THERMAL CONTROL

From Big Medical Encyclopedia

THERMAL CONTROL (Greek therme warmth, heat + lat. regulare to direct, arrange) — physiological function of maintenance of constant body temperature by means of regulation of a thermolysis and heat production of an organism.

Normal at the person temperature of a brain, blood and internals on average apprx. 37 °. Fiziol. a limit of fluctuations of this temperature apprx. 1,5 °. Change of temperature of blood and internals on 2 — 2,5 ° from the average level is followed by disturbance fiziol. functions. Body temperature of the person is higher 43 ° almost is not compatible to life. Possibility of implementation normal fiziol. functions with so narrow temperature range defines value T. for maintenance of human life and gomoyotermny animals.

Special value T. gets in connection with development by the person of polar regions with a severe frigid climate, hot droughty districts, penetration into space, use of an ocean floor. An important role is played by T. in a wedge, practice at emergence natural (see. Fever ) and artificial feverish reaction (see. Pyrotherapy ), creation of the managed hypothermia (see. Hypothermia artificial ).

At temperature increase of the environment, direct action of a caloradiance, increase in rate of heat production of an organism (muscular work) in T. it is carried out by means of reactions of change of a thermolysis. These reactions in general carry the name of physical thermal control. Its major part is the vascular T., edges consists in change of a krovenapolneniye of skin and speed of a volume blood-groove through skin by change of the tone of its vessels regulated by a sympathetic nervous system. Increase in a krovenapolneniye of skin increases its heat conductivity and respectively a thermolysis of an organism due to direct carrying out (conduction) heat through skin. Increase in speed of a volume blood-groove increases transfer of heat from deep parts of a body to a surface due to strengthening of a teplomassopereno-s or convection. It is difficult to establish an exact quantitative ratio between two of these processes in a thermolysis of a live organism. However in the sum their efficiency is quite high. At the person the maximum vasodilatation of skin from a condition of the maximum narrowing reduces the total value of heat insulation of an integument on average by 6 times. Not all sites of a surface of skin it is equivalent participate in T. Hands have special value. From them it can be taken away to 60% of heat production of standard metabolism though the area of brushes makes apprx. only 6% of the general surface of an integument. During the muscular work sites of skin over the working muscles are of particular importance. A part of blood from the working muscles directs directly in veins of these sites of skin that considerably facilitates heat output from muscles by convection.

In process of approach of ambient temperature to body temperature efficiency of vascular T. other reaction of physical T falls and comes into effect. — sweating. Process of water percolation through an epithelium and the subsequent its evaporation carries the name of imperceptible perspiration. Due to this process about 20% of heat production of standard metabolism are absorbed. Imperceptible perspiration is not regulated and depends on ambient temperature a little. Therefore at threat of overheating the sympathetic nervous system stimulates work of sweat glands in skin. At intensive functioning of sweat glands about 1,5 l of sweat an hour and more are allocated. If to consider that for evaporation of 1 g of water from the surface of skin 0,58 kcal are spent, then at the maximum sweating from a body about 870 kcal an hour are taken away that it is enough for maintenance of standard temperature of a body at quite hard work in the conditions of the elevated temperature of the environment.

At fall of temperature of the environment and threat of cooling first of all sweating stops and there is vasoconstriction of skin. If temperature of skin continues to fall and the threat of cooling is not eliminated, joins so-called chemical T., the essence a cut consists in increase in heat production of an organism at the expense of special forms of sokratitelny activity of skeletal muscles and increase in physiological activity of other bodies (in particular a liver). At rather weak cooling in muscles at their visible rest periodically there are single reductions of separate fibers. This phenomenon received the name of a thermoregulatory muscle tone. On EMG to it there corresponds uniform peak electric activity with an amplitude of separate peaks of 10 — 50 mkv. This type of thermoregulatory activity of muscles can increase the general heat production of a human body and animals for 20 — 40%. At further cooling there is a cold muscular shiver. Against the background of a thermoregulatory tone it is shown by periodic series of bystry reductions. The size of separate peaks on EMG at a shiver reaches 200 — 500 mkv. Heat production of a human body at the Cold muscular shiver increases by 2 — 3 times and more. The thermoregulatory muscle tone and a shiver during the cooling most clearly come to light in muscles of the head, a neck and a shoulder girdle. The impulses causing a shiver are carried out to muscles on motor nerves. The spinal ways which are carrying out IMPULSES FOR the COLD shiver and any activity are various since after destruction of pyramidal paths in the paralyzed muscles it is possible to cause a cold shiver.

After long adaptation to cold heat production of muscular contractions at the Cold shiver and a thermoregulatory tone increases by 1,5 — 2 times. This process is stimulated with noradrenaline and thyroxine. Besides, after adaptation to cold the so-called nesokratitelny thermogenesis caused by generally very high level of metabolism and heat production of brown fatty tissue develops, weight a cut after adaptation to cold considerably increases. High intensity of oxidizing processes is stimulated in cells of brown fatty tissue with a sympathetic nervous system.

Management of all reactions, to-rye allow to maintain constant body temperature in various conditions, it is carried out by the special nerve centers localized in a brain. These centers obtain information on conduction paths from the thermosensitive neurons which are located in various parts of c. N of page, and from peripheral thermoreceptors.

For a peripheral thermoesthesia thermoreceptors of skin have the greatest value (see. Thermoreceptors ), the representing free nerve terminations. Distinguish cold thermoreceptors with a maximum of frequency of an impulsation at a temperature of skin of 25 — 30 ° and thermal — with a maximum apprx. 40 °. Thermoreceptors are located in surface layers of skin directly under an epithelium, and also in deep skin layers and in walls of hypodermic blood vessels. Impulses from thermoreceptors of skin go to spinal ganglions. From the second neuron located in back horns of a spinal cord, the fibers which are carrying out impulses from thermoreceptors come over to the opposite side of a spinal cord and as a part of a spinotalamichesky path reach ventrobazalny kernels of a thalamus. A part of the nerve fibrils which are carrying out temperature signals does not cross and goes to the highest centers through gray matter of back horns. In a spinal cord specific cold and thermal thermosensitive neurons are also found, to-rye perceive local changes of temperature and transmit the corresponding signals to the highest centers. After switching to neurons of a thalamus a part of the fibers bearing temperature information goes to a back hypothalamus, and other part reaches touch zones of bark from where on the descending ways the corresponding signals can be also sent to a hypothalamus. Specific thermosensitive neurons are found on average a brain and bark. Assume that as system T. amygdala and a hippocampus participate. However main center T. the hypothalamus is. In a front hypothalamus the greatest value for T. the front pre-optical area has, a large number of nervous cells of 8x8 in size — 40 x 12 microns of a polygonal, spindle-shaped or ovoidny form contains edges. Consider that this area of a hypothalamus regulates processes of a thermolysis since destruction it causes sharp overheating of an organism in experimental animals. In a back hypothalamus the most important role in T. belongs ventro-and to dorsomedial kernels. They contain nervous cells of also various form of 7 X 8 in size — 11 X 13 microns. Kernels of a back hypothalamus consider the center of heat generation as destruction leads them to a resistant hypothermia.

In a hypothalamus distinguish 3 groups of the neurons connected with T. K to the first group carry considerable number of the neurons answering with increase in an impulsation direct cooling or warming; their sensitivity to local changes of temperature is very high. The second group is made by neurons, to-rye do not answer local changes of temperature, but react to an impulsation from thermoreceptors. The third group of neurons obtains information both from local thermosensitive elements, and from peripheral thermoreceptors; these neurons integrate temperature signals from various thermosensitive structures of a body and participate in development of the incentives managing specific thermoregulatory reactions. Most the importance in integration of a thermoesthesia belongs to kernels of a back hypothalamus, to-rye contain especially many such neurons. Specific transmitters of excitement for neurons of the center T. acetylcholine, serotonin or noradrenaline serve. Specific changes of excitability of neurons of the center T. can be caused by changes of ion concentration of sodium and calcium in cerebral cavities. Structure and functions of the center T. and all system in general are displayed in Hammel's schemes (N. by Hammel), K. P. Ivanov, J. Bligh.

Assume that system T. reacts to changes of the sum of temperature of the central and peripheral points of a body and the main subject to its regulation is average body temperature, maintenance a cut is carried out with high precision. At the person in a zone of temperature comfort (t ° 28 — 31 ° for the naked person) vascular reaction of T. develops at change of average body temperature on only 0,1 ° or less.

In fauna distinguish go-moyotermny and poikilothermic animals (see Gomoyotermny animals, P oykiloter mny animals). Birds and mammals, including the person treat the first. They are capable to maintain constant body temperature at the level of 36 — 41 ° at quite considerable temperature variations of the environment. This ability is defined by a high level of heat production of standard metabolism, heat-insulating properties of a cover of a body and existence special fiziol. reactions of T., described above. The T is most perfect. at the person, primacies and predatory though at them it depends on the sizes of a body, heat insulation, behavior and other factors. At the lowest mammals the lowered level of heat production at standard metabolism takes place and the lowered body temperature. At marsupials body temperature makes apprx. 35 °, at single-pass (an echidna, a duck-bill) apprx. 33 °. However these animals possess intensive reactions of T. and also maintain body temperature on a fixed level.

Life activity of poikilothermic animals substantially is defined by ambient temperature though many of them possess fiziol. the reactions counteracting overheating or overcooling of an organism. Reactions of expansion and vasoconstriction of skin, heat output by overbreathing (polypnea) are found in lizards, e.g. Practically all poykplotermny animals possess behavioural T., edges consists in the active choice of ambient temperature.

T. at gomoyotermny animals and the person in the post-natal period has nek-ry features. At premature children is right after the birth of T. practically is absent and develops only in 1 — 2 month. The children born in time show reactions of chemical and physical T. in several hours after the birth though these reactions are not so perfect, as at adults. Newborn mammals (mature and nezrelorozhdayushchiyesya) find reactions of T. almost right after the birth, but these reactions are still ineffective and cannot maintain constant body temperature even at rather small temperature variations of the environment. In the course of further development efficiency of reaction of T. quickly increases. Other feature of T. at newborn gomoyotermny animals consists in use for the strengthened heat production of brown fatty tissue, edges it is localized in mezhlopatoch-ache also axillary areas, on the course of large blood vessels. An intensive physical activity at threat of cooling is characteristic of a number of newborn mammals that, perhaps, reflects phylogenetic more ancient adaptive mechanism T. — active search of adequate ambient temperature.

At zimnespyashchy mammals (groundhogs, gophers, sleepyheads, etc.) the gomoyotermiya is periodically broken: body temperature them during hibernation only by several degrees exceeds a freezing point, during the periods between hibernations these animals have the constant body temperature inherent to gomoyotermny organisms, and find intensive reactions of T. Osobennostyyu T. at feverish reaction (see Fever) the elevated temperature of a body is, edges long time can remain. The mechanism of this phenomenon consists in reduction of the threshold of so-called cold neurons of a hypothalamus and in increase in a threshold of its so-called thermal neurons.

Disturbances of T. can arise at damage of the central and peripheral offices of a thermoesthesia — hemorrhages, tumors in the field of a hypothalamus, nek-ry infections (e.g., a leprosy), and also a traumatic break of conduction paths.

See also Thermolysis , Heat production .



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K. P. Ivanov.

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