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At least 973 records · Page 54Linked to original sources
PubMed1975

Human sensitivity to gravity (on the problem of gravipreferendum).

Experiments were carried out in a centrifuge with an arm of 7.25 m, the cabin being equipped with a self-contained system of control. During an exposure to head-to-feet accelerations (+gz) of 2 g and an onset rate of 0.1 g s-1 the test subject controlled the centrifugation himself and selected the acceleration values he was instructed to attain. The subjects were shown to be capable of assessing integrally the value of accelerations and estimating it with an error of 0.1-0.12 g. During an exposure to chest-to-back accelerations (+gx) limited to a value of 12 g and a gradient of 0.3 g s-1 the ability to select actively the rate of an acceleration increase was studied. Repeated centrifugations resulted in the formation and strengthening of the skill of maintaining a physiologically optimal rate of an acceleration increase. Onset rates were determined for every level of acceleration. When calculated as a mean, physiologically optimal values of the onset rate for 12 g were 0.16-0.18 g s-1. The findings give insight into important properties of the physiological structure of "gravipreferendum", which is the capability of man to differentiate between gravity levels and to select actively optimal rates of an increase in acceleration.

Acceleration↗
PubMed1977

Prevention of human deconditioning during prolonged immersion in water.

A 56-day immersion experiment in which two subjects participated was carried out. During the experiment the preventive effect of periodic acceleration combined with exercise and water-salt intake was assessed. Simulating an increased gravitational field, exposure to acceleration increased the static component of the load upon the musculo-skeletal system, increased the gradient of the blood hydrostatic pressure, activated mechanisms responsible for the venous return to the heart, stimulated systems regulating antidiuretic and antisodiumdiuretic reflexes. Involvement of these mechanisms restored haemodynamic parameters, fluid-electrolyte balance and blood coagulability. The prophylactic effect of acceleration was enhanced if combined with exercise and supplemented water-salt intake.

Acceleration↗
PubMed1977

Body size and chronic acceleration.

Body composition studied as a function of acceleration (1-4.7 G) in mice and rats showed fat-free body mass (FFBM) to be a predictable function of G-force while corroborating the known lability of body fat. Of nine studied components of FFBM only skeletal muscle, liver and heart contributed to the observed changes induced by delta G. (Body water /FFBM) was independent of delta G. When FFBM (as a percentage of 1 G controls) was plotted against G for mice, rats and monkeys (1-4.7 G) and men (0-1 G), the mass of the fat-free compartment passed through a maximum at 1 G. The data distribution in the figure suggested possible effects of body size and of age.

Acceleration↗
PubMed1977

Gravitational influences upon the maintenance requirements of rabbits.

The gravitational component of the feed requirement for the maintenance of body mass has been reported previously for chickens. Similar measurements are reported here for chronically accelerated Polish rabbits, which have mature body masses around 2 kg, approximately the same as for the single-comb white Leghorn chickens used in the earlier experiments. The gravitational influences upon the maintenance requirement and upon body composition were found to be essentially similar for rabbits and chickens, indicating a general phenomenon for small homoiotherms. The principal response in both groups is a greater feed maintenance requirement which increases linearly with the applied acceleration field. The nature of this response is similar to that which follows lesioning of the lateral hypothalamus in rats, though no causal connection between the two phenomena is necessarily implied by these present studies. In monkeys a qualitatively different response of increased body fat is observed.

Acceleration↗
PubMed1977

The effect of increased gravitational stress on bone.

To assess the effects of an altered gravitational state on the physical properties of bone, fifteen sexually mature chickens were centrifuged for 18 weeks. The gravitational forces were gradually increased to allow the chickens to adapt to the increased load. For the last 4 weeks of the experiment, the animals were subjected to a force of 3 x gravity. After sacrifice the tibias were removed and fractured in torsion. Bone diameters, cortical thickness and ash content of the tibias were determined. Histological preparations of the bone specimens were also examined. When the centrifuged animals were compared with control chickens of comparable size and weight, there was no significant change in bone strength or fracture pattern, although the centrifuged birds had smaller mid-tibial shaft diameters and thicker cortices. The mean percentage bone ash for the centrifuged birds was 68.2% which is an accepted value for dry bone ash of adult chickens. Bone density was the same for both centrifuged and control birds. The birds lost an average of 465 grams of body mass during their 18 weeks on a centrifuge.

Adaptation, Physiological↗
PubMed2002

[Effects of repeated +Gz exposures on the expression of c-fos protein in rabbit brain].

OBJECTIVE: To investigate the effect of repeated +Gz exposures on the expression and distribution of c-fos protein immunoreactivity in rabbit brain. METHOD: Twenty rabbits were divided randomly into 4 groups (n=5 in each group). Animals were anesthetized and then exposed to +4 Gz until 30 s after the arterial pressure at eye level dropped to 0 kPa. The exposure was repeated for 3 times with 30 min intervals. The expression of c-fos protein in rabbit brain was examined at 0 h, 1 h and 6 h after +Gz exposure by immunohistochemistry method. RESULT: c-fos protein was obviously expressed in cortex, the third ventricle, hippocampus and dentate gyrus immediately after repeated +Gz exposures, and strongly expressed 1 h after the exposure, and then had a tendency to decrease 6 h after the exposure. CONCLUSION: Three +Gz exposures may cause time-dependent c-fos expression in rabbit brain, which may be involved in +Gz-induced brain damage.

Aerospace Medicine↗
PubMed2002

[+Gz protection of the on board oxygen generating system (OBOGS) and anti-G equipment].

OBJECTIVE: To investigate the +Gz protection of the on board oxygen generating system (OBOGS) and anti-G equipment. METHOD: Physical and physiological tests of the OBOGS and anti-G equipment were accomplished using human centrifuge. +Gz tolerance of ten male subjects with or without the anti-G suit, pressure breathing for G (PBG) and OBOGS were determined. RESULT: The performance of the OBOGS and anti-G equipment fulfilled the requirements. The +Gz protections of the anti-G suit, PBG, and anti-G system were 2.08 G, 1.92 G, and 3.92 G, respectively. CONCLUSION: The +Gz protection of the OBOGS and anti-G equipment satisfy the performance requirements of high performance aircraft.

Acceleration↗
PubMed2002

[Pathologic changes induced by sustained acceleration].

This paper mainly reviewed sustained +Gz acceleration-induced pathologic changes in various organ systems. Some of them are reversible and some may result in damage. Degree of the pathologic changes are related to G levels, G onset rates, duration of G exposure and individual difference factors. With raise of aircraft performance, acceleration problem is becoming more and more serious. Aviation medical workers should pay good attention to these changes and the use of high performance anti-G measures in order to ensure safety and performance of the pilots.

Acceleration↗
PubMed1976

Human tolerance to acceleration after exposure to weightlessness.

The major role in the genesis of varying human tolerance to decelerations that follow weightlessness is evidently played by hypodynamic and hydrostatic factors. Long disuse of compensatory antigravity mechanisms in weightlessness may bring about their deconditioning and reduction of their functional capabilities, and may finally affect general tolerance of crewmembers to decelerations. Laboratory experiments demonstrated changes in the human tolerance to Gx accelerations of varying duration (from 3 to 100 days) and tested the efficacy of different countermeasures. A decrease in the human tolerance to +Gx is on the average -2.0g. It should be noted that an elongation of simulated weightlessness (from 7 to 100 days) caused no further decrease in the +Gx tolerance. Our investigations helped to assess the threshold of human tolerance to accelerations after an exposure to simulated weightlessness and to delineate the value of real risk. The tolerance limit to +Gx accelerations which followed simulated weightlessness of the above duration ranged from 9.5 to 13.0g, averaging 11.6 +/- 1.6g. The information on the tolerance of Soviet and American astronauts to decelerations shown during re-entry in real space flights give support to the laboratory results and predictions.

Acceleration↗
PubMed1976

Deconditioning during prolonged immersion and possible countermeasures.

Test subjects covered with a waterproof highly elastic cloth were exposed to 13-day water immersion up to the neck. They were divided into two groups. The first (control) group consisting of six persons was exposed to immersion alone and the second (experimental) group was exposed daily to accelerations of 0.6-2 Gz for 60-90 min during the last 6 days of immersion. Before and after immersion all the test subjects were exposed to +3 Gz for 5 min which served as a provocative test. These experiments give evidence that the use of dry immersion allows experimentation during prolonged immersion without concomitant complications. Variations in the physiological parameters (cardiovascular system, fluid-electrolyte balance, blood-coagulatory system) are indicative of the preventive effect of periodic accelerations during 13-day immersion.

Acceleration↗
PubMed1976

Hypergravitation and sympatho-adrenergic reactivity.

The sympatho-adrenergic reaction of the organism subjected to hypergravitation was investigated in rats exposed to +6 Gz. The electro- and cardiotachograms recorded telemetrically were correlated with the adrenal catecholamine content. The determinations were made in controls and in rats treated with hexamethonium and atropine administered separately or together. The rats reacted to acceleration initially by a short bradycardia followed by tachycardia. In one case there was a sustained bradycardic response. Hexamethonium (C6) lowered the resting heart rate, attenuated the initial bradycardia and reduced the consecutive tachycardia. In some rats during centrifugation a bradycardic and arhythmic response was found sometimes ending in cardiac arrest. Hexamethonium stored the catecholamines in the adrenals as a result of their increased concentration. Atropin brought about resting tachycardia, disappearance of bradycardia from the very beginning of centrifugation and a more accentuated tachycardia. Atropin and hexamethonium administrated together diminished the response to acceleration.

Acceleration↗
PubMed1976

A study of the cumulative effects of repeated exposures to radial accelerations.

After exposure to transverse accelerations of 4-10 g changes in the vascular system of the eye and immunobiological resistance of the body persisted from 5 to 15 days. The data give evidence that the state of retinal vessels is the most informative index of the cumulative effect on the body and its recovery.

17-Hydroxycorticosteroids↗
PubMed1976

Weight and shape.

In his Discorsi of 1638, Galileo argued that large terrestrial vertebrates would have to grow very thick legs in order to avoid the decreasing cross-sectional area/weight ratios of large objects geometrically similar to small prototypes. Since then, scaling theory (especially the principle of decreasing relative surface area in large forms) has been one of the most widely appreciated and, paradoxically, least studied areas of biology. The two major themes of scaling theory have been: (1) That small and large animals live in different adaptive "worlds" regulated by forces dominant at their size (surface forces for insects, gravity for large organisms). These forces place limits on the size of organic designs (breathing through external tracheae and presence of exoskeleton constrain insects to be small; buckling strength limits the height of trees; scaling of kinetic energy at something close to l(5) sets maximum height of terrestrial bipeds). (2) That small and large animals have characteristic differences in form and function conditioned by the scaling of surfaces and volumes (large animals have relatively smaller brains, thicker legs, lower metabolism, longer life, more convoluted internal surfaces for gas exchange, digestion and circulation). We know that organic form is adapted to body size, but a major issue for space research involves the degree of purely genetic determination for such adaptation. If, as D'Arcy Thompson argued for the trabeculae of the human femur, adaptations to large size require the immediate action of gravitational forces, then prolonged weightless flight will provoke reversion by removal of the necessary stimulus. In any case, we must know what scaling theory predicts (and what animals actually display) in order to predict the potential problems of prolonged weightlessness.

Animals↗
PubMed1976

Physiological effects of sustained acceleration.

A review is given of the literature concerning the physiological and pathophysiological changes in humans, caused by sustained acceleration. After definition of the acting forces and an introduction into terminology, circulatory and respiratory mechanisms are described which are active under sustained acceleration. The origin of visual disturbances associated with acceleration is discussed. Acceleration tolerance is influenced by magnitude, duration, direction, and rate of application of G-forces together with environmental conditions and the condition of the subject. Acceleration protection may be achieved by technical devices (anti-g suits), voluntary manoeuvres and change of posture. Recent work dealing with high sustained +Gz (HSG) is reviewed and the associated problems are discussed.

Acceleration↗
PubMed1976

Physiological changes associated with long-term increases in acceleration.

The long-term retention of organisms in increased acceleration fields is an experimental approach towards understanding the biological consequences of earth gravity. Such treatment, called chronic acceleration, simulates a change in gravity and requires exposed organisms to adapt physiologically to the new environment. Information from chronic acceleration supplements that from space physiology in understanding gravitational physiology. Many of the responses in long-term exposure to increased acceleration are those which would be anticipated from the imposed symmetrical loading. For example, increased requirements for posture and locomotion induce appropriate changes in musculo-skeletal organs. Displacement of body fluids and increased hydrostatic pressures lead to greater blood volumes and increased tissue hydration. However, there are also specific acceleration effects which cannot be so directly interpreted. Among these are decreases of mature body size and of depot fat, which are proportional in degree to field strength. The role of chronic acceleration research in the development of gravitational biology and its relationship to earth-orbital experiments are considered. The applicability of chronic acceleration studies with human subjects towards planning of deep space exploration is also discussed.

Acceleration↗