Hypothermia: Resulting from central failure such as anorexia nervosa, cerebrovascular accident, CNS trauma, hypothalamic dysfunction, metabolic disorders, or pharmacologic effects.

  • Stages based on Temperature:

    • Mild hypothermia: 32-35°C

    • Moderate hypothermia: 28-32°C

    • Severe hypothermia: below 28°C

  • Stages based on Clinical Features:

    • HT 1: Clear consciousness with shivering

    • HT 2: Impaired consciousness without shivering

    • HT 3: Unconsciousness

    • HT 4: Reversible/Apparent death

    • HT 5: Irreversible hypothermia/real death

11. Mechanisms of Response to Cold

  • Short-Term Mechanisms:

    • When skin detects low temperature:

    • Initiation of shivering.

    • Inhibition of sweating.

    • Vasoconstriction to prevent heat loss.

    • Local reflexes activated, intensity controlled by central inputs.

  • Central Thermoregulation:

    • Peripheral and central signals transmitted to the posterior hypothalamus for integration to drive physiological responses.

12. Thermogenesis Strategies

  • Non-Shivering Thermogenesis:

    • Activation of brown adipose tissue (BAT).

    • BAT contains high numbers of mitochondria that generate heat.

    • Sympathetic activation leads to norepinephrine release, oxidizing excess food and generating heat.

  • Shivering Thermogenesis:

    • The primary motor center for shivering is located in the dorsomedial portion of the posterior hypothalamus.

    • Inhibited by high temperature signals and activated by cold signals, resulting in increased muscle tone and heat production (4-5 times more than normal).

13. Thyroxine as a Long-Term Thermal Regulator

  • Influence of Temperature on Thyroxine Release:

    • Colder temperatures stimulate hypothalamus to release thyroxin, increasing metabolic rate and body heat.

    • Warmer temperatures inhibit thyroxin release, reducing metabolic rate to lower body temperature.

  • Mechanism of Action:

    • Decreased temperature leads to an increase in thyrotropin-releasing hormone (TRH) production from the hypothalamus.

    • TRH stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH), which in turn leads to increased thyroxin output from the thyroid gland.

  • Activation of Uncoupling Proteins:

    • Thyroxin activates uncoupling proteins, raising basal metabolic rates over time, requiring weeks of exposure to cold.

14. References

  • Guyton and Hall, Textbook of Medical Physiology, Thirteenth Edition

  • Linda S. Costanzo, Lauralee Sherwood, Human Physiology: From Cells to Systems, Eighth Edition

  • Walter F. Boron, Emile L. Boulpaep, Medical Physiology, Third Edition

  • Additional references available via Blackboard

15. Conclusion

  • Thank you for your attention!

  • Any Questions?