Body Temperature

Dr. Shafali Singh

Learning Objectives

• List the mechanisms by which heat is produced in and lost from the body.

• Comment on the differences in temperature in the hypothalamus, rectum, oral cavity, and skin.

• List the temperature-regulating mechanisms, and describe the way in which they are integrated under hypothalamic control to maintain normal body temperature.

• Applied

Humans maintain a normal body temperature at a set point of 37° C (98.6° F)

• Core Temperature: The temperature of the deep tissues of the body—the “core” of the body—remains very constant, within ±1°F (±0.6°C)

• Range from less than 97°F (36°C) to over 99.5°F (37.5°C). • The average normal core temperature is generally

considered to be between 98.0° and 98.6°F when measured orally and about 1°F higher when measured rectally

• Skin temperature: in contrast to the core temperature, rises and falls with the temperature of the surroundings.

Body Temperature Is Controlled by Balancing Heat Production Against Heat Loss

• When the rate of heat production in the body is greater than the rate at which heat is being lost, heat builds up in the body and the body temperature rises.

• Conversely, when heat loss is greater, both body heat and body temperature decrease.

Heat Production

Heat Production(1) basal rate of metabolism of all the cells of the body; extra rate of metabolism2) caused by muscle activity 3)caused by the effect of thyroxine(and, to a less extent, growth hormone and testosterone) on the cells; 4) caused by the effect of epinephrine, nor-epinephrine, and sympathetic stimulation on the cells;5) caused by increased chemical activity in the cells themselves(6) extra metabolism needed for digestion, absorption, and storage of food( thermogenic effect of food)

• Thyroid Hormones- are thermogenic: Their actions on target tissues result in heat production.

• Major actions of thyroid hormone are stimulation of Na+-K+ ATPase, increased O2

consumption, increased metabolic rate, and increased heat production.

Heat Loss

Heat is transferred from the deeper organs( liver, brain, and heart, and in the skeletal muscles during exercise and tissues) to the skin, where it is lost to the air and other surroundings. Therefore, the rate at which heat is lost is determined almost entirely by two factors: (1) how rapidly heat can be conducted from where it is pro-duced in the body core to the skin and (2) how rapidly heat can then be transferred from the skin to the surroundings.

Heat Loss

Body heat is lost by: • Radiation and conduction -70 %• Vaporization of sweat- 27 %• Respiration- 2 %• Urination and defecation -1 %

Q. When an individual is naked in a room in which the air temperature is 21°C and the humidity 80%,the greatest amount of

heat is lost by-

a) Elevated metabolismb) Respirationc) Urinationd) Vaporizatione) Radiation and conduction

Effect of high and low atmospheric temperatures of several hours’ duration on the internal body “core” temperature

Regulation of BodyTemperature—Role of the

Hypothalamus

• The temperature of the body is regulated almost entirely by nervous feedback mechanisms, and almost all these operate through temperature-regulating centers located in the hypothalamus

Role of the Anterior Hypothalamic-Preoptic Area in Thermostatic Detection of Temperature

• Anterior hypothalamic-preoptic area has been found to contain large numbers of heat-sensitive neurons as well as about one third as many cold-sensitive neuron

• The heat-sensitive neurons increase their firing rate 2- to 10-fold in response to a 10°C increase in body temperature.

• This results in profuse sweating (skin blood vessels over the entire body become greatly dilated).

sweat glands are innervated by cholinergic nerve fibers (fibers that secrete acetylcholine but that run in the sympathetic nerves along with the adrenergic fibers)

Posterior Hypothalamus Integrates the Centraland Peripheral Temperature Sensory Signals

• The posterior hypothalamus integrate body temperature information from sensory receptors (primarily cold receptors) in the skin, deep tissues, spinal cord, extrahypothalamic portions of the brain, and the hypothalamus itself( anterior hypothalamic-preoptic area ).

• Each of these five inputs contributes about 20% of the information that is integrated.

• There are threshold core temperatures for each of the main temperature-regulating responses and when the threshold is reached the response begins. The threshold is 37 °C for sweating and vasodilation, 36.8 °C for vasoconstriction, 36 °C for nonshivering thermogenesis, and 35.5 °C for shivering.

• The reflex responses activated by cold are controlled from the posterior hypothalamus. Posterior hypothalamic stimulation causes shivering

• Those activated by warmth are controlled primarily from the anterior hypothalamus .Stimulation of the anterior hypothalamus causes cutaneous vasodilation and sweating

• If core temperature is below the set-point temperature,then heat-generating and heat-retaining mechanisms are activated.

• If core temperature is above the set-point temperature,then heat-dissipating mechanisms are activated

Temperature-Regulating Mechanisms

Mechanisms activated by cold • Shivering • Hunger • Increased voluntary activity • Increased secretion of

norepinephrine and epinephrine

• Decreased heat loss • Cutaneous vasoconstriction • Curling up • Horripilation

Mechanisms activated by heat • Increased heat loss • Cutaneous vasodilation • Sweating • Increased respiration • Decreased heat production

(shivering and chemical thermogenesis, are strongly inhibited.)

• Anorexia • Apathy and inertia, fanning

Q.If core body temperature rises above normal, which of the following would occur to cool the body?(1) dilation of vessels in the skin, (2) increased

radiation and conduction of heat to the environ-ment, (3) increased metabolic rate, (4) evaporation of perspiration,(5) increased secretion of thyroid hormones.

• (a) 3, 4, and 5 (b) 1, 2, and 4 (c) 1, 2, and 5• (d) 1, 2, 3, 4 and 5 (e) 1, 2, 4, and 5

Concept of a “Set-Point” for Temperature Control

• There is a critical body core temperature of about 37.1°C (98.8°F)

• This crucial temperature level is called the “set-point” of the temperature control mechanism.

• That is, all the temperature control mechanisms continually attempt to bring the body temperature back to this set-point level.

Feedback Gain for Body Temperature Control

• Feedback gain is a measure of the effectiveness of a control system.

• The feed-back gain of the temperature control system is equal to the ratio of the change in environmental temperature to the change in body core temperature minus 1.0 .

• Experiments have shown that the body temperature of humans changes about 1°C for each 25° to 30°C change in environmental temperature

Abnormalities of BodyTemperature Regulation

Fever

• Means a body temperature above the usual range of normal

• Caused by abnormalities in the brain itself or by toxic substances that affect the temperature-regulating centers

Pathophysiology of fever

Endogenous pyrogens ,IL-1

Resetting the Hypothalamic Temperature-Regulating Center in Febrile Diseases

• When the set-point of the hypothalamic temperature-regulating center becomes higher than normal, all the mechanisms for raising the body temperature are brought into play, including

• Heat conservation and increased heat production

Effects of changing the set-point of the hypothalamic temperaturecontroller.

Fever, within limits, is beneficial because it:-inhibits the growth of many micro-organisms.-increases formation of neutrophils in bone marrow-increases antibody production, thus increasing immunity. -Helpful in, anthrax, pneumococcal pneumonia, leprosy, and various fungal, rickettsial, and viral diseases.-slows growth of some tumors.

Very high temperatures are harmful: -Temperature above 41 0C for prolonged periods leads to some permanent brain damage. -Temperature above 43 0C, death results.

Treatment

• The commonest antipyretic is aspirin(acetyl-salicylic acid).

• It inhibits the enzyme cyclooxygenase, thereby inhibiting prostaglandin secretion.

• It acts directly on hypothalamus.

Heat exhaustion

• Occur as a consequence of the body's responses to elevated environmental temperature.

• Normally, the response to increased temperature includes vasodilation and sweating in order to dissipate heat.

• However, if the sweating is excessive, it can result in decreased ECF volume, decreased blood volume, decreased arterial pressure, and fainting.

Heatstroke

• When the body temperature rises beyond a critical temperature, into the range of 105° to 108°F, the person is likely to develop heatstroke.

• Body temperature increases to the point of tissue damage.

• Is characterized by high core temperature and the development of serious neurological disturbances with a loss of consciousness and, frequently, convulsions.

• Heat stroke occurs in two forms, classical and exertional. • In the classical form, the primary factor is environmental heat stress that

overwhelms an impaired thermoregulatory system, and most patients have preexisting chronic disease.

• In exertional heatstroke, the primary factor is high metabolic heat production. Patients with exertional heatstroke tend to be younger and more physically fit (typically, soldiers and athletes) than patients with the classical form. Rhabdomyolysis, hepatic and renal injury, and disturbances of blood clotting are frequent accompaniments of exertional heatstroke.

• The traditional diagnostic criteria of heatstroke—coma, hot dry skin, and rectal temperature above 41.3C (106F)—are characteristic of the classical form; however, patients with exertional heatstroke may have somewhat lower rectal temperatures and often sweat profusely

malignant hyperthermia

• Various mutations of the gene coding for the ryanodine receptor , lead to excess Ca2+ release during muscle contraction triggered by stress.

• This in turn leads to contractures of the muscles, increased muscle metabolism, and a great increase in heat production in muscle.

Periodic fevers also occur in humans with mutations in the gene

• for pyrin, a protein found in neutrophils;• for mevalonate kinase, an enzyme involved in

cholesterol synthesis;• for the type 1 TNF receptor, which is involved

in inflammatory responses

Acclimatization to Heat.

• Soldiers on duty in the tropics and miners.• A person exposed to heat for several hours each day while

performing a reasonably heavy workload will develop increased tolerance to hot and humid conditions in 1 to 3 weeks.

• approximately twofold increase in the maximum rate of sweating, an increase in plasma volume, and diminished loss of salt in the sweat and urine to almost none

• the last two effects result from increased secretion of aldosterone by the adrenal glands.

Hypothermia

• It is a lowering of core body temperature to 35°C(95°F) or below.

• Causes of hypothermia include an overwhelming cold stress(immersion in icy water), metabolic diseases (hypoglycemia, adrenal in-sufficiency, or hypothyroidism), drugs (alcohol, antidepressants, sedatives, or tranquilizers), burns, and malnutrition.

Hypothermia

• Hypothermia is characterized by : sensation of cold, shivering, confusion, vasoconstriction, muscle rigidity, bradycardia, acidosis, hypoventilation, hypotension, loss of spontaneous movement, coma, and death (usually caused by cardiac arrhythmias).

• The elderly are at greater risk

Exposure of the Body to Extreme Cold

• Unless treated immediately, a person exposed to ice water for 20 to 30 minutes ordinarily dies because of heart standstill or heart fibrillation.

• There is loss of Temperature Regulation at Low Temperatures. –below about 85°F

the reason for this diminished temperature regulation is that the rate of chemical heat production in each cell is depressed. Also, sleepiness develops (later followed by coma), which depresses the activity of the central nervous system heat control mechanisms and prevents shivering

Frostbite.

• When the body is exposed to extremely low temperatures, surface areas can freeze; the freezing is called frostbite.

• This occurs especially in the lobes of the ears and in the digits of the hands and feet.

• If the freeze has been sufficient to cause extensive formation of ice crystals in the cells, permanent damage usually results, such as permanent circulatory impairment as well as local tissue damage.

• Often gangrene follows thawing, and the frostbitten areas must be removed surgically.

Artificial Hypothermia. • Induced by administering a strong sedative to depress the reactivity

of the hypothalamic temperature controller and then cooling the person with ice or cooling blankets until the temperature falls.

• The temperature can then be maintained below 90°F for several days to a week or more by continual sprinkling of cool water or alcohol on the body.

• Such artificial cooling has been used during heart surgery so that the heart can be stopped artificially for many minutes at a time.

• Cooling to this extent does not cause tissue damage, but it does slow the heart and greatly depresses cell metabolism, so that the body’s cells can survive 30 minutes to more than 1 hour without blood flow during the surgical procedure..

Q. immediately following the break of fever(while the core temp is falling):a. The set point gradually return to normalb. There will be sweating and peripheral vasodilatationc. There will be shivering and peripheral vasoconstrictiond. The person will feel subjectively cold

Q. Heat stroke is charateristically associated with

a. Profuse sweatingb. Elevated basal metabolic ratec. Hot dry skind. a and b only are correcte. b and c only are correct

The hypothalamic set-point temperature normally averages about 98.6°F. Which of the following factors can alter the set-point level for core temperature control?

Skin temperature Pyrogens Antipyretics ThyroxinA) No Yes Yes NoB) No Yes Yes YesC) Yes Yes Yes YesD) Yes No No YesE) Yes Yes Yes No

Most of the heat loss from an unclothed person at room temperature occurs by which of the following mechanisms?

A) Conduction to airB) Conduction to objectsC) ConvectionD) EvaporationE) Radiation

Which of the following mechanisms causes heat loss from a normal person when the environmental temperature is 106°F and the relative humidity is less than 10%?

A) ConductionB) ConvectionC) EvaporationD) Radiation

A scuba diver explores an underwater lava flow wherethe water temperature is 102°F. Which of the followingprofiles best describes the mechanisms of heat lossthat are effective in this man? Evaporation Radiation Convection

ConductionA) No No No YesB) No No No NoC) Yes Yes No YesD) No Yes No YesE) Yes Yes Yes Yes

A 54-year-old man is admitted to the emergencydepartment after being found lying in his yard near arunning lawnmower on a hot summer day. His bodytemperature is 106°F, blood pressure is normal, andheart rate is 160 beats/min. Which of the following setsof changes is most likely to be present in this man?Sweating Hyperventilation Vasodilation of skinA) No No NoB) No Yes YesC) Yes No YesD) Yes Yes NoE) Yes Yes Yes

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