PRESSOR REACTIONS

From Big Medical Encyclopedia

PRESSOR REACTIONS (Latin pressio pressure) — the reactions of temporary increase in arterial pressure arising in response to an exercise stress, emotional pressure, pain and other types of irritations.

Allocate two types P. of river — local and the general, or system. The general P.' mechanisms of river it is much more difficult, than local. If local P. of river arise as results of reflex reduction of a gleam of limited number of vessels, then the general P. of river is reached as a result of interaction of several fiziol. mechanisms — changes of a gleam of blood vessels, level of cordial activity, redistribution of blood in various areas of an organism, change of rheological properties of blood, etc. Increase in frequency and force of cordial reductions leads to increase of minute volume, and reduction of a gleam of vessels — to increase in peripheric resistance in a circulatory bed.

Pressor influences are carried out by vasoconstrictive department of the vasomotor center (see) located in a myelencephalon at the bottom of the fourth ventricle. Impulses from a vasomotor center arrive to nervous cells of sympathetic department of a spinal cord, and from them on vasoconstrictor nerves to ring muscles of vessels (noradrenaline which is released in nerve terminations contacts cellular receptors of a membrane of muscle fiber and causes reduction of the corresponding muscle). Frequency of nervous impulses in tonic active vasoconstrictive fibers fluctuates from 0,3 to 8 impulses in 1 sec. Under the influence of this impulsation the muscular wall of vessels resides in a condition of tonic tension. For the first time it was shown in A. P. Walther's experiences (1842) and K. Bernard (1851). The tone of a vasomotor center depends on the exciting afferent impulsation which is continuously coming from chemoceptors of an aortic arch and a carotid sine (these receptors are very sensitive to decrease in partial tension of oxygen and carbon dioxide gas, strengthening of carbon monoxide, nicotine etc.). Excitement of a vasomotor center leads to narrowing of arterioles, reduction of a gleam of vessels and increase in arterial pressure (see). Similar receptors are found also in vessels of internals (a spleen, kidneys, adrenal glands). Increase in the ABP is caused also by the so-called interfaced vascular reflexes arising at irritation of any sensory nerve or large number of receptors of a body surface. Items of river can be received not only reflex, but also direct impact on structures oblong and a diencephalon. E.g., the electric irritation (see) causes the clear emotional and negative behavioural reaction which is followed by the river expressed to P. in an animal ventromedialny kernel of a hypothalamus (the ABP at the same time can increase by 40 — 50 mm of mercury.).

Under natural conditions at emotional pressure (see. Emotional stress ) descending excitement from a limbiko-reticular complex are transferred to bulbar department of a vasomotor center. Thanks to it tonic sympathetic influences on arterioles and heart amplify that leads to increase in the ABP. The ABP level remains raised until intensity of these vozbuzhdeniye remains. As a result of activity of protective depressor mechanisms of self-control of the ABP is returned to initial level (see. Depressory reactions).

In P.'s formation by the river the important role belongs also a nek-eye to humoral factors, in particular adrenaline (see) and to vasopressin (see). Adrenaline exerts the stimulating impact on action of the heart, strengthening and speeding up its work, narrows arteries and arterioles of skeletal muscles, abdominal organs, lungs, skin. Vasopressin preferential narrows arterioles and capillaries. The renin-angiotenzinnaya system has powerful pressor effect. The renin which is formed in a periarterial pad in kidneys in itself is not active and does not exert impact on vessels. However in a circulatory bed it promotes release of angiotensin I (see. Angiotensin ). Then physiologically inactive angiotensin I passes into angiotensin II. These transformations are most expressed during the passing of blood through lungs, however are observed also in other bodies. Angiotensin II possesses the expressed vasoconstrictive action and increases systolic and diastolic pressure. Besides, angiotensin II increases secretion of glucocorticoids and Aldosteronum by direct impact on cortical substance (bark) of adrenal glands. In normal conditions accumulation of angiotensin II in blood does not happen since it is unstable and quickly collapses group of enzymes — angiotensinases. Along with direct vasoconstrictive effect angiotensin II even in small, subthreshold, quantities in relation to direct vasoconstriction gradually strengthens influence of a sympathetic nervous system on heart and vessels.

In a human body the antagonism of pressor and depressory reactions is constantly observed, however normal cumulative effect of depressor influences is always higher than pressor. Along with the beginning of action of various extreme factors and especially after their elimination the special mechanisms of self-control directed to decrease in the ABP to the level necessary for normal metabolism turn on. It is provided with activity of special functional system (see).

Bibliography: Anokhin P. K. and Shumilina A. I. The analysis of afferent function of an aortal nerve in the conditions of the changing blood pressure, Fiziol, shurn. USSR, t. 33, No. 3, page 275, 1947; Konradi G. P. Regulation of a vascular tone, JI., 1973; K. V Pike perches. System mechanisms of an emotional stress, M., 1981; Physiology of blood circulation, Physiology of heart, under the editorship of V. N. Chernigovsky, L., 1980; Folkov B. and Neil E. Blood circulation, the lane with English, M., 1976; X and yu t and V. M. N, Sonina R. S. and Bast basket-in and E. V. Central organization of vasculomotor control, M., 1977; Neu-mansC. and. N e i 1 E. Reflexogenic areas of the cardiovascular system, L., 1958.

V. I. Badikov.

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