CNS

body temperature regulation

Normal Body Temperatures
We have Core Temperature and Skin Temperature
Core Temperature: The temperature of the deep tissues of the body, the “core” remains very constant(±0.6°C) day in and day out, except when a person develops a febrile illness.
Skin temperature: rises and falls with the temperature of the surroundings. The skin temperature is the important when we refer to the skin’s ability to lose heat to the surroundings.
Normal core temperatures measured orally,(36°C-37.5°C) and about0.6°C higher when measured rectally.

Body temperature is controlled by balancing heat production against heat loss

When the rate of heat production in the body is greater than the rate of heat loss, the body temperature rises.
Conversely, when heat loss is greater, body temperature decrease.

Heat Production

Heat production is a principal by-product of metabolism.
The most important of these factors are:
(1) basal metabolic rate of all the cells of the body;
(2) extra rate of metabolism caused by muscle activity, including muscle contractions caused by shivering;
(3)extra metabolism caused by the effect of thyroxin
(4) Extra metabolism caused by the effect the sympathetic stimulation on the cells.
)5) extra metabolism caused by increased chemical activity in the cells themselves.


Heat Loss:
Most of the heat produced in the body is generated in the deep organs, especially in the liver, brain, and heart, and in the skeletal muscles during exercise.
Then this heat is transferred to the skin, where it is lost to the air and other surroundings.
The rate of heat loss is determined by two factors:
(1) how rapidly heat can be conducted from body core to the skin
(2) how rapidly heat can be transferred from the skin to the surroundings.

Insulator System of the Body

The skin, the subcutaneous tissues, and especially thefat of the subcutaneous tissues act together as a heatinsulator for the body. The fat is important because itconducts heat only one third as readily as other tissues.
Blood vessels are distributed profusely beneath theskin.A highrate of skin flow causes heat to be conducted from thecore of the body to the skin and then conductance into the air with great efficiency,whereas reduction in the rate of skin flow can decreasethe heat conduction from the core to very little. Therefore, the skin is an effective controlled “heatradiator” system, and the flow of blood to the skin is a most effective mechanism for heat transfer from thebody core to the skin.
Heat conduction to the skin by theblood is controlled by the degree of vasoconstriction. Thisvasoconstriction is controlled by thesympathetic nervous system.

How Heat Is Lost from the Skin Surface?

The various methods by which heat is lost from theskin to the surroundings. Include:
1-Radiation; in a nude person sitting inside at normal room temperature. The human body radiates heat rays in all directions and radiated from the walls of rooms and other objects toward the body.
2-Conduction: about 3 %, are normally lost from the surface of the body to solid objects, such as a chair or a bed by direct conduction.
3-Convection:in a nude person seated in a comfortable room without gross air movement, about 15 %of total heat loss occurs by conduction to the air and then by air convection away from the body.

4-Evaporation: When water evaporates from the body surface, heat is lost. Even when a person is not sweating, water still evaporates insensibly from the skin and lungs at a rate of about 600 to700 ml/day.
Evaporation Is a Necessary Cooling Mechanism. As long as skin temperature is greater than the temperature of the surroundings, heat can be lost by radiation and conduction. But when the temperature of the surroundings becomes greater than that of the skin, the body gains heat by both radiation and conduction. Under these conditions the only means by which the body can rid itself of heat is by evaporation.


Therefore, anything that prevents adequate evaporation when the surrounding temperature is higher than the skin temperature will cause the internal body temperature to rise. This occurs occasionally in human beings who are born with congenital absence of sweat glands(These people can stand cold temperatures as normal people can, but they are likely to die of heatstroke because they cannot prevent a rise in body temperature when the air temperature is above that of the body).

Therefore, anything that prevents adequate evaporation when the surrounding temperature is higher than the skin temperature will cause the internal body temperature to rise. This occurs occasionally in human beings who are born with congenital absence of sweat glands(These people can stand cold temperatures as normal people can, but they are likely to die of heatstroke because they cannot prevent a rise in body temperature when the air temperature is above that of the body).

Effect of Clothing on Conductive Heat Loss.

A usual suit of clothes decreases the rate of heat loss to about half that from the nude body because the rate of heat loss from the body by conduction and convection is greatly depressed.
The effectiveness of clothing in maintaining body temperature is almost completely lost when the clothing becomes wet, because the high conductivity of water increases the rate of heat transmission through cloth 20-fold or more.

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Mechanism of Sweat Secretion
The sweat gland is a tubular structure consisting of two parts:
• a deep subdermal coiled portion that secretes the sweat, and
• a duct portion that passes outward.
The cholinergic sympathetic nerve fibers elicit the secretion. The secretory portion of the sweat gland secretes a fluid called the primary secretion or precursor secretion (an active secretory product of the epithelial cells lining the coiled portion).
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The composition of sweat is similar to that of plasma, except that it does not contain plasma proteins(protein-free secretion).
The concentration of sodium is about 142 mEq/L and that of chloride is about104 mEq/L, with much smaller concentrations of the other solutes of plasma.


Acclimatization of the Sweating Although a normal, unacclimatized person seldom produces more than about 1 liter of sweat per hour, when this person is exposed to hot weather for 1 to 6 weeks, he begins to sweat more profusely, often increasing maximum sweat production to as much as 2 to 3 L/hour. Evaporation of this much sweat can remove heat from the body at rate more than 10 times the normal basal rate of heat production. This increased effectiveness of the sweating mechanism is caused by a change in the internal sweat gland cells themselves to increase their sweating capability.
Also associated with acclimatization is a further decrease in the concentration of sodium chloride in the sweat, which allows progressively better conservation of body salt. Most of this effect is caused by increased secretion of aldosterone by the adrenocortical glands, which results from a slight decrease in sodium chloride concentration in the extracellular fluid and plasma.

Regulation of BodyTemperature—Role of theHypothalamus

The temperature of the body is regulated by nervous feedback mechanisms, and all these operate through temperature-regulating centers located in the hypothalamus.
For these feed backmechanisms to operate, there must also be temperature detectors to determine when the body temperature becomes either too high or too low.
The anteriorhypothalamicpreoptic area contain large numbers of heat-sensitive neurons and cold-sensitive neurons. These neurons are functioning as temperature sensors for controlling body temperature. The heat-sensitive neurons increase their firing rate in response to increase in body temperature. The cold-sensitive neurons, by contrast, increase their firing rate when the body temperature falls ,it is clear that the hypothalamicpreoptic area has the capability to serve as a thermostaticbody temperature control center.

Temperature receptors

The skin receptors and deep receptors
The skin has both cold and warmth receptors. There are far more cold receptors than warmth receptors. Therefore, peripheral detection of temperature mainly concerns detecting cold instead of warm temperatures

Deep body temperature receptors

Are found mainly in the spinal cord, in the abdominal viscera, and in or around the great veins in the upper abdomen and thorax.
These deep receptors function differently from the skin receptors because they are exposed to the body core temperature rather than the body surface temperature. Yet, like the skin temperature receptors, they detect mainly cold rather than warmth. It is probable that both the skin and the deep body receptors are concerned with preventing hypothermia—that is, preventing low body temperature.

Posterior Hypothalamus Integrates the Central and Peripheral Temperature

sensory signals arise in peripheral receptors, contribute to body temperature control mainly through the hypothalamus.
The area of the hypothalamus that they stimulate is located bilaterally in the posterior hypothalamus approximately at the level of the mammillary bodies.
When the hypothalamic temperature centers detect that the body temperature is either too high or too low, they institute appropriate temperature-decreasing or temperature-increasing procedures are the following.


Temperature-Decreasing Mechanisms
When the Body Is Too Hot, The temperature control system uses the following to reduce body heat:
-Vasodilation of skin blood vessels: This is caused by inhibition of the sympathetic centers in the posterior hypothalamus that cause vasoconstriction.
-Sweating. An additional 1°C increase in body temperature causes enough sweating to remove 10times the basal rate of body heat production
-Decrease in heat production by inhibiting The mechanisms that cause excess heat production, such as shivering and chemical thermogenesis.

Temperature-Increasing Mechanisms

When the body is too cold:
-Skin vasoconstriction throughout the body. This is caused by stimulation of the posteriorhypothalamic sympathetic centers
-Piloerection. means hairs “standingon end.” Sympathetic stimulation causes the piloerectormuscles attached to the hair follicles to contract, which brings the hairs to an upright stance.
-Increase in thermogenesis (heat production
-promoting shivering. During maximum shivering, body heat production can rise to four to five times normal.
-sympathetic excitation of heat production
-thyroxine secretion

Behavioral Control of BodyTemperature

Aside from the subconscious mechanisms for body temperature control, the body has another temperature-control mechanism that is even more potent. Whenever the internal body temperature becomes too high, signals from the temperature-controlling areas in the brain give the person a psychic sensation of being overheated.
Conversely ,whenever the body becomes too cold, signals from the skin and probably also from some deep body receptors elicit the feeling of cold discomfort. Therefore, the person makes appropriate environmental adjustments to re-establish comfort, such as moving into a heated room or wearing well-insulated clothing in freezing weather.

“Set-Point”for Temperature Control

A critical body core temperature of about 37.1°C.
above this level, the rate of heat loss is greater than that of heat production, so the body temperature falls
At temperatures below this level, the rate of heat production is greater than that of heat loss.
This crucial temperature level is called the “set-point” of the temperature control mechanism.


Abnormalities of Body Temperature Regulation
Fever
Fever, which means a body temperature above the usual range of normal. Some causes of fever they include :bacterial diseases, brain tumors, and environmental conditions that may terminate in heatstroke
Many proteins, breakdown products of proteins, and certain other substances, released from bacterial cell membranes, can cause the set-point of the hypothalamic thermostat to rise. Substances that cause this effect are called pyrogens.
Drugs such as aspirin that reduce fever are called antipyretics.
Aspirin impedes the formation of prostaglandins from arachidonic acid.

Characteristics of Febrile Conditions

Chills: When the set-point of the hypothalamic temperature-control center is suddenly changed from the normal level to higher than normal the body temperature usually takes several hours to reach the new temperature set-point.
The usual responses that cause elevation of body temperature occur. During this period, the person experiences chills and feels extremely cold, even though the body temperature may already be above normal. Also, the skin becomes cold because of vasoconstriction,and the person shivers. Chills can continue until the body temperature reaches the hypothalamic set-point. Then the person no longer experiences chills.

Crisis, or “Flush”:

The sudden change of events in a febrile state is known as the “crisis” or, the “flush.”

In this situationis analogous to excessive heating of the anteriorhypothalamic-preoptic area, which causes intense sweating and the sudden development of hot skin because of vasodilation everywhere.

Heatstroke

When the body temperature rises beyond a critical temperature, into the range of41°Cto 42°C, heatstroke is developed.
(dizziness, abdominal distress ,vomiting, sometimes delirium, and eventually loss of consciousness) These symptoms are often exacerbated by adegree of circulatory shock brought on by excessive loss of fluid and electrolytes in the sweat.
The hyperpyrexia itself is also exceedingly damaging to the body tissues, especially the brain.
sponge or spray cooling of the skin is likely to be more effective for rapidly decreasing the body core temperature.


Exposure of the Body toExtreme Cold

A person exposed to ice water for 20 to 30 minutes ordinarily dies because of heart standstill or heart fibrillation. If warmed rapidly by the application of external heat, the person’s life can often be saved.
Once the body temperature has fallen below about 30°C, the ability of the hypothalamus to regulate temperature is lost. Frostbite. When the body is exposed to extremely low temperatures, surface areas can freeze; the freezing is called frostbite. This occurs especially in the lobes of the ears and in the digits of the hands and feet.