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Body Temperature And Its Regulation . Normal Body Temperature :
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Body Temperature And Its Regulation 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.
Physiological variations in body temperature • Normally, the body temp. undergoes a regular circadian fluctuation of about 0.60C being lowest in the morning & highest in the evening. • In Woman there is a monthly cycle of temp. variation characterized by a rise in basal temp. of about 0.50 C at the time of ovulation &during the second half of the menstrual cycle.A similar rise occurs during the first trimester of pregnancy.
3. In children temp. regulation is less precise and they may normally have a temp. that is 0.50C above the normal for adults. 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.
The balance between heat production & heat loss : • The body temp. is kept constant when the rate of heat production in the body is equal to the rate of heat loss. • Heat is produced in the body by the basal rate of metabolism, contraction of skeletal muscles, food ingestion and extra metabolism caused by the slow but prolonged effect of thyroxin on the cells and the rapid but short lived effect of epinephrine, nor epinephrine and sympathetic effects on the cells.
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 : • Conduction of heat from the deeper organs and tissues to the skin. • Transfer of heat from the skin to the surrounding.
A.) Heat Conduction to the Skin • The skin and the subcutaneous tissues especially the fat form a heat insulator system for the body. This system maintains the normal core temp., although the temp. of the skin may approach the temp.of the surrounding.Blood vessels penetrate the fatty subcutaneous tissues and are distribution beneath the skin. The amount of heat reaching the skin from the deep tissues depends upon the blood flow into the cutaneous blood vessels which is determined by the degree of vasoconstriction of the vessels.This vasoconstriction in turn is controlled almost entirely by the sympathetic nervous system.The rate of heat conduction to the skin is known as “tissue conductance.”
B.) Heat loss from the skin surface 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.
2. Conduction : • It means heat exchange between objects at different temp. that are in contact with one another. Only minute amount of heat are normally lost from the body by direct conduction to other objects such as a chair or bed (3%). On the other hand large amounts of heat are lost by conduction to air.
3. Convection : • It is the removal of heat from the body by convection air currents.Heat must first be conducted to the air and then carried away by the convection current.About 12% of heat loss from the body occurs by conduction to the air and then by convection.When the body is exposed to wind heat loss by convection is greatly increased.
4. Evaporation : • Evaporation of 1g water removes about 0.6 Kcal. Of heat. Even when a person is not sweating a certain amount of water still evaporates from the skin and lungs at a rate of about 600 ml/day. This is known as the insensible water loss. • Sweating provides a very important way of heat loss from the body which can be regulated. As long as skin temp. is greater than the temp. of surrounding, heat can be lost by radiation & conduction, but when the temp. of the body gains heat by radiation& conduction.
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.
Temperature regulating centres : • The temp. of the body is regulated almost entirely by temp. regulating centers located in the hypothalamus. • The anterior hypothalamus-preoptic area : 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.)
Peripheral receptors for detection of Temperature • These receptors play an important role in temp. regulation. They are present in the following sites: 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.
B) Deep body tissues : • Mainly in the spinal cord, in the abdominal viscera, and around the great veins. These receptors detect body core temp. rather than the body surface temperature. • Yet . Like the skin receptors they detect cold. Therefore, both the skin and deep body receptors are concerned with preventing low body temperatures.
Temp.– regulating mechanisms : • When the body core temp. either increase above or decreases below almost exactly 37o C, several thermoregulatory responses take place to bring it back to this temp. Therefore, this critical temp. level is called the”set-point” of the temp. control system. • The temp. regulating mechanisims include autonomic,somatic endocrine & behavioral changes.When the body temp. increases above the critical temp,(the set-point in the hypothalamus),one group of these change take place to increase heat loss & decreases heat production.
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 : • Increase heat loss : • Cutaneous vaso dilatation : 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.
2. Sweating : • When the anterior hypothalamus-preoptic area is stimulated by heat, impulses are transmitted pathways to the spinal cord and then through the sympathetic cholinergic fibers to the sweat glands to increase their secretion.This is turn increases the evaporative heat loss. Sweat gland can also be stimulated by epinephrine or nor epinephrine circulating in the blood. This is important during exercise.
Acclimatization of the sweating mechanism : • When a normal person is exposed to hot weather for 1 to 6 weeks two changes take place which are called acclimatization of the sweating mechanism. • These are : • Sweat production increases to as much 2 liters/hour. • Decreased concentration of sodium chloride in the sweat caused by increased secretion of aldosterone.
3. Behavioral responses : B) Decrease heat production : • Strong inhibition of mechanisms which cause excess heat production. • Anorexia. • Apathy and inertia II. Temp. increasing mechanisms activated by cold • Decrease heat loss : 1.)Cutaneous vasoconstriction : Caused by stimulated of the posterior hypothslsmus sympathetic centers. This decreases the rate of heat transfer to the skin.
2. Pilo erection • Which is caused by sympathetic stimulation of the erector pili muscles.This mechanism is not important in the human being and is manifested by “goose skin”, but in lower animals upright projection of the hairs allows them to entrap a thick layer of “ insulator air” next to the skin. • 3.)Behavioral responses.(to put on suitable clothes.)
B)Increase heat production: • Shivering : 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.
3. Increased secretion of nor epinephrine and epinephrine • This can cause an immediate increase in the rate of cellular metabolism. This effect is called “chemical thermo genesis” and it results mainly from uncouple oxidative phosphorylation. This process occurs in brown fat. Adults do not have a significant amount of this type of fat, therefore chemical thermo genesis increases the rate of heat production only 10%.On the other hand, infants have some brown fat in the interscapular space & chemical thermo genesis can increase the rate of heat production as much as 100% which is very important factor in maintaining their normal body temp.
4)Increased thyroxin secretion • Exposure to cold increase the production of thyrotropin-releasing hormone by the hypothalamus which stimulates the secretion of the thyroid stimulating hormone (TSH) from the anterior pituitary gland. • TSH in turn stimulates increased output of thyroid gland.Thyroxin increase the rate of cellular metabolism throughout the body I.e.it is another mechanism of chemical thermo genesis .However, this increase in metabolism requires several weeks for the thyroid gland to hypertrophy before it reaches its new level of thyroxin secretion. Moreover, it is of little significance in adult humans.
Abnormalities Of Body Temp. Regulation 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.
Pathogenesis Of Fever : • Substances which elevate the set point are called “pyrogens” and they include bacterial toxins,components of various micro organisms and products of tissue degeneration.These substances act on cells of the immune system including monocytes,macrophages and kupffer cells to produce cytokines that act as endogenous pyrogens (EPS)eg. Interleukin 6 (IL-6), tumour necrosis factor or (TNF), IL-1 and several interferons.
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. • Fever, within limits, is presumably beneficial because it may inhibit the growth of many micro organisms and increase antibody production. • However very high temp. are harmful.When the rectal temp. is over 410C for prolonged periods, some permanent brain damage results.
Hyperthermia : • It means an elevation of body temp. higher than the thermoregulatory set point, usually due to an exogenous cause. • It may be caused by : • Increased heat production e.g. by severe muscular exercise or high environmental temp., which exceeds the normal capacity of heat loss mechanism. • Impaired heat loss e.g. due to high humidity or drugs that impair sweating such as anticholinergics.
3) Brain tumors compressing the hypothalamus. • When the rectal temp. is over 430C, the person develops “heat stroke” which commonly causes death because : • There is a limit to the rate at which the body can loss heat even with maximal sweating. • When the hypothalamus becomes excessively heated,its heat regulating ability becomes greatly depressed and sweating diminishes. As a result a high body temp. tends to perpetuate itself.
Symptoms of heat stroke include dizziness, abdominal distress, delirium and eventually loss of consciousness and death.These manifestations result from two causes : • Direct damaging effect of very high body temperature on all body tissues especially the brain. • Some degree of circulatory shock caused by excessive loss of fluid and electrolytes in the sweat.
Treatment of hyperthermia exceeding 41.1oC must be immediate • Removal from direct sunlight, removal of clothing, wetting the body surface and fanning are simple measures which can be undertaken on the spot.If these are not enough the most effective action is to immerse the patient in the ice water bath while monitoring core temperature to be certain that a state of hypothermia is not induced.
Hypothermia : • It is defined as core body temperature of less than 350C. • Hypothermia is caused by exposure to cold especially when associated with other conditions such as advanced age, decreased metabolic rate, CNS diseases, malnutrition, drugs as alcohol and paralysis. • Effects of hypothermia on the body include depressed mental status followed by loss of consciousness, shivering which stops below 320C, very slow respiration, low heart rate, decreased blood pressure and arrhythmias .
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. • As humans tolerate body temp. of 210-240C without permanent ill effects, “induced hypothermia” has been used extensively in surgery especially heart and brain operations. The circulation can be stopped for relatively long periods because the oxygen need of the tissue are greatly decreased.Also the blood pressure is low and bleeding in minimal.