Regulation of Body Temperature (4)

Heat Production Sources: At rest, heat is primarily produced by the liver, heart, brain, kidneys, and endocrine organs; inactive skeletal muscles contribute 20-30% of body heat.
Heat Generation During Activity: During vigorous exercise, heat production by skeletal muscles can be 30-40 times that at rest.
Temperature Maintenance: Body temperature is a balance of heat production and heat loss.
Average Body Temp: Approximately 37°C (range of 35.8-38.2°C), fluctuating about 10°C over 24 hours (lowest in the early morning and highest in the evening).
Reaction Rates: For every 10°C increase in temperature, the rate of biochemical reactions increases by approximately 10%.
Critical Temperatures: Temperatures too high can depress neuron function, denature proteins, cause convulsions at 41°C, and can be fatal at 43°C.
Body Cooling: Tolerated better than overheating, utilized in contexts like heart surgery.

Core (Internal Organs): Includes organs within the skull, thoracic and abdominal cavities, and has the highest temperature.
Shell (Surface Structures): Includes skin and subcutaneous tissue, with a lower temperature.
Temperature Measurement: Rectal temperature is about 0.4°C higher than oral temperature; serves as a better indicator of core temperature.
Heat Transfer: Blood acts as a major heat transfer medium between the core and shell. When the shell is warmer than the environment, heat is lost. To conserve heat, blood flow to the skin is minimized.
Shell Temperature Fluctuations: Can vary between 20°C and 40°C based on activity level and environmental temperature.

Radiation:
- Loss of heat through infrared waves, contributing to ~50% of body heat loss.
- Examples: A cool room warming up with people inside; body absorbing heat from sunlight.
Conduction:
- Heat transfer between objects in direct contact, requiring molecule-to-molecule contact.
- Examples: Heat from a warm bath to the body; heat from seated position.
Convection:
- Warm air rises and is displaced by cooler, denser air which absorbs heat effectively.
- Enhanced by air movement (e.g., wind or fans).
Evaporation:
- Absorption of heat by water molecules to transition from liquid to gas (heat of vaporization).
- A significant cooling mechanism where approximately 0.5 kcal of heat is removed from the body per 1 gram of vaporized water.
- Continuous insensible loss occurs from lungs and skin (~10% of basal heat production). Active evaporation happens through sweating during heat stress.
- Examples of Temperature Rise: Emotional states and vigorous exercise can raise body temperature significantly.

Hypothalamus: Major integrating center for thermoregulation with two key centers:
- Heat-loss center: Activated when body temperature rises.
- Heat-promoting center: Activated when body temperature drops.
Thermoreceptor Input: Central thermoreceptors play a more crucial role than peripheral ones in regulating heat loss mechanisms.

Vasoconstriction:
- Sympathetic nervous system constricts cutaneous blood vessels, conserving blood in deep body areas and reducing heat loss.
Shivering:
- Involuntary muscle contractions to generate heat when vasoconstriction is insufficient.
Increased Metabolic Rate:
- Cold stimulates norepinephrine release, increasing metabolic heat production notably in infants due to brown adipose tissue.
Enhanced Thyroxine Release:
- Cold exposure triggers hypothalamus to release hormones that increase metabolic rate and body heat production.
Behavioral Modifications:
- Adjustments like wearing warm clothing, consuming warm liquids, and changing physical activity levels.

Vasodilation:
- Turn off vasoconstriction allowing heat loss through radiation and conduction.
Enhanced Sweating:
- Activated by sympathetic response in extreme heat, dependent on humidity for effectiveness.
Behavioral Modifications:
- Engaging in less activity, seeking shade, using fans, and wearing light clothing.

Occurs when normal heat loss mechanisms fail, leading to a core temperature of around 41°C.
Can depress hypothalamic function leading to heat-control suspension. Resulting in a positive feedback loop of increasing temperature and metabolic rate, potentially leading to heat stroke and organ damage.