Lecture #190: Physiology: Thermoregulation

What are the four main objectives of thermoregulation physiology?

Compare core body temperature and skin temperature under different conditions, describe homeostatic mechanisms for stable core temperature including heat production and dissipation, understand CNS regulation of body temperature, and describe the mechanism of fever.

Why must internal body temperature be tightly regulated?

Internal body temperature is regulated within a narrow range despite environmental variation because failure of thermoregulatory mechanisms can cause hyperthermia or hypothermia, and extreme temperatures limit survival.

What is core body temperature?

Core body temperature is the relatively stable internal temperature that protects the function of internal organs.

What is skin temperature?

Skin temperature is the temperature at the body surface and rises or falls with the surrounding environmental temperature.

How does core temperature differ from skin temperature in cold or warm environments?

Core temperature remains relatively stable to protect organs, while skin temperature fluctuates more directly with environmental temperature.

What was historically considered normal body temperature?

For a long time, 98.6°F or 37°C was considered normal based on Carl Wunderlich’s 1868 report of average temperature.

What is today’s average oral body temperature in healthy adults?

Today’s average oral body temperature is closer to 97.9°F, with a range of about 97.3°F to 98.2°F.

How does body temperature vary throughout the day?

Body temperature is lowest in the early morning and increases by about 0.9–1.8°F by late afternoon.

When is body temperature usually lowest and highest?

Minimum temperature usually occurs between 3 AM and 5 AM, while maximum temperature usually occurs between 4 PM and 6 PM.

How does time of day affect fever thresholds?

A morning oral temperature greater than 98.9°F may be considered fever, while afternoon fever and overall fever is defined around 99.9°F.

How does sex affect body temperature?

Body temperature tends to be higher in women.

How does age affect body temperature?

Body temperature tends to decrease with age.

How does weight affect body temperature?

Body temperature tends to increase with weight.

How does height affect body temperature?

Body temperature tends to decrease with height.

What temperature defines pediatric fever?

Pediatric fever is defined as 100.4°F.

When should a doctor be called for fever in a newborn to 3-month-old?

A doctor should be called for a temperature greater than 100.4°F in a newborn to 3-month-old.

When should a doctor be called for fever in a 3- to 6-month-old?

A doctor should be called for temperature greater than 102°F, or 100.4–102°F if the infant is unusually irritable, lethargic, or uncomfortable.

When should a doctor be called for fever in a 6- to 24-month-old?

A doctor should be called for temperature greater than 102°F lasting longer than one day without other signs, or greater than 102°F with signs such as cold, cough, or diarrhea.

When should a doctor be called for fever in a child older than 2 years?

A doctor should be called for any fever lasting over 72 hours.

What is conduction?

Conduction is heat transfer through direct contact.

What is convection?

Convection is heat transfer through movement of air or water.

What is radiation?

Radiation is heat transfer through electromagnetic waves without direct contact.

What is evaporation?

Evaporation is heat transfer through evaporation of water, especially sweat.

What is basal or resting metabolic rate?

BMR or RMR is the metabolic rate under standard resting conditions and reflects heat produced as a byproduct of metabolism needed to maintain chemical reactions in the brain and internal organs.

What are standard conditions for measuring BMR/RMR?

Standard conditions include a 12-hour fast, morning measurement, 30 minutes of rest, and a comfortable temperature around 77°F.

How is BMR measured or estimated?

BMR can be measured by direct or indirect calorimetry and estimated based on sex, weight, height, and age.

Which body region produces the most heat at rest?

The trunk viscera produce the most heat at rest, about 56% of resting heat production despite being about 34% of body mass.

How much resting heat is produced by the brain?

The brain is about 2% of body mass but produces about 16% of heat at rest.

How do age and sex affect basal metabolic rate?

Metabolic rate is proportional to body surface area, highest in infancy, increased in pregnancy, and higher in males than females.

How do catecholamines affect metabolic rate?

Catecholamines increase metabolic rate; pheochromocytoma can cause weight loss and heat intolerance.

How does thyroxine affect metabolic rate?

Thyroxine increases metabolic rate; hyperthyroidism causes weight loss and heat intolerance, while hypothyroidism causes weight gain and cold intolerance.

How does progesterone affect body temperature?

Progesterone increases body temperature after ovulation.

What is the thermic effect of food?

The thermic effect of food is a 10–20% increase in metabolic rate after a meal, with fats and proteins being more effective than carbohydrates.

What is the main source of heat during exercise?

Skeletal muscle becomes the main source of heat during exercise.

How much can metabolic rate increase during exercise?

Metabolic rate can increase up to 10-fold during exercise.

How does exercise affect circulating blood temperature?

Working muscles can warm circulating blood by about 1–2°F.

Which tissue produces most heat during exercise?

Muscle and skin produce about 90% of heat during exercise.

How does skin help with thermoregulation?

Skin and subcutaneous tissue provide insulation, skin blood flow transfers heat from the core to the surface for radiation, and eccrine sweat glands increase heat loss by evaporation.

How is cutaneous blood flow regulated?

Cutaneous blood flow is regulated through the superficial arteriovenous plexus and the sympathetic nervous system using adrenergic vasoconstriction and cholinergic vasodilation.

What does cutaneous vasodilation do?

Cutaneous vasodilation increases blood flow to the skin, transferring heat from the core to the body surface and promoting heat dissipation.

What does cutaneous vasoconstriction do?

Cutaneous vasoconstriction reduces blood flow to the skin, conserving heat and reducing heat loss.

Where are eccrine sweat glands found?

Eccrine glands are found in non-glabrous skin including the face, trunk, and limbs.

What are the two parts of an eccrine sweat gland?

An eccrine sweat gland has a deep subdermal coil that produces primary secretion and a duct in the dermis and epidermis that reabsorbs sodium and chloride.

What is the composition of primary sweat secretion?

Primary sweat secretion resembles blood plasma minus the proteins.

What determines final sweat composition?

Final sweat composition depends on sweat production rate and flow, which are determined by the level of cholinergic sympathetic activation.

What type of neurons innervate sweat glands?

Sweat glands are innervated by post-ganglionic cholinergic sympathetic neurons.

How does excessive sweating activate aldosterone?

Excessive sweating reduces plasma volume, which activates aldosterone production by the adrenal gland.

How does aldosterone help during heat acclimation?

Aldosterone increases sodium and water reabsorption in the kidney and upregulates ENaC in sweat gland luminal cells to increase sodium reabsorption from sweat.

How does heat acclimation affect salt loss?

An unacclimated person may lose 15–30 grams of salt per day, but after 4–6 weeks of acclimation, salt loss decreases to about 3–5 grams per day.

What are skin thermoreceptors?

Skin thermoreceptors sense body surface temperature, are more sensitive to cold, and include more cold receptors than warmth receptors.

What are hypothalamic temperature-sensitive neurons?

Hypothalamic temperature-sensitive neurons are the principal mechanism for sensing core temperature, setting the temperature threshold, and detecting deviations from that threshold.

What behavioral responses help regulate temperature?

Behavioral responses include seeking a cooler or warmer environment, adding or removing clothing, changing posture, and otherwise changing the environment.

What happens when body temperature is too high?

Heat-loss mechanisms are activated, including vasodilation of skin blood vessels and sweating.

What happens when body temperature is too low?

Heat-conserving and heat-producing mechanisms are activated, including vasoconstriction of skin blood vessels, shivering, and chemical thermogenesis.

What is chemical thermogenesis?

Chemical thermogenesis is heat production through brown adipose tissue, which contains abundant lipid droplets, mitochondria, sympathetic innervation, and UCP1.

How does UCP1 produce heat?

UCP1 uncouples the electrochemical proton gradient from ATP synthesis, causing energy to dissipate as heat.

What is shivering thermogenesis?

Shivering thermogenesis is heat production from increased basal skeletal muscle tone and involuntary, clonic, rhythmic contractions mediated by motor neurons.

What are the three levels of hierarchical thermoregulatory response?

Thermoregulatory responses include behavioral responses, autonomic responses such as sweating, shivering, and vasoconstriction or vasodilation, and endocrine responses that increase metabolic heat production.

What is the hypothalamic temperature set point?

The temperature set point is a homeostatic hypothalamic mechanism that activates heat production or dissipation to keep core temperature constant, similar to a central thermostat.

What non-thermal factors can affect the hypothalamic set point?

Exercise, ovulation, and pyrogens can affect the hypothalamic temperature set point.

What is fever?

Fever is an increase in the hypothalamic set point, causing activation of heat production and reduction of heat dissipation.

What are pyrogens?

Pyrogens are substances that increase the hypothalamic temperature set point.

Why do chills occur at fever onset?

Chills occur because the elevated set point makes the body respond as if it is cold, causing cutaneous vasoconstriction, cold skin, and shivering.

What happens during fever crisis or resolution?

When the cause of the high set point is removed, the set point drops to normal, causing cutaneous vasodilation and sweating.