Body Temperature And Its Regulation . Normal Body Temperature :
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Normal Body Temperature :
The temperature of the deep tissues of the body which is known as the core temperature remains almost exactly constant except when a person develops a febrile illness.On the other hand the skin temperature rises & falls with the temperature of the surroundings.
The core body temperature can be measured either orally or rectally. The oral temperature is normally 0.50C lower than the rectal temp.,
but is affected by many factors including ingestion of hot or cold fluids & mouth breathing.The average normal oral temperature in young adults measured in the morning is 37C with a range between 36.3 and 37.1 C.
4. During exercise, excess heat is produced in the body and the rectal temp. can normally rise to as high as 400C .
5. Emotional excitement slightly increases the body temp. probably due to unconscious tensing of muscles.
6. When the metabolic rate is high the body temp. is chronically elevated by as much as 0.50C and vice verse.
7. Constitutional hyperthermia.
Most of the heat produced in the body is generated in the organs especially in the liver, the brain the heart and the skeletal muscles especially during exercise. Therefore, heat loss from the body occurs in two steps :
1. Radiation : It means transfer of heat as infrared electromagnetic rays from one object to another at a different temp. with which it is not contact.Human body radiates heat rays in all directions but is also exposed to heat rays radiated from the surrounding. Therefore, heat is lost by this method when the temp. of the surrounding is less than the body temp. At normal temp. about 60% of the total heat loss from a nude person occurs by radiation.
Under these conditions,the only means by which heat loss can occur is evaporation.Evaporation of sweat is decreased by increased humidity of the environment.5. Small amounts of heat are removed in the urine and Feces.
The preoptic & anterior hypothalamic nuclei contain two types of neurons :
A) Heat – Sensitive neurons(receptors) which are present in large numbers.They increase their rate of firing as the temp. rises.
B)Cold – sensitive neurons (receptors) which are less in number than the heat sensitive neurons.Their firing rate increase when the body temp. falls.Both of these types of cells function as temp. sensor for controlling body temp.
2. The posterior hypothalamus :The posterior hypothalamus contains a special area located bilaterally,approximately at the leval of the mammary bodies. This area receives signals from the anterior hypothalamus -preoptic area and from peripheral receptors where they are combined to provide mainly the heat producing and heat conserving reactions.
(it is the regulating C., it is the thermostat.)
A) The skin :- Where both cold and warmth receptors are present.However, there are far more cold receptors than warmth receptors. Therefore, skin receptors mainly concerns detection of cold rather than warm temp. of the body surface.
On the other hand, when the body temp. decreases below the set point. Another group of change takes place to decrease heat loss & increase heat production.
Temp. decreasing mechanisms activated by heat :
This occurs in almost all areas of the body.It is caused by inhibition of the sympathetic centers in the posterior hypothalamus that causes vasoconstriction. Cutaneous vasodilatation increase the rate of heat tranfer to the skin.
B) Decrease heat production :
II. Temp. increasing mechanisms activated by cold
1.)Cutaneous vasoconstriction :
Caused by stimulated of the posterior hypothslsmus sympathetic centers. This decreases the rate of heat transfer to the skin.
It is an involuntary response of the skeletal muscles which is controlled by an area in the posterior hypothalamus called the primary motor center for shivering .This area is normally inhibited by signals from the heat center in the anterior hypothalamus but is excited by cold signals from the skin and spinal cord.Therefore, this center becomes activated when the body temp. falls even slightly below 370C.
It then transmits signals down the brain stem and spinal cord to the anterior motor neurons to increase the tone of the skeletal muscles throughout the body. When the tone rises above a certain critical level, shivering begins.During Maximum shivering,body heat production can rise to as high as five times normal.
2. Semiconscious general increase in motor activity.
Fever ( pyrexia) :It means an elevation of core body temp. above the level which is normally maintained by the individual. It results when the set point of the hypothalamic temp. control system is elevated to a new point above 370C. Consequently, all the mechanisms for raising the body temp. are activated including shivering and cutaneous vasoconstriction. Within few hours the body Temperature approaches the new set point temperature which rarely exceeds 41.10C.
Therefore cytokines activate the preoptic area of the hypothalamus most probably through local release of prostaglandins.They may inter act directly with neutral tissues.Drugs that reduce the level of fever are called “antipyretics”e.g. aspirin.They act by blocking the synthesis of prostaglandins.
Symptoms of heat stroke include dizziness, abdominal distress, delirium and eventually loss of consciousness and death.These manifestations result from two causes :
At rectal temp. of about 280C, ability to spontaneously return the temp. to normal is lost. However, if the individual is rewarmed with external heat, he survives and returns to a normal state.