Normal Body Temperature

There are 3 major sources of heat within the body:

1. Cellular respiration.
2. Muscular activity.
3. Ingestion of food.

The normal range of body temperature varies based on the person, age, activity, and time of day. The generally accepted ranges are between 97º to 99º with the average generally accepted temperature being 98.6º F (37º C). Normal variations among human beings follow a bell-curve distribution.

The normal daily temperature varies typically 0.9°F (0.5°C).

Normal body temperature varies over the course of the day, controlled in the thermoregulatory center located in the hypothalamus. The body is normally able to maintain a fairly steady temperature because the hypothalamic thermoregulatory center balances the excess heat production (derived from metabolic activity in muscle and the liver) with heat dissipation (from the skin and lungs).

(With environmental challenges, however, humans are unable to maintain the narrow range of body temperature without the aid of clothing and protective environments.)

Heat is lost from the body by

• radiation, sending out energy, such as heat, in waves into space;
• conduction, the transfer of heat from one material to another through direct contact;
• convection, the carrying away of heat by air water and other liquids;
• evaporation, and
• respiration.

ORAL TEMPERATURE: The maximum normal oral temperature at 6 AM is 98.9°F (37.2°C), and the maximum level at 4 PM is 99.9°F (37.7°C); therefore, a morning reading > 98.9°F (37.2°C) or an afternoon temperature of > 99.9°F (37.7°C) would be considered a fever.

RECTAL TEMPERATURE: Generally 1.0°F (0.6°C) higher than oral readings.

Oral readings are lower probably because of mouth breathing, which can make a difference in patients with respiratory infections and rapid breathing.

CORE BODY TEMPERATURE: Also called core temperature, it is the operating temperature of an organism, specifically in deep structures. It is this core temperature which affects management via the hypothalamus. Core temperature is normally maintained within a narrow range so that essential enzymatic reactions can occur. Compatibility with life has a narrow temperature range, so significant core temperature elevations (hyperthermia) or depressions (hypothermia) that are prolonged is life-threatening.

Generally, oral, rectal, gut, and core body temperatures, although slightly different, are well-correlated, with oral temperature being the lowest of the four.

The temperature in the lower esophagus and the tympanic membrane are close to core temperature. Oral temperatures are generally about 0.7 - 1.0°F (0.4 - 0.55°C) lower than rectal temperatures.

Rectal Temp - 1°F = Oral Temp.

 

Terms: 

• HYPOTHERMIA: Core body temp < 94ºF indicates hypothermia,
• HYPERTHERMIA: 101-104ºF indicates hyperthermia, and
• HYPERPYREXIA: a temp between 104-107ºF is called hyperpyrexia.

98-99/100 ºF is generally normal.

Women, in the second half of their menstrual cycle (after ovulation) generally have a 1/2-1° rise (due to the pyrogenic properties of progesterone), which can be used to determine when ovulation occurred in infertility patients. 

 

The Hypothalamus and How Body Heat is Lost or Generated

ANTERIOR HYPOTHALAMUS: The temperature regulatory center is in the anterior hypothalamus, which controls several mechanisms to maintain a constant body temperature. All temperature control in the body is maintained by neural feedback mechanisms that operate through the hypothalamus.

The hypothalamus functions as the thermostat in the body, raising and lowering temperature to maintain a set-point temperature, causing an increase or decrease in body temperature back to this set-point when necessary, using

• thyroxine,
• sympathetic stimulation,
• respiration,
• skeletal muscles,
• the liver, and
• food.

Hot Enough For You?

With hyperthermia, external heat transfer mechanisms are via:

• radiation (passive transfer from a warm area --you-- to a colder area--the air around you),
• vasodilation (via thyroxin/T4),
• breathing (about 20% of our body heat is lost due to exhaling), and
• evaporation (perspiration).

THYROXINE (T4): thyroid hormone that controls blood vessel dilation through sympathetic stimulation. The more dilated vessels are, the more heat can escape from the skin.

Normal respiration exhales warmed air, thus removing some heat from the body, and the act of breathing in cold air actually reduces body temperature.

The active process of breaking down food also causes chemical reactions within the body that raise the body temperature and can also affect the body temperature based on the temperature of what is ingested.

Is It Me, or Is It  Cold in Here?

With hypothermia, heat is conserved via:

• vasoconstriction;
• cessation of sweating;
• shivering to increase heat production in the muscles; skeletal muscles start to shiver involuntarily to increase heat production when the core body temperature starts to lower;
• secretion of norepinephrine, epinephrine, and thyroxine.

The metabolic activity in the liver contributes substantially to heat production within the body.

 

Metabolic Terms

METABOLISM: the chemical processes that occur within a living organism in order to maintain life.

CATABOLISM: the breakdown of complex molecules in living organisms to form simpler ones, together with the release of energy, and is a destructive form of metabolism.

ANABOLISM: the synthesis or creation of complex molecules in living organisms from simpler ones together with the storage of energy, and is a constructive form of metabolism.

BASAL METABOLIC RATE: the rate at which the body uses energy while at complete rest to keep vital functions going, such as breathing and keeping warm. (It is associated with basal metabolic temperature)

KILOCALORIE: unit of energy equal to 1,000 calories and also represents the amount of energy required to raise the temperature of a liter of water 1.0° C at sea level.

CALORIE: the unit of heat energy used to measure energy in food.

 

Physiology of Fever

SICKNESS RESPONSE: Fever is part of the sickness response. In humans, the suite of responses that comprise sickness behavior is manifested as:

• Fever
• Malaise
• Anorexia
• Depression
• Listlessness (fatigue, sleepiness, and loss of interest in social interactions).

The sickness response is helpful in recovery. The decreased activity that accompanies depression and fatigue helps to conserve energy for those processes focused on healing. The anorexia response is part of shunting blood from the gastrointestinal tract to other areas more essential for recovery. Like heat in chemistry, fever makes biochemical reactions go more quickly, acting as a catalyst. If someone who's sick wants to get better, he'll get better more quickly if all of the healing reactions go faster, which is what heat as a catalyst does.

The sickness response, especially the fever response, is mediated by pro-inflammatory cytokines, which are produced by cells in the immune system--T-cells and macrophages in the circulation and by glial cells in the central nervous system.

Inflammatory Cytokines: small proteins that initiate the inflammatory cascade that stimulates the sickness response. There are also mechanisms to protectively curtail the inflammation via anti-inflammatory cytokines, and the balance between the two makes recovery possible.

PYROGENS: Fever is caused by the release of cytokines designated as pyrogens, molecules that are part of the immune system, produced by immune cells.

"Prostaglandin E2": This cytokine is called a "pyrogen." Prostaglandin E2 acts on the hypothalamus which generates the systemic response back to the rest of the body, raising the temperature in various ways.

ADVANTAGES OF FEVER:

• increased mobility of leukocytes,
• enhanced recycling of leukocytes through phagocytosis,
• decreased effects of endotoxins and
• increased proliferation of T cells.

DISADVANTAGES OF FEVER:

• dehydration due to an increase in fluid consumption,
• loss of vitamins and minerals,
• the possibility of seizures, and the
• potential for vital organs to shut down.

When the hypothalamic set-point is reset downward, the heat loss processes are accelerated through vasodilation and sweating. The resetting of the set-point downward can be due to either a reduction in the concentration of pyrogens or the use of antipyretics.
 

Common Mistakes with Antipyretic Administration:

Underdosing: Parents sometimes give the same dose of acetaminophen (APAP) or ibuprofen that they administered six months ago. As children grow, their weight increases, requiring adjusted dosages. Underdosing can lead to ineffective fever management.

Inappropriate Insulation: Children often seek warmth when they have a fever, leading them to burrow into blankets. While it is important to keep the child comfortable, over-insulation can prevent heat from dissipating and worsen the fever.

Timing of Medication: Parents may not give the medication sufficient time to work before concluding it is ineffective. It is important to follow recommended intervals and allow time for the medication to take effect.

Use of Suppositories: For children who are vomiting, acetaminophen suppositories are a good option. However, parents should be aware that rectal stimulation can sometimes provoke a bowel movement, which may expel the suppository before it is absorbed.

Tips for Proper Antipyretic Use:

• Always base the dosage of antipyretics on the child's current weight and age. Consult dosage charts or a healthcare provider to ensure the correct amount.
• Maintain a balance between keeping the child warm and avoiding excessive insulation. Light clothing and blankets are usually sufficient.
• Monitor the child’s response to the medication and follow the recommended dosing schedule.
• Be cautious with rectal suppositories and understand that they may not always stay in place, especially if the child has a bowel movement soon after insertion.

 

FEVER and SETPOINT: In a fever, as part of the sickness response, the set point may be raised from 97º to 99°F, that is, from normothermia to febrile levels.

Elevated prostaglandin E2 (PGE2) levels in the hypothalamus are the trigger for raising the set point. Once the hypothalamic set-point is raised, this activates neurons in the vasomotor center to commence vasoconstriction and warm-sensing neurons to slow their firing rate and increase heat production in the periphery. This vasoconstriction can produce a noticeable cold sensation in the hands and feet as blood is shunted away from the periphery to the internal organs, essentially decreasing heat loss from the skin, and the patient feels cold, although the core temperature is rising. In most fevers, this is sufficient to raise core body temperature 1 or even 2°C.