The short answer is: homeostasis
?Homeostasis is the ability or tendency of an organism or cell to
maintain internal equilibrium by adjusting its physiological
processes. Your normal body temperature, for example, is around 98.6
deg F. If you start to get cold you shiver. Shivering is a
physiological process designed to keep your body temperature from
dropping below 98.6 deg F. Also, you may put a coat on. This will also
help keep your body temperature from dropping, but it is not a
physiological process. Your body has many physiological processes
designed to maintain many equilibriums.
The hypothalamus is a group of neurons, at the base of your brain, and
forms the primary link between the nervous system and the endocrine system.
Among the homeostatic processes controlled by the hypothalamus are:
thirst,hunger, daily (circadian) rhythms, body temperature and blood
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?Temperature sensation rests with receptors in three key sites:
?peripheral thermoreceptors: present in skin as free nerve endings of
A and C type fibres
?central thermoreceptors: two types of receptor are found in the
preoptic area of the anterior hypothalamus. One group responds to
relative warmth, the other to relative cold.
o spinal cord
o abdominal viscera
o great veins?
?There are two key physiological mechanisms for increasing heat
loss from the body of Man:
Generally, both act in tandem to lower core temperature under the
integration of the hypothalamus. However, there may be situations when
either mechanism is inefficient or disordered e.g. high external
humidity or anticholinergic drugs for sweating, and the use of
alpha-adrenoceptor blocking drugs for vasodilatation.
Decreased heat production is a theoretical means of decreasing
temperature. In practice, the metabolic rate is unchanged; the only
contribution to decreasing temperature is an inhibition of shivering
and non-shivering thermogenesis. This is not as important as sweating
?Homeostasis: the tendency for individuals to maintain their internal
environment (e.g., body temperature) fairly constant.?
?One of the most obvious examples of homeostasis is body temperature,
which in humans is normally very close to 98.6 _F or 37 _C.?
?In humans temperature homeostasis is controlled by the
thermoregulatory centre in the hypothalamus. It receives input from
two sets of thermoreceptors: receptors in the hypothalamus itself
monitor the temperature of the blood as it passes through the brain
(the core temperature), and receptors in the skin monitor the external
temperature. Both pieces of information are needed so that the body
can make appropriate adjustments. The thermoregulatory centre sends
impulses to several different effectors to adjust body temperature:
(See chart on site)
?Heat is sensed by the skin and the hypothalamus, as both contain
thermo receptors. External environment temperature is sensed by the
skin, and internal environment temperature is sensed by the
When it is cold outside, messages are sent from the
themoreceptors in the skin or from deep thermal receptors or via the
blood to the cerebrum and the hypothalamus. The cerebrum makes the
person aware of being cold, and can cause behavioral changes which are
voluntary to, for example, put on a sweater. When the message has
reached the hypothalamus, a series of reactions follow. TRH (thyroid
releasing hormone) is released by the hypothalamus, its target organ
being the anterior lobe of the pituitary gland. When TRH reaches the
pituitary gland, it releases TSH (thyroid stimulating hormone) into
the blood stream. The target organ of TSH is the thyroid gland. Upon
receiving TSH, the thyroid then produces thyroxin. Thyroxin increases
cellular metabolism to make heat.
Other things that happen:
-vasoconstriction (blood diverted from skin to keep heat)
-skin hairs raised (erector pilli muscle contracted)
-increased metabolic rate
When it is too warm, messages are sent in the same way to
the hypothalamus. This causes:
-increased sweating to release heat via water
-vasodilation (blood diverted to skin to lose heat)
-skin hairs lowered,
-reduced metabolic rate
Fever and Temperature
?If the body temperature is above 37,2 C and is associated with
sweating, hyperventilation, and vasodilatation in the skin, we speak
of fever. At the beginning, gradual increase in body temperature is
observed together with muscle shivering, vasocontriction in the skin,
and piloerection. This situation is called chills. Increased body
temperature is achieved by lowered loss of heat. Vasoconstriction in
the skin and subcutaneous tissue is the cause of pale color and
dryness, the affected person has a feeling of coldness. At the same
time the production of heat in the organism increases. The muscle
tonus increases, the spasms accur. Spasms may occur mainly in
children. When the vasodilatation starts in the skin, the feeling of
warmth and sweating occurs.
Fever may be provoked by many stimuli. Most often, they are bacteria
and their endotoxins, viruses, yeasts, spirochets, protozoa, immune
reactions, several hormones, medications, and synthetic
polynucleotides. These substances are commonly called exogenic
pyrogens. Cells stimulated by exogenic pyrogens form and produce
cytokines called endogenic pyrogens. Endogenic pyrogens centrally
affect the thermosensitive neurons in the preoptic area of the
hypothalamus increase the production of heat and decrease in heat
loss. The body temperature increses until it reaches the set point.
This information is transferred by temperature of blood that flows
around the hypothalamus. The decrease of temperature is controlled by
activation of mechanisms regulating increased outcome of heat to the
surrounding area. Increased outcome continues in favourable case until
the new equilibrium is achieved.
The most important endogenic pyrogens are IL-1, IL-6 and cachectin
also called the tumour necrosis factor- (TNF- ). These are
glycoproteins that also have other important effects. They are
produced especially by monocytes and macrophages but also by
endothelial cells and astrocytes. Also the interferons , and
display the pyrogenic activity.?
?Fever is a natural reaction during a number of illnesses. In
several cases, absence of the natural reaction is more alarming sign
than the presence of fever itself. Fever is usually accompanied by
different general symptoms, such as sweating, chills, sensation of
cold, and other subjective sensations. Missing of these symptoms
during high temperature may be a sign of a serious illness.
The main task in heat production has thermogenesis caused by the
effect of thyroid hormones. Hormones of thyroid gland stimulate -ase
found in cytoplasmic membranes. Increased production of heat is
achieved by increasing the metabolic processes in which energy is
released in the form of heat.?
?Heat is lost from an organism in several ways. The biggest loss is by
conduction. It depends on the gradient between the body temperature
and the temperature of the surrounding environment. The second way is
by radiation. The third way is by evaporation. It is used especially
during increased production of heat. Distribution of heat is done by
blood circulation. Heat goes from each cell to the surrounding liquid
and afterwards to the circulated blood. Modulating factor of heat loss
is the amount of blood that circulates through the body surface. The
big flow through the subcutaneous area and the skin secures the income
of heat that may be given to the environment through the body surface.
Sweating helps delivering the heat. Sweat glands are controlled by
cholinergic impulses through the sympathetic fibers.
During intensive sweating, up to one liter of sweat may be formed.
When the humidity of the environment is higher, a loss of heat by
sweating is easier. When it is necessary to accumulate the heat in an
organism, adrenergic stimuli cause reduction of the blood flow through
the skin. The skin becomes an isolator decreasing the heat loss to
minimum. Control mechanisms regulate the production of heat and its
loss. Production and handover (loss) of heat are controlled from the
center in the hypothalamus. It works on the principle of negative
feedback control and includes:
1. Receptors registrating central temperature
2. Effector mechanisms composed of vasomotors, metabolic effectors,
and controls of sweat glands
3. Structures recording whether the actual temperature is not too high or too low
Increased central temperature activates mechanisms enabling the heat
loss. Low central temperature activates mechanisms enabling the
accumulation of heat. These mechanisms work as the thermostat.?
Causes of fever:
1. infections caused by bacteria, ricketsia, chlamydia, viruses, and parasites
2. immune reactions, including the defects in collagen, immunological
abnormalities and acquired immunodeficiency
3. destruction of tissues, such as trauma, local necrosis
(infarction), and inflammatory reaction in tissues and vessels
(flebitis, arteritis), pulmonary infarction, cerebral and myocardial
infarction, and rhabdomyolysis
4. specific inflammations (sarcoidosis, granulomatous hepatitis)
5. inflammation of intestine and intraabdominal inflammatory processes
6. neoplastic processes with the participation of lymphoendothelial
system and hemopoetic system, solid tumours (Grawitz tumour of the
kidney, carcinoma of the pancreas, pulmonary and skeletal tumours,
hepatoma) Fever is present in complications of solid tumours, usually
in metastases that are associated with necrosis of the tumour,
obstruction of ducts, or with infection
7. acute metabolic failures such as arthritis urica, porfyria,
Addison's crisis, thyreotoxic crisis, and feochromocytoma
8. administration of some drugs
9. dehydration or admistration of salts. That's why fever occurs
together with diarrhea.
10. administration of foreign proteins (e.g. globulinum
antitetanicum-antitoxic fraction of horse serum) may be the reason of
?How much heat does the body produce?
The Basal Metabolic Rate (BMR) is defined as the heat production of a
human in a thermoneutral environment (33°C or 91°F) at rest mentally
and physically more than 12 hours after the last meal. The standard
BMR for a 70 kg man is approximately 1.2 W/kg, but it can be altered
by changes in active body mass, diet, endocrine levels. It is probably
not affected by living in hot climates .
The range of endogenous heat production, M, dependent on the work
being performed, age, sex, size, physical fitness, and level of
activity, is about 40 to 800 W/m² (or 1-21 W/kg for a standard man)
. If deep body temperature is altered, either by heat storage from
being in a warm environment, or by febrile disease (having a fever),
then M changes as well. In cold environments, for example, shivering
induced by the body can increase heat production up to four or five
times the normal resting level. Further increase can be induced by
?Substances that cause fever are known as "pyrogens." There are two
types of pyrogens; exogenous and endogenous. Those that originate
outside the body, such as bacterial toxins, are called "exogenous"
pyrogens. Pyrogens formed by the body's own cells in response to an
outside stimulus (such as a bacterial toxin) are called "endogenous"
Researchers have discovered that there are several "endogenous"
pyrogens. These are made up of small groups of amino acids, the
building blocks of proteins. These natural pyrogens have other
functions in addition to inducing fever; they have been named
"cytokines". When cytokines are injected into humans, fever and chills
develop within an hour. Interferon, tumor necrosis factor, and various
interleukins are the major fever producing cytokines.
The production of fever is a very complex process; somehow, these
cytokines cause the thermoregulatory center in the hypothalamus to
reset the normal temperature level. The body's initial response is to
conserve heat by vasoconstriction, a process in which blood vessels
narrow and prevent heat loss from the skin and elsewhere. This alone
will raise temperature by two to three degrees. Certain behavioral
activities also occur, such as adding more clothes, seeking a warmer
environment, etc. If the hypothalamus requires more heat, then
Low Body Temperature
?Hypothyroidism is more common than is typically diagnosed. It is
possible to have some laboratory values be returned in the normal
range when you have low thyroid symptoms. Oral temperatures during the
day of less than 98.0°F (36.7°C) are very suspicious.?
?Wilson?s Syndrome is a state of chronic low body temperature. It may
be genetic, or triggered by stress or inactivity, as well as diet,
selenium or zinc deficiency, heavy metals or starvation. It appears to
be part of the body's method of conserving fuel: Wilson?s Syndrome
occurs when someone drops into this ?conservation? mode and is not
able to later recover back to the normal ?productive? mode of
functioning. An interesting side note is that Dr. Fahrenheit
considered normal body temperature to be 100°F. Some say the present
average temperature is 98.2°, and may be as much as 1½° lower than it
was 150 years ago.
The difficulty arises when these low metabolism conditions continue
for too long, various repair functions are deferred too long, and the
body begins to show signs and symptoms. For every degree of body
temperature lost, biochemists say the loss in efficiency of enzymatic
processes can be up to 20%. Chronic low body temperature is related to
many syndromes and symptoms, including: allergies, apathy, chronic
fatigue, "brain fog", "personal failure", depression, dizziness,
hypoglycemia, lethargy, passive/aggressive syndromes, skin and joint
conditions, sleep disorders, sexual dysfunction, past sexual abuse,
yeast problems, porphyria, and many other poorly-defined chronic low
health states. Obviously, not all chronic problems are related to
chronic low body temperature but it would be prudent to measure one's
body temperature from time to time to see if this is a possibility. It
is interesting to note that a common time of death is in the very
early morning, when the body temperature is at its lowest.?
?What are the signs of anemia? (Iron deficiency type)
Anemia takes some time to develop. In the beginning, you may not have
any signs or they may be mild. But as it gets worse, you may have
? not doing well in work or school
? low body temperature?
Immunity and temperature
?A second defense offered by innate immunity is a fever. The human
body, under normal conditions, maintains an internal temperature of
98.6oF. The body is, however, capable of generating an immune response
that intentionally raises the internal temperature that results in a
fever. This is one example of an innate immune response that helps the
immune system defend the body against pathogens.?
Homeostasis and Negative Feedback
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thermoregulation + body temperature
homeostasis + body temperature
immunity + body temperature
anemia + body temperature