In a healthy individual, body temperature is kept constant in a very small range despite of big differences in temperature of the surroundings and also those in physical activity. Very perfect regulation of body temperature, necessary for optimal progress of enzymatic reactions, is developed in all homoiotermic animals. It doesn't apply to poikilothermic animals. During the most variable changes in human organism, the body temperature may increase. 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. The greatest importance is splitting
of ATP when 54kJ are released from one mol
of ATP. Skeletal muscles, liver, splanchnic organs, and brain
are the biggest producers of heat in an organism. In the heat
production the muscles have especially important role. Because
of their weight, they are able to produce very large amount of
heat very quickly. Increased production of heat takes place in
skeletal muscles during increased physical activity. During the
digestion, an increased production of heat occurs also in the
GIT. Constant body temperature is achieved by perfect nervous
regulation. Nervous system maintains the optimal intensity of
metabolism and at the same time regulates the amount of heat
loss. In early postnatal development, the thermoregulation is
inadequate because of immature CNS. Fever is always achieved by
reset the center of thermoregulation to higher values.
Hyperthermia means overheating of an organism caused only by
exogenic causes (e.g. hot environment, hot bath). In this case
the center of thermoregulation doesn't change its setting up.
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:
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.
In healthy individuals, the body temperature (oral
temperature) is somewhere between 36,5
and 37,5
. It
slightly increases during the day since the morning (from 6:00
a.m.). The peak is reached at 6:00 to 10:00 p.m. The lowest
temperature is between 2:00 and 4:00 a.m.
Diurnal variation
depends on the activity throughout the day. Diurnal variations
don't change in persons that work at night and sleep during the
day. Such a diurnal variation is also kept when fever occurs. Fever
reaches the peak in the evening, and in the morning
even a very sick patient may have almost normal temperature. Body
temperature changes are more intensive in young person than in old people. The
temperature may slightly or temporarily increase in hot
environment. Physical activity may also increase the body
temperature. In extreme effort, the increase may be very high.
The temperature in marathon runners may increase to 39
to 41
.
The temperature may increase slightly if vasodilatation,
hyperventilation, and other compensation mechanisms fail. Small
increase in temperature may occur if the surrounding temperature
is lower or the jogging is done early in the morning.
Organism uses simple
mechanism for temperature regulation.
It is the blood flow through the skin and subcutaneous area.
Vasoconstriction allows the increased accumulation of heat, and
vasodilation secures its quick loss. Changes in temperature up
to 3
don't cause an interuption of physiological
functions. Spasms may occur during high fever in children. If
the body temperature is increased over 42,2
, irreversible
changes in the brain occur. In humans the temperature usually
doesn't overcome 41,1
. Uncontrolled decrease in
temperature below 32,8
is accompanied by confusions and
gradual loss of consciousness. If the decrease continues under
30
, the fibrilation of ventricles occur that is the sign
of fatal termination of this condition.
Brown fat that differs from the white one in structure and
sites of location has an important function in thermogenesis in newborns and
children. It is found between scapulas, on the neck, in axils,
around the aorta and the kidneys. It is highly vascularized, and
it has large mitochondria in its cells. One could say that while
the white fat acts as feather-bed, the brown one is an
electrical pillow. Receptors of cold conduct the information to
the center of thermoregulation. From this center the impulses run
in the sympathetic nerve fibers and lead to the release of
norepinephrine in the brown fat. Norepinephrine activates the
enzyme lipase. Activated lipase splits the fat to glycerol
and free fatty acids (FFA) and the heat is released. Glycerol and FFA
remain in the cell and can perform the resynthesis after some time. An adult
person has little brown fat.