Transcript Temp.

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.,
The average normal oral temperature in
young adults measured in the morning is
37C with a range between 36.3 and 37.1 C.
Physiological variations in body
temperature
1. Normally, the body temp. undergoes a
regular circadian fluctuation of about
0.60C being lowest in the morning &
highest in the evening.
2. 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.
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
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.
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 :
A. Conduction of heat from the deeper
organs and tissues to the skin.
B. Transfer of heat from the skin to the
surrounding.
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
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.
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.
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.
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 temperature of the surroundings
becomes greater than that of the skin,
instead of losing heat, the body gains heat
by radiation& conduction.
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.
Temperature regulating
centres :
•
The temp. of the body is regulated almost
entirely by temp. regulating centers located
in the hypothalamus.
1. 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.
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.
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.
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.
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.)
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 the”set-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.
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 :
A) Increase heat loss :
1) 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.
2. Sweating :
• When the anterior hypothalamuspreoptic 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.
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 :
A) Sweat production increases to as much 2
liters/hour.
B) Decreased concentration of sodium chloride
in the sweat caused by increased secretion of
aldosterone.
II. Temp. increasing mechanisms activated by
cold
A) 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.
2. Pilo erection
• Which is caused by sympathetic stimulation of
the erector pili muscles.This mechanism is not
important in the human being, 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.)
B) Increase heat production:
1) 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.
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.
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.
4) Increased thyroxin secretion
• Exposure to cold increase the production of thyrotropinreleasing 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.
Concept of a "Set-Point" for Temperature Control
That is, all the temperature control mechanisms continually attempt to bring the body
temperature back to this set-point level .
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
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.
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
“antipyretics”e.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.
Hyperthermia :
•
•
1)
2)
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.
•
When the rectal temp. is over 430C, the
person develops “heat stroke” which
commonly causes death because :
A) There is a limit to the rate at which the body
can loss heat even with maximal sweating.
B) 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.
Symptoms of heat stroke include
dizziness, abdominal distress, delirium
and eventually loss of consciousness and
death.These manifestations result from
two causes :
1) Direct damaging effect of very high
body temperature on all body tissues
especially the brain.
2) Some degree of circulatory shock
caused by excessive loss of fluid and
electrolytes in the sweat.
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.
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
.
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.