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Thermo-regulation
Normal range of body temperature
Humans are homeotherms i.e. body temperature is maintained by reflexes which are integrated in
the hypothalamus as well as voluntary acts.
The homeothermic response is an excellent example of homeostasis.
The human body normally maintains a body temperature between 36 and 37.50C
Core temperature refers to the temperature of the body below the skin and subcutaneous tissue.
Shell temperature refers to the temperature of the body at the surface i.e. skin and subcutaneous
tissue.
Too high a core temperature kills by denaturing body proteins.
Too low a core temperature causes cardiac arrhythmias that result in death.
Most of the heat produced by the body comes from oxidation of food.
Heat is produced mainly by the liver and muscles.
The rate at which this heat is produced (the metabolic rate or MR) is measured in kilocalories.
Resting metabolic rate varies with age and sex.
Homeostasis and maintenance of body temperature
Homeostasis means 'staying the same'. The internal environment of a healthy body remains the
same under conditions of outside environmental changes.
Homeostasis is maintained by adaptive mechanisms in the body regulated by the nervous and
endocrine systems.
Although external temperatures may vary, our bodies remain within less than a degree of the normal
36.50C.
Negative feedback produces a response of warmth that is opposite to the initiating stimulus of
cold (or a response of cooling that is opposite to the initiating stimulus of heat).
The three major components of this negative feedback system are

Temperature receptors (sense existing body temp)

Effector organ systems (control heat production or heat loss)

Integrator or Controller (compares the sensed temperature with a 'reference' temperature)
If the body temperature is too high or too low, the controller activates the appropriate effector
system, returning the body temperature to normal.
The role of the hypothalamus in temperature regulation
Body temperature is regulated by mechanisms that attempt to keep heat production and heat loss in
balance.
The structure with overall responsibility for temperature regulation is the hypothalamus which is
responsive to changes in the temperature of the blood and impulses from temperature receptors in
for example the skin (peripheral thermoreceptors).
The hypothalamus thus acts as the body’s thermostat.
The heat promoting centre when stimulated sets in operation responses that raise body
temperature
The heat losing centre when stimulated sets in operation responses that lower body temperature.
How the body conserves and produces heat (e.g. when in cold environment)
Endocrine system response
Cooling of specific area in hypothalamus (preoptic area)
 increase in secretion of thyrotropin releasing hormone (TRH - from hypothalamus)
 stimulates secretion of thyroid stimulating hormone (TSH - from pituitary)
 stimulates secretion of thyroxine (from thyroid gland)  increases cellular metabolism and
body heat.
Sympathetic nervous system response
Stimulation of the sympathetic nervous system
 Increased secretion of adrenaline and noradrenaline from adrenal glands also helps raise
level of cellular metabolism (chemical thermogenesis).
(Increased metabolism  increased heat production)
Effectiveness of chemical thermogenesis seems to be related to amount of brown fat in the
body (brown fat contains lots of mitochondria and is capable of a high level of cellular
metabolism).
Adult bodies contain little or no brown fat, infants contain more.
Shivering (effector is skeletal muscle)
The hypothalamus also stimulates shivering.
The primary motor centre for shivering in the hypothalamus stimulates the brain stem, spinal cord
and motor neurones that ultimately cause an increase in muscle tone.
Shivering can raise heat production by as much as 5 times above normal
Peripheral vasoconstriction
The temperature centres in the hypothalamus cause the constriction of peripheral blood vessels,
thereby reducing heat loss through the skin (decrease in flow of warm blood to the skin)
Piloerection
Hypothalamus causes hair in the skin to stand on end ( increase in muscle activity of the skin 
slight increase in body temperature
Adipose tissue
Offers insulation
Behaviour
Putting on clothing, increasing activity, posture.
Mechanisms for losing body heat
Basic mechanisms of reducing body heat (necessary due to excess heat production e.g. exercise,
hot environment or fever) are Radiation, Convection, Conduction.
Definition of terms (ways in which heat is lost from the body)
Transfer of heat from the body to an object without physical contact = Radiation
Transfer of heat from the body to an object with physical contact = Conduction
Cool air making contact with the body, being warmed and carried away by convection currents =
Convection
The conversion of a liquid to a vapour in which the evaporating substance (e.g. perspiration)
removes heat from the body = Evaporation
How the body loses heat (e.g. when in hot environment)
Sweating and evaporation
Perspiration (from sweat glands) is increased  increased evaporation of sweat  cooling of the
skin
Sweat loss can vary between 500ml and 12L (12L of sweat requiring 7000 kcal)
Peripheral vasodilation
Blood vessels in the skin dilate (vasodilation)  increased volume of warm blood flows from the
core to the skin so that heat is transferred to the skin from the blood about 8 times faster than
normal (caused by inhibition of nerve centres in the hypothalamus that usually cause the
constriction of blood vessels)  the skin becomes warm and excess heat is lost to the environment
Shivering and other muscular activities that increase body heat are inhibited.
Behaviour (removing clothing, decreasing activity, posture).
Heat is also lost via urine, faeces and the respiratory tract.
Summary of ways in which the body gains and loses heat
Heat gain (and conservation)
Vasoconstriction
Sympathetic stimulation
Increased metabolism
Shivering
Hormones
Fat
Behavioural responses
Heat loss
Vasodilation
Sweating,
Behavioural responses