Human Body Temperature Regulation

• Body temperature is a critical physiological parameter, and its study involves understanding core and skin temperatures.

Objectives

• Define core and skin temperatures and note their differences.
• Discuss temperature variations in the hypothalamus, rectum, oral cavity, and skin.
• List and define the mechanisms of heat transfer from the skin to the environment.
• Explain the feedback control of internal body temperature.
• Understand the short-term responses to cold for increasing heat production and minimizing heat loss and to heat for decreasing heat production and maximizing heat loss.
• Describe adaptations to cold and warm environments.
• Understand the mechanisms of fever.

Core Temperature

• The internal core temperature is homeostatically maintained, surrounded by an outer shell.
• It is the temperature within the central core, including abdominal and thoracic organs, the central nervous system, and skeletal muscles, remains fairly constant.
• Core temperature varies daily with circadian rhythm and monthly in women; temperature of deep tissues is constant, varying less than 1°F (+0.6°C).
• Core temperature of deep tissues is relatively constant within ±0.6°C under normal conditions.
• A nude person can be exposed to temperatures as low as 12.8°C or as high as 55°C in dry air while still maintaining an almost constant core temperature.

Skin Temperature

• The skin and subcutaneous fat constitute the outer shell.
• The temperature within the shell is generally cooler and may vary substantially, depending on environmental conditions.

Normal Core Temperature

• Oral temperature ranges in healthy people from 36°C to greater than 37.5°C.
• Average normal core temperature is between 36.7°C and 37°C when measured orally, approximately 0.6°C higher when measured rectally.
• Rectal temperature represents the core temperature and varies least with environmental changes.
• Oral temperature is normally 0.5°C lower than rectal temperature, affected by factors like ingestion of hot or cold fluids, gum chewing, smoking, and mouth breathing.
• Extremities are generally cooler than the rest of the body.
• Scrotum temperature is carefully regulated at 32°C.

Factors Affecting Body Temp

• Food ingestion and emotion increase norepinephrine.
• Thyroid hormone release affects body temp.
• During menstruation and pregnancy, core temperature averages 0.5°C higher in the last half of the cycle.
• Time of day affect body temp.
• Exercise is a factor.
• Elderly individuals have lower temperatures, with a midday average of 36.4°C.
• Temperature regulation is less precise in young children, potentially leading to temperatures slightly above the established norm for adults (0.5°C).

Heat Production

• Muscular activity, including shivering, produces heat.
• Basic metabolic processes generate heat.
• Food intake results in specific dynamic action, producing heat.
• Thyroxine and other hormones like growth hormone and testosterone affect heat production.
• Epinephrine, norepinephrine, and sympathetic stimulation contribute to heat

Heat Loss

• Heat is transferred from deeper organs and tissues to the skin, where it is lost to the air and surroundings.
• The rate of heat loss depends on how rapidly heat is conducted from the body core to the skin and how rapidly heat is transferred from the skin to the surroundings.

Mechanisms of Heat Loss

• Radiation: Emission of heat to and from the skin through electromagnetic waves, proportional to the temperature difference between the body surface and environment.
• Conduction: Intermolecular thermal heat transfer between the skin and air. It occurs more rapidly when immersed in water.
• Convection: Heat loss or gain through the movement of air or water over the body. Air carries heat away from the body by convection.
• Evaporation: Water evaporation from skin and the respiratory tract carries a large amount of heat, due to the energy required to transform water from liquid to gas.

Role of the Hypothalamus

• It's the temperature control center.
• The preoptic area of the anterior hypothalamus is heat-sensitive and cold-sensitive.
• Skin and deep body temperature receptors mainly detect cold temperatures
• Functions to prevent hypothermia.
• The posterior hypothalamus receives input from the anterior hypothalamus and peripheral temp receptors to elicit mainly heat producing and heat conserving reactions.

Mechanisms to Decrease Temperature

• Vasodilation: Transfers heat to the skin, inhibited by sympathetic centers in the posterior hypothalamus.
• Sweating: Evaporative heat loss stimulated by the preoptic area of the hypothalamus via sympathetic cholinergic fibers, with circulating epinephrine and norepinephrine during exercise.
• Decreased heat production: Mechanisms like shivering and chemical thermogenesis are inhibited.

Mechanisms to Increase Temperature

• Vasoconstriction: Impedes heat transfer to the skin, stimulated by sympathetic centers in the posterior hypothalamus.
• Piloerection: Usually not important in humans.
• Increased heat production: Shivering involves synchronous, rhythmic contractions of antagonistic muscles.
• Non-shivering thermogenesis: Sympathetic stimulation increases levels of norepinephrine and epinephrine, with increased thyroxine during long-term cold exposure.

Concept of a "Set-Point"

• The body attempts to maintain a critical core temperature of about 37.1°C.
• Thermoregulation attempts to bring the body temperature back to this set point level.

Pyrogens

• Pyrogens are circulating factors that cause fever.
• Bacteria may release exogenous pyrogens, such as lipopolysaccharides (endotoxins).
• The unifying cause of fever during infection is the release of small proteins called endogenous pyrogens from macrophages and other immune cells.
• Endogenous pyrogens includes cytokines such as interleukins and tumor necrosis factor.
• These endogenous pyrogens can activate vagal afferents to the hypothalamus or directly alter the hypothalamic set point for temperature.
• Pyrogens can signal the hypothalamus despite the blood-brain barrier.

Fever Resetting

• Pyrogens can directly reset the set-point

• Bacteria and lipopolysaccharides are pyrogens

• Pyrogens from degenerating tissues

• Pyrogens can indirectly reset-point

• Interleukin-1 is released from phagocytes.

• IL-1 raises the set-point by increasing prostaglandin production (mainly E2).

Heat Stroke

• Occurs when body temperature rises above 42-44°C.
• Malfunction of the preoptic temperature control causes sweating to cease.
• Rising body temperature increases metabolism resulting in ore heat being generated.

Heatstroke Symptoms

• High body temperature of 40°C or higher.
• Altered mental state or behavior includes confusion, agitation, slurred speech, irritability, delirium, seizures and coma.
• Alteration in sweating, in heatstroke, brought on by hot weather, causing skin to be hot and dry to the touch, or moist in heatstroke from strenuous exercise.
• Other symptoms include nausea and vomiting, flushed skin, rapid breathing, racing heart rate, and headache.

Explore the regulation of human body temperature, including core and skin temperatures and the mechanisms of heat transfer. Understand short-term responses to heat and cold. Learn about adaptations to different environments and the mechanisms behind fever.