Body Temperature And Its Regulation

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Body Temperature And Its Regulation

• Normal Body Temperature
• The temperature of the deep tissues of the body
  which is known as the core temperature remains
  almost exactly constant except when a person
  develops a febrile illness.On the other hand the
  skin temperature rises falls with the
  temperature of the surroundings.
• The core body temperature can be measured either
  orally or rectally. The oral temperature is
  normally 0.50C lower than the rectal temp.,

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but is affected by many factors including
ingestion of hot or cold fluids mouth
breathing.The average normal oral temperature
in young adults measured in the morning is 37C
with a range between 36.3 and 37.1 C.
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Physiological variations in body temperature

• Normally, the body temp. undergoes a regular
  circadian fluctuation of about 0.60C being lowest
  in the morning highest in the evening.
• In Woman there is a monthly cycle of temp.
  variation characterized by a rise in basal temp.
  of about 0.50 C at the time of ovulation during
  the second half of the menstrual cycle.A similar
  rise occurs during the first trimester of
  pregnancy.

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3. In children temp. regulation is less precise
and they may normally have a temp. that is 0.50C
above the normal for adults.

• 4. During exercise, excess heat is produced in
  the body and the rectal temp. can normally rise
  to as high as 400C .
• 5. Emotional excitement slightly increases the
  body temp. probably due to unconscious tensing of
  muscles.
• 6. When the metabolic rate is high the body temp.
  is chronically elevated by as much as 0.50C and
  vice verse.
• 7. Constitutional hyperthermia.

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The balance between heat production heat loss

• The body temp. is kept constant when the rate of
  heat production in the body is equal to the rate
  of heat loss.
• Heat is produced in the body by the basal rate of
  metabolism, contraction of skeletal muscles, food
  ingestion and extra metabolism caused by the slow
  but prolonged effect of thyroxin on the cells and
  the rapid but short lived effect of epinephrine,
  nor epinephrine and sympathetic effects on the
  cells.

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Most of the heat produced in the body is
generated in the organs especially in the liver,
the brain the heart and the skeletal muscles
especially during exercise. Therefore, heat loss
from the body occurs in two steps

• Conduction of heat from the deeper organs and
  tissues to the skin.
• Transfer of heat from the skin to the surrounding.

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A.) Heat Conduction to the Skin

• The skin and the subcutaneous tissues especially
  the fat form a heat insulator system for the
  body. This system maintains the normal core
  temp., although the temp. of the skin may
  approach the temp.of the surrounding.Blood
  vessels penetrate the fatty subcutaneous tissues
  and are distribution beneath the skin. The amount
  of heat reaching the skin from the deep tissues
  depends upon the blood flow into the cutaneous
  blood vessels which is determined by the degree
  of vasoconstriction of the vessels.This
  vasoconstriction in turn is controlled almost
  entirely by the sympathetic nervous system.The
  rate of heat conduction to the skin is known as
  tissue conductance.

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B.) Heat loss from the skin surface

• 1. Radiation It means transfer of heat as
  infrared electromagnetic rays from one object to
  another at a different temp. with which it is not
  contact.Human body radiates heat rays in all
  directions but is also exposed to heat rays
  radiated from the surrounding. Therefore, heat is
  lost by this method when the temp. of the
  surrounding is less than the body temp. At normal
  temp. about 60 of the total heat loss from a
  nude person occurs by radiation.

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2. Conduction

• It means heat exchange between objects at
  different temp. that are in contact with one
  another. Only minute amount of heat are normally
  lost from the body by direct conduction to other
  objects such as a chair or bed (3). On the other
  hand large amounts of heat are lost by
  conduction to air.

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3. Convection

• It is the removal of heat from the body by
  convection air currents.Heat must first be
  conducted to the air and then carried away by
  the convection current.About 12 of heat loss
  from the body occurs by conduction to the air and
  then by convection.When the body is exposed to
  wind heat loss by convection is greatly increased.

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4. Evaporation

• Evaporation of 1g water removes about 0.6 Kcal.
  Of heat. Even when a person is not sweating a
  certain amount of water still evaporates from the
  skin and lungs at a rate of about 600 ml/day.
  This is known as the insensible water loss.
• Sweating provides a very important way of heat
  loss from the body which can be regulated. As
  long as skin temp. is greater than the temp. of
  surrounding, heat can be lost by radiation
  conduction, but when the temp. of the body gains
  heat by radiation conduction.

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Under these conditions,the only means by which
heat loss can occur is evaporation.Evaporation of
sweat is decreased by increased humidity of the
environment.5. Small amounts of heat are removed
in the urine and Feces.
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Temperature regulating centres

• The temp. of the body is regulated almost
  entirely by temp. regulating centers located in
  the hypothalamus.
• The anterior hypothalamus-preoptic area
• The preoptic anterior hypothalamic nuclei
  contain two types of neurons
• A) Heat Sensitive neurons(receptors) which are
  present in large numbers.They increase their rate
  of firing as the temp. rises.

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B)Cold sensitive neurons (receptors) which are
less in number than the heat sensitive
neurons.Their firing rate increase when the body
temp. falls.Both of these types of cells function
as temp. sensor for controlling body temp.

• 2. The posterior hypothalamus The posterior
  hypothalamus contains a special area located
  bilaterally,approximately at the leval of the
  mammary bodies. This area receives signals from
  the anterior hypothalamus -preoptic area and from
  peripheral receptors where they are combined to
  provide mainly the heat producing and heat
  conserving reactions.
• (it is the regulating C., it is the thermostat.)

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Peripheral receptors for detection of Temperature

• These receptors play an important role in temp.
  regulation. They are present in the following
  sites
• A) The skin - Where both cold and warmth
  receptors are present.However, there are far more
  cold receptors than warmth receptors. Therefore,
  skin receptors mainly concerns detection of cold
  rather than warm temp. of the body surface.

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B) Deep body tissues

• Mainly in the spinal cord, in the abdominal
  viscera, and around the great veins. These
  receptors detect body core temp. rather than the
  body surface temperature.
• Yet . Like the skin receptors they detect cold.
  Therefore, both the skin and deep body receptors
  are concerned with preventing low body
  temperatures.

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Temp. regulating mechanisms

• When the body core temp. either increase above or
  decreases below almost exactly 37o C, several
  thermoregulatory responses take place to bring it
  back to this temp. Therefore, this critical temp.
  level is called theset-point of the temp.
  control system.
• The temp. regulating mechanisims include
  autonomic,somatic endocrine behavioral
  changes.When the body temp. increases above the
  critical temp,(the set-point in the
  hypothalamus),one group of these change take
  place to increase heat loss decreases heat
  production.

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On the other hand, when the body temp. decreases
below the set point. Another group of change
takes place to decrease heat loss increase heat
production.

• Temp. decreasing mechanisms activated by heat
• Increase heat loss
• Cutaneous vaso dilatation
• This occurs in almost all areas of the
  body.It is caused by inhibition of the
  sympathetic centers in the posterior hypothalamus
  that causes vasoconstriction. Cutaneous
  vasodilatation increase the rate of heat tranfer
  to the skin.

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2. Sweating

• When the anterior hypothalamus-preoptic area is
  stimulated by heat, impulses are transmitted
  pathways to the spinal cord and then through the
  sympathetic cholinergic fibers to the sweat
  glands to increase their secretion.This is turn
  increases the evaporative heat loss. Sweat gland
  can also be stimulated by epinephrine or nor
  epinephrine circulating in the blood. This is
  important during exercise.

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Acclimatization of the sweating mechanism

• When a normal person is exposed to hot weather
  for 1 to 6 weeks two changes take place which are
  called acclimatization of the sweating mechanism.
• These are
• Sweat production increases to as much 2
  liters/hour.
• Decreased concentration of sodium chloride in the
  sweat caused by increased secretion of
  aldosterone.

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3. Behavioral responses

• B) Decrease heat production
• Strong inhibition of mechanisms which cause
  excess heat production.
• Anorexia.
• Apathy and inertia
• II. Temp. increasing mechanisms activated by cold
  
• Decrease heat loss
• 1.) Cutaneous vasoconstriction
• Caused by stimulated of the posterior
  hypothslsmus sympathetic centers. This decreases
  the rate of heat transfer to the skin.

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2. Pilo erection

• Which is caused by sympathetic stimulation of the
  erector pili muscles.This mechanism is not
  important in the human being and is manifested by
  goose skin, but in lower animals upright
  projection of the hairs allows them to entrap a
  thick layer of insulator air next to the skin.
  
• 3.) Behavioral responses.(to put on suitable
  clothes.)

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B) Increase heat production

• Shivering
• It is an involuntary response of the
  skeletal muscles which is controlled by an area
  in the posterior hypothalamus called the primary
  motor center for shivering .This area is normally
  inhibited by signals from the heat center in the
  anterior hypothalamus but is excited by cold
  signals from the skin and spinal cord.Therefore,
  this center becomes activated when the body temp.
  falls even slightly below 370C.

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It then transmits signals down the brain stem and
spinal cord to the anterior motor neurons to
increase the tone of the skeletal muscles
throughout the body. When the tone rises above a
certain critical level, shivering begins.During
Maximum shivering,body heat production can rise
to as high as five times normal.

• 2. Semiconscious general increase in motor
  activity.

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3. Increased secretion of nor epinephrine and
epinephrine

• This can cause an immediate increase in the rate
  of cellular metabolism. This effect is called
  chemical thermo genesis and it results mainly
  from uncouple oxidative phosphorylation. This
  process occurs in brown fat. Adults do not have a
  significant amount of this type of fat, therefore
  chemical thermo genesis increases the rate of
  heat production only 10.On the other hand,
  infants have some brown fat in the interscapular
  space chemical thermo genesis can increase the
  rate of heat production as much as 100 which is
  very important factor in maintaining their normal
  body temp.

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4) Increased thyroxin secretion

• Exposure to cold increase the production of
  thyrotropin-releasing hormone by the hypothalamus
  which stimulates the secretion of the thyroid
  stimulating hormone (TSH) from the anterior
  pituitary gland.
• TSH in turn stimulates increased output of
  thyroid gland.Thyroxin increase the rate of
  cellular metabolism throughout the body I.e.it is
  another mechanism of chemical thermo genesis
  .However, this increase in metabolism requires
  several weeks for the thyroid gland to
  hypertrophy before it reaches its new level of
  thyroxin secretion. Moreover, it is of little
  significance in adult humans.

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Abnormalities Of Body Temp. Regulation

• Fever ( pyrexia) It means an elevation of core
  body temp. above the level which is normally
  maintained by the individual. It results when the
  set point of the hypothalamic temp. control
  system is elevated to a new point above 370C.
  Consequently, all the mechanisms for raising the
  body temp. are activated including shivering and
  cutaneous vasoconstriction. Within few hours the
  body Temperature approaches the new set point
  temperature which rarely exceeds 41.10C.

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Pathogenesis Of Fever

• Substances which elevate the set point are called
  pyrogens and they include bacterial
  toxins,components of various micro organisms and
  products of tissue degeneration.These substances
  act on cells of the immune system including
  monocytes,macrophages and kupffer cells to
  produce cytokines that act as endogenous pyrogens
  (EPS)eg. Interleukin 6 (IL-6), tumour necrosis
  factor or (TNF), IL-1 and several interferons.

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Therefore cytokines activate the preoptic area
of the hypothalamus most probably through local
release of prostaglandins.They may inter act
directly with neutral tissues.Drugs that reduce
the level of fever are called antipyreticse.g.
aspirin.They act by blocking the synthesis of
prostaglandins.

• Fever, within limits, is presumably beneficial
  because it may inhibit the growth of many micro
  organisms and increase antibody production.
• However very high temp. are harmful.When the
  rectal temp. is over 410C for prolonged periods,
  some permanent brain damage results.

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Hyperthermia

• It means an elevation of body temp. higher than
  the thermoregulatory set point, usually due to an
  exogenous cause.
• It may be caused by
• Increased heat production e.g. by severe muscular
  exercise or high environmental temp., which
  exceeds the normal capacity of heat loss
  mechanism.
• Impaired heat loss e.g. due to high humidity or
  drugs that impair sweating such as
  anticholinergics.

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3) Brain tumors compressing the hypothalamus.

• When the rectal temp. is over 430C, the person
  develops heat stroke which commonly causes
  death because
• There is a limit to the rate at which the body
  can loss heat even with maximal sweating.
• When the hypothalamus becomes excessively
  heated,its heat regulating ability becomes
  greatly depressed and sweating diminishes. As a
  result a high body temp. tends to perpetuate
  itself.

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Symptoms of heat stroke include dizziness,
abdominal distress, delirium and eventually loss
of consciousness and death.These manifestations
result from two causes

• Direct damaging effect of very high body
  temperature on all body tissues especially the
  brain.
• Some degree of circulatory shock caused by
  excessive loss of fluid and electrolytes in the
  sweat.

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Treatment of hyperthermia exceeding 41.1oC must
be immediate

• Removal from direct sunlight, removal of
  clothing, wetting the body surface and fanning
  are simple measures which can be undertaken on
  the spot.If these are not enough the most
  effective action is to immerse the patient in the
  ice water bath while monitoring core temperature
  to be certain that a state of hypothermia is not
  induced.

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Hypothermia

• It is defined as core body temperature of less
  than 350C.
• Hypothermia is caused by exposure to cold
  especially when associated with other conditions
  such as advanced age, decreased metabolic rate,
  CNS diseases, malnutrition, drugs as alcohol and
  paralysis.
• Effects of hypothermia on the body include
  depressed mental status followed by loss of
  consciousness, shivering which stops below 320C,
  very slow respiration, low heart rate, decreased
  blood pressure and arrhythmias .

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At rectal temp. of about 280C, ability to
spontaneously return the temp. to normal is lost.
However, if the individual is rewarmed with
external heat, he survives and returns to a
normal state.

• As humans tolerate body temp. of 210-240C without
  permanent ill effects, induced hypothermia has
  been used extensively in surgery especially heart
  and brain operations. The circulation can be
  stopped for relatively long periods because the
  oxygen need of the tissue are greatly
  decreased.Also the blood pressure is low and
  bleeding in minimal.