Introduction
Normal Body Core Temperatures
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The normal core body temperature range can vary from individual to individual, and can also be influenced by age, activity, and time of day: 36.1 °C (97°F) to 37.2 °C (99 °F).
During strenuous exercise, the temperature can temporarily rise to 40 °C (104 °F).
When the body becomes exposed to extreme cold, the temperature can fall below 35.6 °C (96.1 °F).
An unclothed person can be exposed to temperatures as low as 12.8 °C (55 °F) or as high as 54.4 °C (130 °F) in dry air and still maintain almost constant core temperature.
Skin Temperature
In contrast to core temperature, skin temperature (shell) rises and falls with the temperature of the surroundings.
Issues of Concern
Thermoregulatory Impairment
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Hypothermia
Hypothermia, defined as a drop in core body temperature below 35 °C (95 °F), initially impairs the body's thermoregulatory capacity. More significant impairment occurs with increasing severity of hypothermia; severe hypothermia is defined as core body temperature dropping below 28 °C (82.4 °F)
Core body temperature below 29.4 °C (85°F) impairs the hypothalamus's ability to regulate body temperature.
Part of the reason for this diminished regulation is that the rate of chemical heat production in each cell is depressed almost 2-fold for each 10°F decrease in body temperature.
Extreme symptoms indicative of severe hypothermia include mental status changes, slurred speech, unconsciousness, ventricular arrhythmias, and gross motor skill impairment. The end-stage presentation includes central nervous system (CNS) depression (coma), which ultimately suppresses all thermoregulatory function of the body (including the ability to "shiver").
Heat Illness Spectrum
Cellular Level
Heat Production
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Heat production is a function of metabolism.
Basal metabolism
Muscle activity, such as shivering and muscle contractions
Extra metabolism caused by the effect of sympathetic stimulation and norepinephrine, epinephrine on the cells
Extra metabolism caused by increased chemical activity in the cells, especially when the cell temperature increases
Extra metabolism caused by thyroid hormone and, to a lesser extent, testosterone and growth hormone in the cells
Extra metabolism is needed for the digestion, absorption, and storage of food
Most of the heat produced in the body is generated in the liver, brain, heart, and skeletal muscles during exercise.
Heat Loss
The rate at which heat is lost is determined almost entirely by:
Development
Brown Fat
Brown fat has an abundant supply of sympathetic nerves that release norepinephrine, which stimulates mitochondrial uncoupling protein (UCP, also called thermogenin) expression and increases thermogenesis. Uncoupled oxidation occurs in this type of fat because it contains many specialized mitochondria. In animals, the amount of brown fat in the tissues is directly proportional to the degree of chemical thermogenesis that occurs.
Infants have a small amount of brown fat in the interscapular space. Chemical thermogenesis can increase heat production in human neonates by 100%.
There is nearly no brown fat in adult humans. In adults, it is rare to increase heat production by more than 10%-15% through chemical thermogenesis.
Organ Systems Involved
Anterior Hypothalamic-Preoptic Area in Thermostatic Detection of Temperature
The anterior hypothalamic-preoptic area contains cold- and heat-sensitive neurons—central thermoreceptors.
Temperature-sensory signals from the central anterior hypothalamic-preoptic area are transmitted to the posterior hypothalamic area.
When the preoptic area is heated, the skin throughout the body immediately breaks out in profuse sweating, and the blood vessels over the entire body surface dilate.
Also, any excess body heat production becomes inhibited.
Posterior Hypothalamus Integrates the Peripheral and Central Temperature Sensory Signals
Temperature-sensory signals from the central anterior hypothalamic-preoptic area are transmitted to the posterior hypothalamic area.
Temperature-sensory signals from peripheral thermoreceptors are transmitted to the posterior hypothalamus.
These signals are integrated to regulate the body's heat-conserving and heat-producing reactions.
Detection of Temperature by Receptors in the Deep Body Tissues
Deep body temperature receptors are in the abdominal viscera, the spinal cord, around the great veins in the thorax, and the upper abdomen.
Deep thermosensitive receptors, like skin temperature receptors, mainly detect cold rather than warmth.
Both the deep body receptors and the skin receptors are probably involved in preventing hypothermia, that is, low body temperature.
Detection of Temperature by Receptors in the Skin
The skin has both warmth and cold receptors.
The warmth receptors in the skin are much less than the cold receptors. Therefore, peripheral temperature detection primarily concerns detecting cool and cold temperatures.
When the skin is chilled over the entire body, immediate reflexes are invoked, including sweating inhibition, shivering, and skin vasoconstriction, to reduce heat loss.
Function
Insulator System of the Body
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The heat insulator of the body is composed of the skin, the subcutaneous tissues, and the adipose tissues.
The subcutaneous fat is important because it conducts heat poorly.
Insulator properties of the female body are better than those of the male body.
Blood Flow to the Skin from the Body Core Imparts Heat Transfer
Blood vessels are profusely distributed under the skin.
The presence of a continuous venous plexus that is supplied by an inflow of blood from the skin capillaries is significant for thermoregulation.
Also, blood is supplied to the plexus directly from small arteries via highly muscular arteriovenous anastomoses in the hands, feet, and ears.
Effect of the environmental temperature on heat conductance from the body core to the skin
The skin is an efficient, controlled “heat radiator” system.
The flow of blood to the skin is the most effective mechanism for heat transfer from the body's core to the environment.
Introductory Physics: The Loss of Heat from the Skin Surface
Heat is dissipated by radiation, evaporation, and conduction.
Evaporation and air conduction are accelerated by convection.
Evaporation Is a Necessary Cooling Mechanism at High Air Temperatures
When the environment is warmer than the skin, the body gains heat by both conduction and radiation.
So, evaporation is the only way the body can rid itself of heat under these conditions.
Therefore, anything that prevents adequate evaporation when the surrounding temperature is higher than the skin temperature will result in a rise in the internal body temperature.
Inadequate perspiration occurs in patients with congenital absence or dysfunction of sweat glands (Ectodermal dysplasia). These patients are in danger of overheating in hot environments.
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Clothing Reduces Conductive and Convective Heat Loss
Clothing entraps air next to the skin, thereby increasing the thickness of the so-called private zone of air adjacent to the skin and also decreasing the flow of convection air currents.
When clothing becomes wet, its ability to maintain body temperature is almost completely lost, because water's high conductivity increases heat transfer through the fabric.
Mechanism
Temperature-Decreasing Mechanisms
Inhibition of the sympathetic centers in the posterior hypothalamus (that control blood vessel tone) causes vasodilation of skin blood vessels.
When the body's core temperature rises above the critical level of 37 °C (98.6 °F), sweating increases the rate of heat loss.
Shivering and chemical thermogenesis are strongly inhibited.
Temperature-Increasing Mechanisms
Stimulation of the posterior hypothalamic sympathetic centers causes vasoconstriction of skin blood vessels.
Also, piloerection will occur, meaning hair “standing on end.” This mechanism is not important in humans.
Increase in thermogenesis by promoting shivering, sympathetic excitation of heat production, and thyroxine secretion
Related Testing
SetPoint for Temperature Control
Artificial Hypothermia
The temperature of a person can be decreased by first administering a strong sedative to depress the hypothalamic temperature controller's reactivity, then cooling the patient with ice or cooling blankets.
The temperature can then be maintained below 90 degrees F for an extended period by a continual sprinkling of cool alcohol or water on the body.
Such artificial cooling has been practiced during heart surgery so that the heart can be artificially stopped for many minutes at a time.
Cooling to this extent does not cause tissue damage. Still, it slows the heart rate and significantly depresses cellular metabolism, allowing the body's cells to survive for 1 hour without blood flow during the surgical procedure.
Pathophysiology
Sweating and Its Regulation by the Autonomic Nervous System
Stimulation of the anterior hypothalamus, a preoptic area in the brain, by excess heat causes sweating.
The sweat glands are innervated by cholinergic nerve fibers that secrete acetylcholine, which runs alongside the sympathetic nerves along with the adrenergic fibers.
Sweat glands can also be stimulated by norepinephrine or epinephrine, which is important during periods of intense physical activity (such as work or exercise).
Mechanism of Sweat Secretion
The sweat gland consists of two parts: the duct that passes through the skin and reabsorbs salt and water, and the deep subdermal coiled gland that secretes the sweat.
Sweat gland secretes a fluid called the precursor secretion; the concentrations of constituents in the fluid are then modified in the duct.
Precursor Secretion
Cholinergic sympathetic nerve fibers stimulate the secretion.
The composition of the precursor secretion is similar to the plasma, except that it does not contain plasma proteins.
The concentration of sodium is about 142 mEq/L, and that of chloride is about 104 mEq/L.
Reabsorption
When sweat glands are weakly stimulated, the precursor fluid passes slowly through the duct, and the concentration of sodium and chloride ions falls to as low as 5 mEq/L.
Reabsorption of sodium and chloride reduces the osmotic pressure of the sweat fluid, leading to water reabsorption. In contrast, other constituents, such as urea, lactic acid, and potassium, become concentrated in the sweat.
When the sweat glands are strongly stimulated, a large amount of precursor fluid forms, and the duct reabsorbs only about half the sodium chloride secreted (resulting in 50 to 60 mEq/L sodium and chloride ions).
Strongly stimulated sweat flows through the glandular tubules so rapidly that little of the water is reabsorbed. In these conditions, the other dissolved constituents of sweat (urea, lactic acid, and potassium) are only moderately increased in concentration compared to plasma.
Hypothalamic Stimulation of Shivering
The primary motor center for shivering is in the dorsomedial portion of the posterior hypothalamus.
Shivering is normally inhibited by signals from the heat center in the anterior hypothalamic-preoptic area, but is excited by cold signals from the spinal cord and skin.
Sympathetic "Chemical" Excitation of Heat Production
An increase in circulating epinephrine and norepinephrine in the blood rapidly increases the rate of cellular metabolism
This effect is called chemical thermogenesis, or non-shivering thermogenesis
No shivering thermogenesis results from the ability of norepinephrine and epinephrine to "uncouple oxidative phosphorylation."
Clinical Significance
Heat Stroke
The ambient conditions significantly impact the body's ability to release heat into the surrounding environment
Clinicians utilize various environmental heat stress indices (such as the Wet Bulb Globe Temperature [WBGT]) to predict the current hazardous potential of the surrounding environment
WBGT combines the ambient temperature, humidity, wind speed, and solar radiation into a calculated value used to estimate the current (or anticipated) risk of heat-related illness and injury
The relative humidity is the most significant contributing factor to the overall heat index score
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Risk factors for heatstroke include:
Obesity
Dehydration (including predisposing risk factors such as illnesses and diarrhea)
History of prior heat illness
Poor physical conditioning status (sedentary lifestyle)
Sleep deprivation
Sweat gland dysfunction
Medications include:
Anticholinergics
Antihistamines
Stimulants
ACE inhibitors
Diuretics
If the air is 100% humidified or if the body is in water, body temperature rises any time the environmental temperature rises above 34.4 C (94 F).
[10]If the air is dry and sufficient convection air currents are flowing, a person can withstand many hours of air temperature at 54.4 C (130 F).
If the person is doing heavy work, the critical environmental temperature above which heatstroke is likely to occur may be as low as 29.4 C to 32.2 C (85 F to 90 F).
When the temperature of the body rises beyond a critical temperature into the range of 40.6 C to 42.2 C (105 F to 108 F), heatstroke is likely to develop.
Hyperpyrexia is exceedingly damaging to the body tissues, especially the brain, and even a few minutes of very high body temperature can sometimes be fatal.
Treatment for Heat Stroke
After following the standard ATLS resuscitation protocol (airway, breathing, circulation), the next step is to cool the body to reduce core body temperature rapidly. On-site whole-body cooling units should be used. As long as the patient is stable, the rapid cooling protocol takes priority even before starting transportation to the local emergency department.[3][9]
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Disclosure: Hani Yousef declares no relevant financial relationships with ineligible companies.
Disclosure: Edris Ramezanpour Ahangar declares no relevant financial relationships with ineligible companies.
Disclosure: Matthew Varacallo declares no relevant financial relationships with ineligible companies.
