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 metabolism
2) 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 %
Most of the heat loss from an unclothed person
at room temperature occurs by which of the
following mechanisms?
A) Conduction to air
B) Conduction to objects
C) Convection
D) Evaporation
E) Radiation
Effect of high and low atmospheric temperatures
of several hours’ duration on the internal body
“core” temperature
Regulation of Body
Temperature—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 2to 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 Central
and 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 heatretaining 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 Body
Temperature 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 TemperatureRegulating 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 temperature
controller.
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 normal
b. There will be sweating and peripheral vasodilatation
c. There will be shivering and peripheral vasoconstriction
d. The person will feel subjectively cold
Q. Heat stroke is charateristically associated with
a.
b.
c.
d.
e.
Profuse sweating
Elevated basal metabolic rate
Hot dry skin
a and b only are correct
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 Thyroxin
A) No
Yes
Yes
No
B) No
Yes
Yes
Yes
C) Yes
Yes
Yes
Yes
D) Yes
No
No
Yes
E) Yes
Yes
Yes
No
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) Conduction
B) Convection
C) Evaporation
D) Radiation
A scuba diver explores an underwater lava flow where
the water temperature is 102°F. Which of the following
profiles best describes the mechanisms of heat loss
that are effective in this man?
Evaporation Radiation
Convection
Conduction
A) No
No
No
Yes
B) No
No
No
No
C) Yes
Yes
No
Yes
D) No
Yes
No
Yes
E) Yes
Yes
Yes
Yes
A 54-year-old man is admitted to the emergency
department after being found lying in his yard near a
running lawnmower on a hot summer day. His body
temperature is 106°F, blood pressure is normal, and
heart rate is 160 beats/min. Which of the following sets
of changes is most likely to be present in this man?
Sweating
Hyperventilation
Vasodilation of skin
A) No
No
No
B) No
Yes
Yes
C) Yes
No
Yes
D) Yes
Yes
No
E) Yes
Yes
Yes
Use the following figure to answer questions. The diagram shows the effects of changing the
set-point of the hypothalamic temperature controller. The red line indicates
the body temperature, and the blue line represents the hypothalamic set-point temperature