Overview

This lecture reviews body temperature regulation, mechanisms of heat loss, the physiological responses to temperature changes, and what happens when thermoregulation fails, such as during fever or extreme temperatures.

Types of Body Temperature

• Core temperature refers to the temperature of internal organs and must be tightly regulated (36–37.5°C or 97–99.5°F).
• Skin temperature fluctuates and helps regulate core temperature.

Heat Production and Loss

• Heat is produced by basal metabolism, muscle activity, hormones (thyroxine, growth hormone, testosterone, epinephrine, norepinephrine), and digestion (thermogenic effect).
• Heat is mainly lost through the skin, with loss rate determined by blood flow and environmental conditions.
• Fat in subcutaneous tissue acts as insulation, slowing heat loss.

Mechanisms of Heat Loss

• Radiation: Infrared heat emission from skin to environment, more pronounced when body is warmer than surroundings.
• Conduction/Convection: Direct transfer to solids/air; convection is enhanced by wind or moving air.
• Evaporation: Water loss from skin (sweating) and lungs; increased by sweating, occurs even without sweat (insensible loss).

Skin and Blood Flow

• Epidermis, dermis, and subcutaneous layers make up the skin; the latter contains fat and blood vessels.
• Venous plexus and arteriovenous anastomoses allow regulation of blood flow, thus altering heat loss by vasodilation (increases heat loss) and vasoconstriction (conserves heat).

Regulation of Body Temperature

• Hypothalamic pre-optic area acts as the body's thermostat.
• Heating this area causes sweating, vasodilation, and reduced metabolism; cooling causes vasoconstriction, piloerection, increased metabolism, and shivering.
• Peripheral receptors, mainly sensitive to cold, provide feedback to prevent hypothermia.
• Shivering increases muscle tone and heat production; non-shivering thermogenesis involves brown fat and uncoupling proteins.
• Thyroxine increases metabolism and heat; its release is stimulated via TRH and TSH in response to cold.

Set Point and Feedback Control

• Body temperature set point is 37.1°C (98.8°F); feedback mechanisms adjust heat production/loss to maintain this.
• Skin temperature can modify set points to prevent overcorrection.
• Behavioral responses also help regulate temperature.

Failures of Temperature Regulation

• Fever (Pyrexia): Set point rises (often due to infection/pyrogens), leading to chills and then sweating as the set point returns to normal; mediated via prostaglandins (inhibited by NSAIDs).
• Hyperthermia/Heat Stroke: Body temperature rises uncontrollably, but set point is normal; can cause shock, organ, or brain damage.
• Hypothermia/Frostbite: Body temperature drops below set point; CNS depression, loss of heat control, and cell death in frozen tissues.

Key Terms & Definitions

• Core Temperature — Inner body temperature regulated within a narrow range.
• Radiation — Heat loss by infrared emission.
• Conduction — Heat transfer to objects in direct contact.
• Convection — Heat loss by movement of air or liquid over skin.
• Evaporation — Loss of heat by water changing to vapor.
• Insensible Loss — Water lost by evaporation not related to sweating.
• Hypothalamic Pre-optic Area — Brain region responsible for temperature regulation.
• Pyrexia — Fever; elevation of set point for body temperature.
• Hyperthermia — Uncontrolled increase in body temperature without a raised set point.
• Frostbite — Freezing of skin/tissues, leading to cell death.
• Antipyretic Drugs — Medications that reduce fever.
• Brown Fat — Tissue that produces heat via non-shivering thermogenesis.

Action Items / Next Steps

• Review definitions: pyrexia, hyperthermia, and mechanisms of heat loss.
• Study figures 74.3, 74.4, 74.8, and 74.9 from the textbook for visual understanding.
• Prepare answers to: What is fever (pyrexia)? What is hyperthermia? Name three mechanisms of heat loss from the body.

Certainly! Here's a more detailed and comprehensive summary of the chapter on body temperature regulation based on the lecture content:

Detailed Summary: Body Temperature Regulation and Dysregulation

Types of Body Temperature

• Core Temperature: Refers to the temperature of internal organs, maintained within a narrow range (36–37.5°C or 97–99.5°F). This is critical for normal biochemical and physiological functions.
• Skin Temperature: More variable and influenced by the environment; it fluctuates widely and plays a role in regulating core temperature by adjusting heat loss.

Heat Production Mechanisms

• Basal Metabolic Rate (BMR): Continuous chemical reactions in the body generate heat.
• Muscle Activity: Physical activity increases heat production.
• Hormonal Influence: Hormones such as thyroxine, growth hormone, testosterone, epinephrine, and norepinephrine increase metabolic rate and heat production.
• Thermogenic Effect of Food: Digestion, absorption, and storage of food produce heat.
• Cellular Metabolism: Heat production increases as cell temperature rises.

Heat Loss Mechanisms

Heat is primarily lost through the skin, and the rate depends on:

• Blood Flow to Skin: Increased blood flow (vasodilation) enhances heat transfer to the skin surface; decreased flow (vasoconstriction) conserves heat.
• Environmental Conditions: Temperature, humidity, and wind affect heat loss.

Three Main Mechanisms of Heat Loss:

1. Radiation: Emission of infrared heat waves from the body to the environment. Heat loss occurs if body temperature is higher than surroundings; heat gain if surroundings are warmer.
2. Conduction and Convection:
   • Conduction: Direct transfer of heat through molecular vibration to objects or air in contact with the skin.
   • Convection: Movement of air or liquid removes warmed molecules from the skin surface, allowing more heat transfer. Wind increases convection.
3. Evaporation: Loss of heat through water vaporization from the skin and lungs.
   • Sweating: Controlled by the sympathetic nervous system; sweat glands secrete a plasma-like fluid that is modified by reabsorption of sodium and chloride.
   • Insensible Water Loss: Continuous evaporation without sweating, influenced by humidity and environmental dryness.

Additional Factors Affecting Heat Loss:

• Insulation by Fat: Subcutaneous fat has low thermal conductivity, slowing heat transfer to the skin.
• Clothing: Traps a layer of air, reducing convection and radiation heat loss. Wet clothes increase heat loss due to water’s higher conductivity.

Skin Structure and Blood Flow Regulation

• Skin layers: epidermis, dermis, and subcutaneous tissue (contains fat and blood vessels).
• Venous Plexus: A network of veins in the skin regulated by arteriovenous anastomoses.
• Vasodilation: Opens arteriovenous anastomoses, increasing blood flow to the venous plexus and enhancing heat loss.
• Vasoconstriction: Reduces blood flow, conserving heat.

Central Regulation of Body Temperature

• Hypothalamic Pre-optic Area: Acts as the body’s thermostat.
  • When heated: triggers vasodilation, sweating, and reduces metabolism.
  • When cooled: triggers vasoconstriction, piloerection (hair standing up to trap air), shivering, and increases metabolism via sympathetic stimulation and thyroxine release.
• Peripheral Thermoreceptors: Located in skin, spinal cord, and abdominal viscera; mainly detect cold to prevent hypothermia by local vasoconstriction, shivering, and inhibiting sweating.
• Shivering Mechanism: Increased skeletal muscle tone leads to rhythmic contractions via stretch reflex oscillations, producing heat.
• Non-Shivering Thermogenesis: Sympathetic stimulation increases metabolism and uncouples oxidative phosphorylation in brown fat, producing heat instead of ATP.
• Thyroxine Role: Cold exposure stimulates hypothalamic release of TRH → anterior pituitary releases TSH → thyroid gland produces thyroxine, which increases metabolism and heat production.

Set Point and Feedback Control

• Normal set point: 37.1°C (98.8°F).
• Body activates heat production below set point and heat loss above it.
• Skin temperature modulates the set point to prevent overcorrection:
  • Low skin temperature raises the core temperature threshold for sweating.
  • High skin temperature lowers the threshold for shivering.
• This negative feedback system has a high gain (~27), ensuring tight temperature control.
• Behavioral responses (e.g., seeking shade or warmth) also contribute to temperature regulation.

Failure of Temperature Regulation

Fever (Pyrexia)

• Set point is elevated (e.g., from 37.1°C to 39°C).
• Caused by pyrogens:
  • Exogenous: Bacterial endotoxins (lipopolysaccharides), viral proteins, cancer products.
  • Endogenous: Cytokines like interleukin-1 released by immune cells.
• Mechanism: Pyrogens stimulate prostaglandin E2 production in hypothalamus, raising set point.
• Symptoms: Initial chills (body tries to raise temperature to new set point), followed by sweating and flushing when set point returns to normal.
• Antipyretic drugs (NSAIDs) inhibit prostaglandin synthesis, reducing fever.

Hyperthermia and Heat Stroke

• Body temperature rises uncontrollably without a change in set point.
• Causes: Excessive heat exposure, strenuous exercise, failure of heat loss mechanisms.
• Symptoms: Dehydration, dizziness, vomiting, circulatory shock, brain damage.
• Heat stroke is a medical emergency.

Hypothermia and Frostbite

• Body temperature falls below set point; regulatory mechanisms fail as CNS depresses.
• Symptoms: Sleepiness, loss of shivering, vasoconstriction failure.
• Frostbite: Freezing of skin and tissues; ice crystals damage cells irreversibly, leading to gangrene if untreated.
• Controlled hypothermia is used clinically to reduce metabolism during surgeries like open-heart surgery.

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