Fluid and electrolyte alterations during prolonged hypokinesia

The physiological and biochemical systems that regulate the level of each electrolyte in blood and other endogenous fluids, and the balance between the consumption and loss of fluid and electrolytes and the total fluid and electrolyte content of the body are significantly affected during hypokinesia (HK; diminished movement). Among the known effects of HK, the fluid and electrolyte deficiency has drawn a great interest due to the higher fluid and electrolyte deficiency with higher than lower fluid and electrolyte consumption. The impossibility of the body to use fluid and electrolytes, after the fluid and electrolyte deficiency has been established, has drawn the greatest interest. The fluid shifting to the thoracic region and the daily body rehydration are considered as the most effective methods to counteract fluid and electrolyte changes. To this end, the objective of this review was to report some of the findings in the fluid and electrolyte deficiency and fluid and electrolyte loss with fluid and electrolyte deficiency during prolonged HK.     

Correlation between vestibular sensitization and leg muscle relaxation under weightlessness simulated by water immersion.

The experiments were designed to determine the contribution of the leg muscle relaxation to the sensitization of the vestibular function under weightlessness, The neuromuscular unit (NMU) discharges were continuously recorded with microelectrodes from the anti-gravitational soleus muscle and its antagonist, the tibialis anterior, of a man standing first upright on the level floor of a dry water tank, and then gradually being immersed in water till it reached his neck; while he was buoyed with an airtube placed under his armpit. In each of the successive states, the caloric nystagmus was evoked, analyzed and compared with the NMU discharge as well as with subjective symptoms associated with the nystagmus. The results indicate that the nystagmogenic activity had a significant correlation with the appearance of the active NMU in the soleus, and they also suggest that the reduction of ascending signals from the antigravity muscles might be one of the causes of atypical vestibular responses occuring in weightlessness.     

A review of muscle atrophy in microgravity and during prolonged bed rest

With the prospect of long duration space missions in Earth orbit or to Mars, there is a need for adequate information on the physiological adaptations that will occur. One consequence of prolonged exposure to microgravity is muscle atrophy (loss of muscle mass). After a long duration space flight, muscle atrophy along with skeletal calcium loss would affect the capacity of astronauts to re-adapt to gravity on return to Earth. Of importance are any countermeasures which can attenuate the adaptive responses to microgravity. Experimentation is difficult in space with small subject numbers and mission constraints. Prolonged bed rest using healthy volunteers is used as an Earth-based model to simulate the muscle atrophy which occurs in the microgravity environment.     

A new application of LBNP to measure orthostatic tolerance before and after 0-g simulation (water immersion)

Lower body negative pressure (LBNP) remains an important device for the generation of orthostatic stress in the space flight environment as well as a tool to measure inflight and postflight changes in orthostatic response.

These applied levels of LBNP have typically not exceeded 50–60 mm Hg negative pressure. Information is incomplete as to the levels of absolute LBNP orthostatic tolerance, and the factors responsible for their variance. A better definition of the tolerance limits for males and females could be expected to aid the evaluation of lower levels of LBNP.

An LBNP device was built to study absolute orthostatic tolerance; additionally, another LBNP device was constructed to permit orthostatic tolerance testing directly after a controlled water immersion period.

Absolute LBNP orthostatic tolerance patterns are analyzed for a group of males and females (series I). A preliminary statement on the variations of LBNP orthostatic tolerance after limited periods of water immersion and bed rest is also provided (series II).     

Electrolytes in femur and plasma of rats during prolonged restriction of motor activity.

Hypokinesia (decreased motor activity) induces insignificant bone mineral changes. The aim of this study was to measure mineralization, density, and also electrolyte content in the femur of rats during prolonged hypokinesia (HK). Studies were done on 144 male Wistar rats (370-390 g) during 15 days period of pre-HK and 90 days period of HK. Rats were equally divided into two groups: hypokinetic rats (HKR) and vivarium control (VCR). The HKR group of rats was kept in small individual cages. Femur mineralization density, ash mineral content, calcium (Ca) and phosphate (P) content, and plasma Ca and P concentration were measured. In the HKR group body weight, femur mineralization, density, ash mineral content, Ca and P concentration decreased significantly (p < or = 0.01) while plasma Ca and P concentration increased significantly (p < or = 0.01) when compared with the VCR group. The measured parameters did not change significantly in the VCR group when compared with the baseline control values. It was concluded that prolonged HK induces a significant reduction in electrolyte concentration accompanied by decreased mineralization, density, and ash mineral content of the femur of rats.     

Effect of fluid and salt supplementation on body hydration of athletes during prolonged hypokinesia

Body hydration decreases significantly during hypokinesia (HK) (diminished movement), but little is known about the effect of fluid and salt supplements (FSS) on body hydration during HK. The aim of this study was to measure the effect of FSS on body hydration during HK. Studies were done during 30 days pre HK period and 364 days HK period. Thirty male athletes aged 24.5 +/- 6.6 yr were chosen as subjects. They were equally divided into three groups: unsupplemented ambulatory control subjects (UACS), unsupplemented hypokinetic subjects (UHKS) and supplemented hypokinetic subjects (SHKS). Hypokinetic subjects were limited to an average walking distance of 0.7 km day-1. The SHKS group took daily 30 ml of water/kg body weight and 0.1 g of sodium chloride (NaCl)/kg body weight. Control subjects experienced no changes in their professional training and routine daily activities. Plasma volume (PV), urinary and plasma sodium (Na) and potassium (K), plasma osmolality, plasma protein, whole blood hemoglobin (Hb) and hematocrit (Hct), plasma renin activity (PRA) plasma aldosterone (PA) levels, physical characteristics, food and fluid intakes were measured. Plasma osmolality, plasma protein, urinary and plasma Na and K, whole blood Hct and Hb, PRA and PA levels decreased significantly (p < or = 0.01), while PV and body weight increased significantly (p < or = 0.01) in the SHKS group when compared with the UHKS group and did not change when compared with the UACS group. Plasma osmolality, plasma protein, urinary and plasma Na and K, PRA and PA, whole blood Hb and Hct levels increased significantly (p < or = 0.01), while PV body weight, food and fluid intakes decreased significantly (p < or = 0.01) in UHKS group when compared with the SHKS and UACS groups. The measured parameters did not change in the UACS group when compared with their baseline control values. It was shown that during HK body hydration decreased significantly, while during HK and FSS body hydration increased significantly. It was concluded that daily intake of FSS prevents the decrease of PV and blunts the increase of activity of the PRA and PA during prolonged HK.     

Chronic hyperhydration and hematological changes in trained subjects during prolonged restriction of motor activity

The objective of this investigation was to evaluate the effect of a daily intake of fluid and salt supplementation (FSS) on the hemoglobin content of endurance trained athletes during hypokinesia (decreased number of steps from 10,000 to 3000 steps per day). The studies were performed on 30 long-distance runners who had a VO2max average of 66 ml kg-1 min-1 and were in the age range of 19-24 years. Prior to their exposure to hypokinesia (HK) of 364 days, all volunteers were on an average of 10,000 steps per day. All volunteers were divided into three equal groups: the first group underwent normal ambulatory life (control subjects), the second group was kept under continuous restriction of motor activity (hypokinetic subjects), and the third group was placed under continuous restriction of motor activity and consumed 26 ml water kg-1 body weight daily and 0.1 g sodium chloride kg-1 body weight in the form of supplementation (hyperhydrated subjects). For simulation of the hypokinetic effect, the number of steps taken per day by the second and third groups of volunteers was restricted to an average of 3000. During the hypokinetic period we determined reticulocytes (Rt), hemoglobin (Hb), hematocrit (Hct), plasma volume (PV), red blood cell (RBC) mass and VO2max. In hyperhydrated volunteers the content of Hb and Hct decreased significantly, while PV, RBC mass and Rt count increased significantly. In hypokinetic volunteers Hb and Hct increased, while PV, RBC and Rt decreased significantly. It was concluded that chronic hyperhydration may be used to attentuate an increase in the Hb content of physically conditioned subjects during prolonged restriction of motor activity.     

基于流固耦合理论的关机水击特性

为了考虑结构变形对关机水击特性的影响,应用Workbench15.0构建双向流固耦合分析系统,模拟关机水击过程,通过压力和流线的分布图分析压力波传播和能量耗散。根据轨控发动机大流量、高室压、快响应的发展趋势,设计了8个工况来分析流量、压力、阀门关闭时间对水击特性的影响。仿真结果表明:在水击发生后,水击的能量只有小部分通过从入口流出和结构变形而耗散,大部分水击能量的耗散是由于流体的粘性损失。流量只对水击峰值压力有影响,且流量越大,水击峰值压力越大。阀门关闭时间的缩短增加了峰值压力和水击频率,减缓了衰减速率。管路背压对水击特性几乎没有影响。因此,在进行轨控发动机高室压水击试验时,在保证流量和关阀时间相同的情况下,减小出口背压,可以得出与高背压一致的水击压力变化曲线。     

镁粉的高温水反应特性研究

采用管式炉模拟发动机的高温环境(600～900℃),研究了镁粉与水的反应特性,得到了反应率与温度、镁粉粒度的关系和添加剂对反应的影响.结果表明,Mg/H2O反应的反应率随温度增加而增大,随粒径减小而增大,添加剂对反应有促进作用,Mg/H2O反应的表现活化能为24.67 kJ/mol.     

Recent advances in the physiology of whole body immersion

Recent investigations have furnished a complete analysis of the hemodynamic events accompanying whole-body immersion. About 700 ml of blood are translocated into the intrathoracic circulation, and heart volume increases by 180 +/- 62 ml. These changes are followed by an increase in stroke volume and cardiac output of over 30%. At the same time a reflex reduction of total peripheral resistance and venous tone occurs. Renin and aldosterone activity are reduced while the 17-hydroxycorticosteroid is not affected. Treatment of the subject with DOCA attenuates but does not extinguish the excess sodium excretion of immersion. This finding strengthens the arguments in favor of an unknown factor enhancing sodium excretion. Finally, the relative activation of the three factors that serve volume control, the excretory function of the kidney, capillary filtration pressure, and the thirst mechanism, is discussed.     

液体火箭发动机关机水击特性仿真

关机水击是引起液体火箭发动机及其试验台故障的常见现象之一.为获得关机水击的主要影响规律,采用一维有限体积法建立了发动机关机水击仿真模型,通过地面试验验证了模型的正确性.针对发动机常见设计变量,开展仿真研究,结果表明:水击增量与推进剂流量、流速成正比;管路足够长时,水击增量与其长度无关,但管路过短时,管路越短,水击增量越...     

着陆作用下月尘激扬的三维离散元分析

基于模拟月尘的细观颗粒形状,采用刚性圆球单元重叠法建立了4种典型不规则模拟月尘颗粒的三维几何模型.根据月尘所处环境和颗粒间接触特性,建立了月尘离散元接触力学模型.通过三轴压缩实验离散元模拟的结果和模拟月尘的实际三轴压缩实验数据的对比,确定模拟月尘颗粒的三维离散元模型参数.采用建立的模型和获得的参数,进行了圆盘模拟体着陆...     

Status of vestibular function after prolonged bedrest.

6 young, healthy, male volunteers were submitted to one week of head down (-4 degrees) bedrest. This position simulates the cerebral hemodynamic conditions in weightlessness. Measurements of vestibular equilibrium and of oculomotor system function were made before and after the prolonged bedrest. Analysis of the results indicates that vestibular responses, as measured by the maximal speed of the slow phase of the provoked nystagmus (caloric and sinusoidal rotatory stimulations), are decreased after prolonged bedrest. This statistically significant diminution requires confirmation with a greater number of cases. The reflex conflicting or interacting with the cervico-ocular and optokinetic reflexes on the one hand and the foveal vision on the other, is one of several possible explanations for the observed changes.     

Thyroid cell growth: sphingomyelin metabolism as non-invasive marker for cell damage acquired during spaceflight

Prolonged spaceflights are known to elicit changes in human cardiovascular, musculoskeletal, and nervous systems, whose functions are regulated by the thyroid gland. It is known that sphingomyelin metabolism is involved in apoptosis (programmed cell death) of thyroid cells induced by UVC radiation, but at present no data exists with regard to this phenomenon, which occurs during space missions. The aim of this study was to analyze, for the first time, the effect of spaceflight on the enzymes of sphingomyelin metabolism, sphingomyelinase, and sphingomyelin synthase, and to determine whether the ratio between the two enzymes might be used as a possible marker for thyroid activity during space missions. Both quiescent thyroid cells and thyroid cells stimulated to proliferate with thyrotropin (TSH) were cultured during the Eneide and Esperia missions on the International Space Station. The results show that during space missions the cells treated with TSH grew only 1.5?±?0.65-fold and, thus, behave similarly to quiescent cells, while on the ground the same cells, maintained in experimental conditions that reproduced those of the flight, grew 7.71?±?0.67-fold. Comparison of the sphingomyelinase/sphingomyelin-synthase ratio and the levels of Bax, STAT3, and RNA polymerase II in proliferating, quiescent, pro-apoptotic, or apoptotic cells demonstrated that thyroid cells during space missions were induced into a pro-apoptotic state. Given its specificity and the small amount of cells needed for analysis, we propose the use of the sphingomyelinase/sphingomyelin-synthase ratio as a marker of functional status of thyroid cells during space missions. Further studies could lead to its use in real time during prolonged spaceflights.     

Effects of prolonged space flight on rat skeletal muscle

The effect of a 20-day space flight on water, Na+, K+, Mg2+, Ca2+ and glycogen contents as well as on activities of glycogen metabolism enzymes--glycogen synthetase and glycogen phosphorylase--of rat skeletal muscles was studied. This data is regarded as an integral test characterizing the state of contractile tissue of the animals at the final stage of flight aboard biosatellites. The measurements indicate that there were no significant changes of cations and glycogen contents nor of the enzymic activities in fast-twitch muscles during the 20-day spaceflight. At the same time dehydration in these muscles was observed, which disappeared on the 25th postflight day. In slow-twitch antigravitational skeletal muscle (m. soleus) there was a decrease of K+ and increase of Na+ in the tissue contents. The changes disappeared at the end of the on-earth readaptation period. From the pattern of these observations, we can conclude that the 20-day space flight leads to some reversible biochemical changes of the rat skeletal muscles. A conclusion can be drawn about necessity of creating, aboard the spaceship, an artificial load on antigravitational skeletal muscles.     

Pathophysiology of motor functions in prolonged manned space flights.

The influence of weightlessness on different parts of the motor system have been studied in crew members of 140 and 175 days space flights. It has been shown that weightlessness affects all parts of the motor system including (i) the leg and trunk muscles, in which severe atonia, a decrease of strength and an increase of electromyographic cost of contraction have been observed, (ii) the proprioceptive elements and the spinal reflex mechanisms in which decreased thresholds accompanied by decreases of maximal amplitude of reflexes and disturbances in cross reflex mechanisms have been found. and (iii) the central mechanisms that control characteristics of postural and locomotor activities. The intensities and durations of disturbances of different parts of the motor system did not correlate to each other, but did correlate with prophylactic activity during space flight. The data suggest a different nature of disturbances caused by weightlessness in different parts of the motor system.     

The effects of prolonged weightlessness and reduced gravity environments on human survival

The manned exploration of the solar system and the surfaces of some of the smaller planets and larger satellites requires that we are able to keep the adverse human physiological response to long term exposure to near zero and greatly reduced gravity environments within acceptable limits consistent with metabolic function. This paper examines the physiological changes associated with microgravity conditions with particular reference to the weightless demineralizatoin of bone (WDB). It is suggested that many of these changes are the result of physical/mechanical processes and are not primarily a medical problem. There are thus two immediately obvious and workable, if relatively costly, solutions to the problem of weightlessness. The provision of a near 1 g field during prolonged space flights, and/or the development of rapid transit spacecraft capable of significant acceleration and short flight times. Although these developments could remove or greatly ameliorate the effects of weightlessness during long-distance space flights there remains a problem relating to the long term colonization of the surfaces of Mars, the Moon, and other small solar system bodies. It is not yet known whether or not there is a critical threshold value of 'g' below which viable human physiological function cannot be sustained. If such a threshold exists permanent colonization may only be possible if the threshold value of 'g' is less than that at the surface of the planet on which we wish to settle.