ACCELERATION

From Big Medical Encyclopedia

ACCELERATION, action on an organism — the change of speed in unit of time rendering depending on size, duration and the direction of influence considerable influence on a human body.

Unlike speed, the edge subjectively is not perceived by the person, At. it is felt as it since arises owing to action of force on any moving body having weight including and on a live organism. Therefore At. in inverse proportion to body weight, it is proportional to force causing it and matches it in the direction.

Preservation of a condition of rest or the uniform rectilinear motion is peculiar to any body (i.e. inertia). When on the way of the movement of a body there is resistance and there is a delay of speed of the movement or dead stop of a body, force of inertia is directed towards the movement. In air and space medicine such influence of force of inertia is designated by the term «overload». The overload shows, in how many time force causing acceleration or braking of the movement of a body is more than its weight. The gravity determined by an attraction of Earth causes free fall of a body with At., equal 9,81 m / сек2, that is At., the equal 9,81 m / сек2, are caused by action of force equal to body weight. Acceleration of gravity is called a gravitational constant and is designated by a letter G (gravitas). Therefore size U., equal, e.g., 5 G, means that the overload will be equal 5 and that body weight in these conditions increases by 5 times in comparison with initial. Therefore, speaking about influence At., mean that mechanical force, edges influences the person at present, and speaking about overloads, mean the relation of the operating force to body weight.

Under the terms of emergence and according to characteristics of impact on an organism distinguish three look At.: rectilinear, radial and angular, or Coriolis's acceleration (see Cory-olisa acceleration).

Rectilinear At. arises at the movement of a body on a straight line during change of speed without change of the direction of the movement (at take off and landing of the airplane or the spaceship, at change of speed of rectilinear flight, at disclosure of a parachute, a landing of the parachutist). On duration of impact on an organism At. divide on long (more than one second) and short-term (less than one second), or shock, acceleration.

Radial, or centripetal, At. arises owing to change of the direction of the movement of a body without change of speed of its movement (at an entrance and an exit of the airplane from a dive, at bends, rotation on the centrifuge, etc.) * It in direct ratio to a square of speed and in inverse proportion to the radius of the movement of a body.

Coriolis's acceleration develops at rotation of a body around any axis and simultaneous change of radius of gyration.

Short-term, or shock, At. arises at ejection (see), crash landing of airplanes, etc. Its effects are shown with the broad range: from small concussions of a body and reversible deformation of separate bodies and fabrics to morfol. disturbances of structure of fabrics.

The main pathogenetic mechanism of impact on an organism long At. the air hunger (see the Hypoxia) arising owing to local or system disturbance of blood circulation (see) is. Portability long At., as well as development of compensatory reactions of an organism, depends on size, duration of influence At., speeds of their increase (a gradient of increase) up to the maximum size, from the direction in relation to a body of the person, the state of health of the person, conditions of the environment, etc.

The description of action of the overloads which are most often found in flight practice is included below.

Influence of overloads at action of acceleration in the direction from the head to legs (-\-Gz). Increase in body weight is the most general consequence of influence of overloads of this type. At +2 Gz deformation of the person is observed, increase in body weight, extremities clearly is felt. At influence - j-3 Gz of people is not able to rise independently from a seat. At further increase of size of overloads the shift of internals, change of blood supply of parts of a body, the deterioration in sight preceding disturbance of consciousness is observed. Deterioration in sight begins with loss of peripheral sight at safety central more often, then the so-called gray veil appears. The size of overloads, at to-rykh appears a gray veil, depends on degree of individual endurance of the person, his pose and some other factors. Usually vision disorders arise during the first 5 — 6 sec. of influence of an overload. Total loss of sight — a so-called black veil follows usually after emergence of a gray veil, however often total loss of sight happens without preliminary loss of peripheral sight. Sight is recovered usually in 3 — 5 sec. after cancellation of an overload.

At continuation of influence of an overload after loss of sight there can come the loss of consciousness; at higher values it can be observed also without emergence of visual disturbances. At the same time falling of a muscle tone, generalized spasms are noted. Recovery of consciousness happens during 30 — 60 sec. after the end of influence of an overload.

Influence of overloads on sight is caused by change of a blood-groove in vessels of eyes and a brain in connection with disturbances of pressure profile of blood in cardiovascular system. Blood in the greatest measure is exposed to shift under influence At., as does not possess strong intercommunications, and blood vessels have high elasticity. In vessels of the lower extremities there is a deposition of blood, transudation of liquid in fabrics and respectively a degrowth of the circulating blood; at sizes of the overloads exceeding +4 Gz there are petekhialny hemorrhages. At +5 Gz difficulty of breath is noted, lung ventilation increases, owing to lowering of a diaphragm and abdominal organs the residual capacity of lungs increases; the termination of a blood-groove in upper parts of lungs at big sizes of accelerations (overloads) leads to their partial atelectasis that is followed by fits of coughing, difficulty of a deep breath, a sensation of discomfort.

Impact At. on an organism increases its oxygen requirement. So, e.g., at +5 Gz oxygen consumption increases three times. Besides, after action At. the increased oxygen consumption is for 3 — 5 min. at the level exceeding initial sizes for 50 — 70%.

Action of radial acceleration in the direction from legs to the head (— Gz) takes place during the performance of a loop of Nesterov, during rotation of the airplane at a corkscrew. The overloads arising at the same time in aviation practice do not reach big sizes. At the same time resistance to them is much lower, than to overloads of type ~\~ GZ. The discomfort arises already at — 2 Gz. At the same time working capacity is considerably broken.

PARTICLE ACCELERATORS 123


Main mechanism fiziol. shifts it is caused by increase in the ABP in vessels of an upper half of a body of the person, including in vessels of the head. The last causes «reddening» of a field of vision (a so-called redout), a severe headache, the century, emergence of petechias on face skin and necks swelled. At — 3Gz there is a feeling of a raspiraniye of eyeglobes, plentiful dacryagogue, sub-conjunctival hemorrhages. At big overloads already in

5 — 6 sec. after the beginning of influence there is a disturbance, and then and a loss of consciousness.

Increase in the ABP in vessels of a neck leads to stimulation of a carotid sine, emergence bradikardip and dilatation of arterioles. Owing to vagal efferent signals the cordial rhythm up to a total atrioventricular block is broken (see the Heart block).

The positive overloads influencing in the direction the head — legs are transferred much easier.

Influence of the overloads directed perpendicularly to a longitudinal axis of a body of the person (+ of £ *) • In aviation practice of an overload of this type meet, e.g., at ejection (braking in an air flow). During the flights in spaceships these overloads reach size from +6 to - f 10 and several minutes (last at start of the rocket).

Because the main main blood vessels of the person are located hl. obr. along a longitudinal axis of a body, and also because of limited opportunities of shift of internals at influence of overloads like +GX, considerable changes of blood circulation do not happen. The main reason limiting portability of an overload is difficulty of breath.

Strictly transverse direction of action of overloads practically does not meet since the trunk of the pilot in a cabin of the aircraft is located under uglokhm 12 — 25 ° in relation to the direction of action of overloads (in such situation the smallest disturbances of activity of respiratory system are noted).

At overloads of type - \-Gz the limit of stability more depends on the speed of increase U. Diapazon of individual stability is ranging from +3 Gz to + 8 Gz. In addition to the speed of turning on of compensatory mechanisms and their power, resistance to action At. depends on many other factors. So, it decreases at fervescence, a hypoglycemia, after alcohol intake, at a hyperventilation, a hypoxia etc.

Protection of an organism against action of long accelerations. Treat number of the most effective actions directed to increase in resistance of a human body to influence of overloads: observance of a work-rest schedule, physical training and training, use of anti-overload devices, comfortable conditions of dwelling.

Increase in endurance at

1 — 2 Gz can be caused by a muscle tension due to increase in return of a venous blood to heart, reduction of deposition of blood in the lower extremities and increases in vnut-ribryushny pressure.

An effective remedy is the forced exhalation at the closed glottis with a simultaneous natuzhivaniye (Valsalva's test). At the same time there is an increase in intrathoracic pressure, a cut is transferred to all circulatory system. Sometimes use so-called method of M-1. In essence it is the modified Valsalva's test: the slowed-down forced exhalation through partially close glottis. After a bystry breath reception can be repeated. Use of reception of M-1 in combination with use of an anti-overload suit allows to increase endurance of a human body to action of overloads on 2 Gz. Approximately on as much increases endurance to At. breath under excessive pressure.

To one of means of increase in endurance of an organism to action of overloads serve the change of a pose of the person reducing vertical distance in the direction from heart to eyes. (Writhe-vaniye) raising of legs, reducing dilatation of vessels of the lower extremities, also promotes moving away of a threshold of approach of a gray veil. The bigger effect can be gained, increasing a tilt angle of a chair, i.e. changing the direction of action of an overload.

The greatest value in protection of a human body against action At. has an irotivoperegruzochny suit. The existing designs of anti-overload suits (the II personal computer) are based on the principle of creation on

vyshenny pressure upon sites of the lower half of a body of the person. Pressure is created in the rubber bags which are built in in a suit in a shin, hips and a stomach. Cameras are connected among themselves and have the general hose. Pressure of the air moving in PPK is regulated automatically depending on the size of the operating acceleration. The PPK cameras create counter-pressure to moving of blood to the lower half of a body, increasing return of a venous blood to heart and by that promoting strengthening of blood supply of a brain. PPK also counteracts the shift of internals, prevents the shift of heart down that also increases portability of overloads. Bibliography: Air medicine, under the editorship of A. N. Babiychuk, page 96, M., 1980;

Fundamentals of bioastronautics and medicine, under the editorship of O. G. Gazenko and M. Calvin, t. 2, book 1, page 141, M., 1975; Sergeyev A. A. Physiological mechanisms of action of accelerations, L., 1967, bibliogr.; Stasevich R. A. and Isakov P. K. Speeds, accelerations, overloads, M., 1956; V. V Sagittariuses. Influence of big overloads on an organism of the pilot, Civil aviation, No. 7, page 23, 1940; Aviation medicine, ed. by J. Ernsting, v. 1, p. 208, L., 1978; Beckman E. L. a. o. Some observations on human tolerance, to accelerative stress, J. Aviat. Med., v. 24, p. 377, 1953. P. K. Isakov.

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