Regulation of Body Temperature

Regulation of Body Temperature

• Core temperature of the deep tissues remains constant at ±1°F (±0.6°C) except during illness
• A nude person can withstand temperatures as low as 55°F or 130°F and maintain a relatively stable core temperature
• °C = (F-32)/1.8

Core Temperature and Skin Temperature

• The skin temperature fluctuates based on the surrounding environment, whereas the core temperature remains relatively stable
• Skin temperature can range from 20-40°C
• Factors influencing skin temperature include skin blood flow and air temperature

Normal Core Temperature

• Normal oral temperature ranges from 96-104°F (35.5-40°C)
• Rectal temperature is typically slightly higher than oral temperature
• Variations exist based on activity levels, time of day, and individual differences

Differences between Skin Temperature and Core Body Temperature

• Skin temperature is more variable than core temperature responding to variations in skin blood flow and air temperature
• Core temperature changes elicit a more significant thermoregulatory response (9x) compared to skin temperature

Body Temperature Is Controlled by Balancing Heat Production Against Heat Loss

• Heat production is a by-product of metabolism
• Factors influencing the rate of heat production include basal metabolic rate, muscle activity, hormone effects, chemical activity, and food digestion
• Basal rate of metabolism for all body cells
• Extra rate of metabolism caused by muscle activity
• Extra metabolism caused by thyroid hormone, growth hormone, and testosterone, and to a lesser extent, epinephrine, norepinephrine , and sympathetic stimulation.
• Extra metabolism from increased chemical activity in the cells, especially during higher temperatures.
• Extra metabolism from digestion, absorption, and storage of food (thermogenic effect of food)

Heat Loss

• The rate of heat loss is determined primarily by how quickly heat is conducted from the core to the skin, and how quickly heat is transferred from the skin to the surroundings
• The rate of blood flow into the skin directly impacts this process. Increased blood flow enhances heat conduction, while reduced blood flow decreases it

Blood Flow to the Skin from the Body Core Provides Heat Transfer

• Blood flow to the skin's venous plexus can vary significantly, from near zero to 30% of total cardiac output
• High skin blood flow efficiently conducts heat from the core to the skin.
• Reduced skin blood flow minimizes heat conduction from the core

Control of Heat Conduction to the Skin by the Sympathetic Nervous System

• Vasoconstriction or dilation of arterioles and arteriovenous anastomoses, controlling blood flow to the skin's venous plexus, is primarily regulated by the sympathetic nervous system
• This control is largely based on body core temperature and environmental temperature

How Heat Is Lost from the Skin Surface

• Heat loss occurs through radiation (60%), conduction to the air (15%), convection by air currents, and evaporation (22%), and conduction to surrounding objects (3%)

Heat Dissipation Pathways

• Heat dissipation varies significantly between sitting indoors at 25°C and walking outside at 30°C
• Radiation accounts for 60% of heat loss for both situations.
• Convection accounts for 15% (indoors) and 10% (outside) of heat loss
• Conduction accounts for a negligible 5% (indoors) and minimal amount (outside) of heat loss
• Evaporation accounts for 20% (indoors) and 90% (outside) of heat loss

Sweating

• Sweating is an active evaporative heat loss mechanism controlled by the sympathetic nervous system
• Sweat rate can be adjusted.
• At normal temperature, 100ml of sweat is produced per day.
• During hot weather, this increases to 1.5 liters and during heavy exercise, up to 4 liters per day.
• Sweat's composition is a dilute salt solution actively secreted by sweat glands
• Evaporation of sweat is essential; otherwise, there's no heat loss if the sweat is dripped or wiped away

Relative Humidity and Sweating

• Relative humidity significantly influences the extent of evaporative heat loss from sweating
• High relative humidity limits the air's ability to absorb moisture, thus reducing sweat evaporation and resulting in little evaporative heat loss in hot, humid environments

Temperature-regulating Mechanisms

• The hypothalamus, possessing two centers, has a crucial role in temperature regulation
• The posterior region of the hypothalamus triggers heat production and conservation, while the anterior region triggers heat loss reflexes in response to warmth

Receptors

• The body has two types of thermosensors: central and skin
• Central thermoreceptors monitor internal body temperature (located in hypothalamus, spinal cord, and viscera)
• Skin thermoreceptors monitor external thermal environment (cold and warmth receptors with sensitivity ranges of 5-45°C and 30-50°C, respectively).

Effector Mechanisms in Temperature Regulation

• Mechanisms activated by cold, such as vasoconstriction, behavioral responses, increased muscle tone, shivering, and increased epinephrine secretion, help increase heat production
• Heat-induced mechanisms, including vasodilation, sweating, decreased muscle tone, and reduced appetite, help increase heat loss

Fever versus Hyperthermia

• Fever and hyperthermia result in temperature increase; however, regulations differ regarding the increase
• In fever, regulation mechanisms drive the increase, whereas, in heat stress, heat gain drives the increase that regulation attempts to counteract

Mechanism of Fever

• Infection triggers elevated temperature set point in the hypothalamus, leading to multiple responses that increase heat production (shivering, heat production in skeletal muscles) and reduce heat loss (vasoconstriction, limiting sweat)

Effects of Changing the Set-Point of the Hypothalamic Temperature Controller

• The hypothalamus alters the body's set point for temperature, triggering varied responses from the body—raising or lowering the set point

Hyperthermia

• Hyperthermia occurs when body temperature rises above the set point, frequently due to exercise leading to heat retention.

Heat Exhaustion and Heat Stroke

• Heat exhaustion is a state of collapse due to plasma volume depletion and skin blood vessels dilation resulting in rapid, weak pulse, hypotension, sweating, and disorientation
• Heat stroke is a severe breakdown in heat regulation with body temperature continuously rising, characterized by hot, dry skin, confusion or unconsciousness and eventually a life threatening situation

Heat Stroke

• Impaired blood circulation to the brain, often caused by dehydration, is a significant factor contributing to heat stroke
• A positive feedback loop exists where rising body temperature stimulates metabolic heat production, leading to further temperature increase.

Acclimatization

• Acclimatization is the body's gradual adaptation to environmental changes including increased plasma volume, earlier sweating onset, increased maximal sweat rate, and altered sweat composition to maintain homeostasis