What is normal body temperature

Normal body temperature varies by person, age, activity, and time of day ¹. The average normal body temperature (core or internal temperature) is generally accepted as 98.6 °Fahrenheit (37 °Celsius) ². Babies and young children have a higher body temperature than older children. This is because their body surface area is larger in relation to their body weight. Their metabolism is more active too. Newborns usually have an average body temperature of 99.5 °F (37.5°C) ³. Some studies have shown that the “normal” body temperature can have a wide range, from 97 °Fahrenheit (36.1 °Celsius) to 99 °F (37.2 °C). Your body temperature normally changes throughout the day ¹.

• During strenuous exercise, body temperature can rise temporarily to as high as 104 °F (40 °C).
• When the body is exposed to extreme cold, the temperature can fall below 98 °F (35.6 °C).
• An unclothed person can be exposed to temperatures as low as 55 °F (12.8 °C) or as high as 130 °F (54.4 °C) in dry air and still maintain almost constant core temperature.
• A temperature over 100.4 °F (38 °C) most often means you have a fever caused by an infection or illness.

Slight changes in core body temperature occur every day, depending upon variables such as circadian rhythm and menstrual periods; but otherwise, the temperature is tightly controlled. When a person is unable to regulate his or her body temperature, various pathologies may occur. The human body has four different methods for keeping itself at its core temperature: vaporization, radiation, convection, and conduction. To keep the body functioning, it must be at its ideal temperature, and for this to happen, physical factors must be sufficient. This includes having enough intravascular volume and cardiovascular function; the body must be able to transport the rising internal heat to its surface for release. The reason that elderly people are at higher risk for disorders of thermoregulation is that they, as a whole, have less intravascular volume and decreased cardiac function.

Core temperature is most accurately measured via rectal probe thermometer ². This is the temperature at which the human body’s systems work together at their optimum, which is the reason the body has such tightly regulated mechanisms. Thermoregulation is crucial to human life. Thermoregulation, by definition, is a mechanism by which mammals maintain body temperature by tightly controlled self-regulation, no matter the temperature of their surroundings. Temperature regulation is a type of homeostasis, which is a process that biological systems use to preserve a stable internal state to survive. Ectotherms are animals that depend on their external environment for their body heat, and endotherms are animals that use thermoregulation to maintain a somewhat consistent internal body temperature to survive, even when their external environment changes. Humans and other mammals and birds are endotherms. Without thermoregulation, the human body would not be able to adequately function and, inevitably, will expire.

Skin Temperature

In contrast to the core temperature, the skin temperature (shell), falls and rises with the temperature of the surroundings.

What causes a fever?

People get a fever when their brain sets the body temperature higher than normal. This may happen as a reaction to germs such as viruses or bacteria, but it can also happen as a reaction to substances that are made by the body, such as prostaglandins. Our body produces prostaglandins to fight off germs.

A body temperature of 100.4°F (38°C) or more is considered to be a fever ³. Temperatures above 103.1°F (39.5°C) are considered to be a high fever, and very high fever is defined as any temperature above 105.8°F (41°C). A temperature between 99.5 °F (37.5°C) and 100.4 °F (38°C) is an elevated body temperature.

The regulation of body temperature doesn’t always work perfectly in younger children. Compared to older children and adults, they also sweat less when it is warm, and it takes longer for them to start sweating. That is why they are more likely to react with a fever.

Heat Production

Heat production is determined by metabolism:

• Basal metabolism
• Muscle activity, by 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 on the cells
• Extra metabolism needed for digestion, absorption, and storage of food
• Most of the heat produced in the body is generated in the liver, brain, heart, and in the skeletal muscles during exercise.

Heat Loss

The rate at which heat is lost is determined almost entirely by:

• How rapidly heat is transferred from the skin to the surroundings
• How rapidly heat is conducted from where it is produced in the body core to the skin

Brown Fat

• Brown fat is richly supplied with sympathetic nerves that release norepinephrine, which stimulates tissue expression of mitochondrial uncoupling protein (UCP, also called thermogenin) and increases thermogenesis. Uncoupled oxidation occurs in this type of fat because it contains large numbers of special mitochondria. In animals, the amount of brown fat in the tissues is directly proportional to the degree of chemical thermogenesis that occurs.
• There is a small amount of brown fat in the interscapular space in infants. Chemical thermogenesis can increase the rate of heat production in human neonates, by one hundred percent.
• There is nearly no brown fat in adult humans. In adults, it is rare to increase the rate of heat production by more than 10% to 15% by chemical thermogenesis.

Body temperature regulation organ systems involved

The brain, or more specifically the hypothalamus, controls thermoregulation. If the hypothalamus senses external temperatures growing too hot or too cold, it will automatically send signals to the skin, glands, muscles, and organs. For example, when the body is in a very hot external environment, or simply undergoing high activity levels such as exercise, it’s temperature will rise, causing the hypothalamus to send signals to the cells of the skin that produce sweat. Sweating is the body’s approach to cooling itself down. As the body’s temperature rises, sweat is expelled, the muscles relax, and body hair lies flat against the skin. These are all ways to release heat and therefore lower the temperature of the body. In contrast, when the body experiences a cold environment, the skeletal muscles tense up leading to the shivering reflex, and the arrector pili muscles, a type of smooth muscle, raise the bodily hair follicles where they are attached. These processes, in turn, create warmth and trap heat, respectively.

Anterior Hypothalamic-Preoptic Area in Thermostatic Detection of Temperature

• The anterior hypothalamic-preoptic area contains cold and heat -sensitive neurons-central thermoreceptors.
• The temperature sensory signals from the central anterior hypothalamic-preoptic area are transmitted into the posterior hypothalamic area.
• When the preoptic area is heated, the skin all over the body immediately breaks out in a profuse sweat, and the blood vessels over the whole-body surface become dilated.
• Also, any excess body heat production is inhibited.

Posterior Hypothalamus Integrates the Peripheral and Central Temperature Sensory Signals

• The temperature sensory signals from the central anterior hypothalamic-preoptic area are transmitted into the posterior hypothalamic area.
• The temperature sensory signals from the peripheral thermoreceptors are transmitted to the posterior hypothalamus.
• These signals are integrated to control the heat-conserving and heat-producing reactions of the body.

Detection of Temperature by Receptors in the Deep Body Tissues

• Deep body temperature receptors are in the abdominal viscera, the spinal cord, around or the great veins in the thorax, and upper abdomen
• The deep thermos-sensitive receptors, like the skin temperature receptors, detect mainly cold rather than warmth.
• It is probable that both the deep body receptors and the skin receptors are concerned with preventing hypothermia, that is, preventing low body temperature.

Detection of Temperature by Receptors in the Skin

• The skin has both warmth and cold receptors.
• The warmth receptors at the skin are much less than cold receptors. Therefore, peripheral detection of temperature mainly concerns detecting cool and cold temperatures.
• When the skin is chilled over the entire body, immediate reflexes are invoked that include sweating inhibition, shivering, skin vasoconstriction to diminish the loss of body heat.

Temperature-Decreasing Mechanisms

• Inhibition of the sympathetic centers in the posterior hypothalamus (that control blood vessel tone), cause vasodilation of skin blood vessels.
• When the body core temperature rises above the critical level of 98.6 °F (37 °C), there is an increase in the rate of heat loss by sweating.
• 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 take place, which means hairs “standing on end.” This mechanism is not important in humans.
• Increase in thermogenesis by promoting shivering, sympathetic excitation of heat production, and thyroxine secretion

Thermoregulatory Impairment

Hypothermia

• Hypothermia, defined as a drop in core body temperature below 95 °F (35 °C), results in initial/mild impairment in the body’s thermoregulatory capacity ⁴. Greater impairment occurs with increasing severity of hypothermia; severe hypothermia is defined as core body temperature dropping below 82.4 °F (28 °C)
• Core body temperature below 85 °F (29.4 °C) impairs the ability of the hypothalamus to regulate body temperature is lost
• 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. 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

• Excessive core body temperature presents along with a clinical spectrum, with heat stroke presenting as an emergency clinical condition defined by a core body temperature exceeding 104 °F (40 °C)
• Other conditions along the heat illness spectrum include:
  • Heat edema
  • Heat syncope
  • Heat-associated cramping
  • Heat exhaustion

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