Simply put, a fever is the body?s response to invading organisms
?To really understand what a fever is, you need to say hello to the
hypothalamus (say: high-po-tha-luh-mus). The hypothalamus is in the
center of your brain. It's a lot like your body's thermostat - that
thing on the wall in your house that you use to set the heat or the
air conditioning. Your hypothalamus knows what temperature your body
should be and will send messages to your body to keep it that way.
Most human beings have a body temperature of around 98.6 degrees
Fahrenheit (37 degrees Celsius). Some people will have a normal
temperature that's a little higher; others will have a normal
temperature that's a little lower. Most people's body temperatures
even change a little bit during the course of the day: It is usually a
little lower in the morning and a little higher in the evening. For
most kids, their body temperature stays pretty much the same from day
to day - until germs enter the picture.?
?Fever is a positive feedback mechanism which acts towards the
direction of change (as opposed to negative feedback which acts
opposite to change to maintain homeostasis). Therefore, fever is the
opposite of thermoregulation. Substances which induce fever are called
pyrogens. Although external pathogens may be the ultimate reason for a
fever, it is the internal or endogenous pyrogens that ultimately cause
the increase in the thermoregulatory set-point.?
?Every person's temperature varies slightly, but the average
"normal" temperature for humans is 37°C (98.6°F). Various things
influence body temperature, for example activity, metabolic rate,
environmental temperature, and infection.
Infection will cause an elevated temperature which is sometimes the
only outside evidence of an infection, but other times is associated
with obvious symptoms to suggest the cause. There is evidence that the
body deals better with infection as a result of creating an elevated
temperature. At the same time the body's immune system comes into
action with special proteins being produced and mobilisation of white
The raised temperature may be associated with shivering and hot sweats
in turn. Shivering is the body's way of elevating the temperature and
is brought about by the temperature regulating centre in the brain
(the hypothalamus). Sweating performs the reverse function, once again
regulated by the hypothalamus. When you have a fever it is as though
the body has temporarily reset its thermostat.
Fever is also associated with headache, other bodily aches and pains,
rapid breathing and rapid heart rate. In the presence of such symptoms
people often prefer to take some treatment to alleviate them, but if
the fever is only mild (eg only one degree above normal), and the
symptoms not too severe, there is no need to take medication just to
return the temperature to normal.?
?Temperature homeostasis is controlled by the hypothalamus, which
acts as a thermostat that sets the body?s target temperature. In
fever, this temperature set point is elevated and the body responds by
implementing internal heat conservation methods to reach this new
temperature. This heat conservation results in an elevation in core
body temperature, a process that is metabolically expensive. However,
although fever is often given a negative connotation, this response
must provide some benefit for the host or it would not have been
preserved throughout human evolution?
?When fever occurs, many physiological stresses take place. Some of
these include increased oxygen consumption as a response to increased
cell metabolism, increased heart rate, increased cardiac output,
increased leukocyte count, and an increased level of C-reactive
protein. Oxygen consumption increases by 13% for every 1°C increase in
body temperature, provided no shivering occurs. If shivering is
present, oxygen consumption may increase by 100% to 200%.1 Some
cytokines released during fever states also induce physiological
stress. These cytokines can trigger accelerated muscle catabolism by
causing weight loss, loss of strength, and negative nitrogen balance.
Physiological stress can be manifested by decreased mental acuity,
delirium, and seizures, which are more frequent in children.?
?An elevated body temperature may not always be a fever. Because many
syndromes can lead to an elevated body temperature, understanding the
pathophysiology of each will provide a framework to guide the
diagnosis. With appropriate assessment, accurate differentiation of
fever from hyperthermia is possible. Appropriate therapy must be
started quickly to address the physical cause of the temperature
elevation. Fever is the body?s response to infection. Awareness of
both the beneficial and injurious effects of fever helps to guide
clinicians in the care of patients.?
?Fever occurs when the body?s immune response is triggered by
pyrogens (fever-producing substances). Pyrogens usually come from a
source outside the body and, in turn, stimulate the production of
pyrogens inside the body. Pyrogens tell the hypothalamus to increase
the temperature set point. In response, our body begins to shiver; our
blood vessels constrict (close); we get under the covers in an attempt
to reach the new temperature that is higher than our baseline.?
?A fever is an increase in body temperature above the normal range.
However, body temperature varies between people, with different levels
of activity and at different times of the day. Modern medical
textbooks differ in their definition of the highest normal body
temperature. Fever generally can be defined as an early morning
temperature greater than 99° Fahrenheit or a temperature greater than
100° Fahrenheit at any time of the day.
A part of the brain called the hypothalamus acts as the body's
thermostat. When all is well in the body, the hypothalamus is reset at
the normal body temperature. Fever develops when the hypothalamus is
set to a higher-than-normal temperature. This resetting of the
hypothalamus is usually caused by small molecules called pyrogens in
the blood. Pyrogens can come from outside the body (external) or can
be produced inside the body (internal). External pyrogens include
toxins (poisons) produced by infectious viruses or bacteria. Internal
pyrogens include abnormal chemicals that are produced by tumors and
proteins that are released during the normal response of the immune
Illustrations and explanations of fever dynamics:
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Hypothalamus + fever
Fever + dynamics