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PubMed2000

Magnetic compensation of gravity forces in (p-) hydrogen near its critical point: application to weightless conditions

We report a study concerning the compensation of gravity forces in two-phase (p-) hydrogen. The sample is placed near one end of the vertical z axis of a superconducting coil, where there is a near-uniform magnetic field gradient. A variable effective gravity level g can thus be applied to the two-phase fluid system. The vanishing behavior of the capillary length l(C) at the critical point is compensated by a decrease in g and l(C) is kept much smaller than the cell dimension. For g ranging from 1 to 0.25 times Earth's gravity (modulus g(0)) we compare the actual shape of the meniscus to the expected shape in a homogeneous gravity field. We determine l(C) in a wide range of reduced temperature tau=(T(C)-T)/T(C)=[10(-4)-0.02] from a fit of the meniscus shape. The data are in agreement with previous measurements further from T(C) performed in n-H2 under Earth's gravity. The effective gravity is homogeneous within 10(-2)g(0) for a 3 mm diameter and 2 mm thickness sample and is in good agreement with the computed one, validating the use of the apparatus as a variable gravity facility. In the vicinity of the levitation point (where magnetic forces exactly compensate Earth's gravity), the computed axial component of the acceleration is found to be quadratic in z, whereas its radial component is proportional to the distance to the axis, which explains the gas-liquid patterns observed near the critical point.

Journal Article↗

PubMed2001

Stability of modulated-gravity-induced thermal convection in magnetic fields.

A stability analysis is presented of modulated-gravity-induced thermal convection in a heated fluid layer subject to an applied magnetic field. The nearest correction to the critical Rayleigh number for both single and multiple frequency oscillating-gravity components is obtained by solving the linearized magnetohydrodynamic equations using the small parameter perturbation technique. The correction depends on both the applied magnetic field and the oscillating frequency. In the absence of an applied magnetic field, the correction depends on the Prandtl number only when the exciting frequency is small. However, it asymptotically approaches zero as the frequency increases, with or without the presence of a magnetic field. The heated fluid layer is more stable with gravity modulation than with any type of wall temperature modulation. The difference becomes smaller with decreasing Prandtl number Pr. This finding is of critical importance in that ground-based experiments with appropriate wall temperature modulations may be conducted to simulate the oscillating-gravity effects on the onset of thermal convection in lower-Prandtl-number fluids. For conducting melts considered for microgravity applications, it is possible to apply an external magnetic field to further inhibit the onset of modulated-gravity-induced thermal convection. This effectiveness increases with the Hartmann number Ha. For large Ha, the nearest correction term R02 approximately Ha2 as the magnetic Prandtl number Pm<<1. However, R02 approximately Ha(4/3) for Ha>>1 and Pm>>1, provided that Ha<0.5pi(Pm/Pr(3/2)), which is satisfied by a majority of space melt experiments. Thus, under normal laboratory conditions applied magnetic fields are more effective in stabilizing a conducting fluid subject to an oscillating-gravity field than one subject to a constant field. If Ha>0.5pi(Pm/Pr(3/2)), R02 approximately -Ha2 for Ha>>1 and Pm>>1 and the magnetic field becomes less effective in stabilizing thermal convection driven by oscillating gravity than that driven by the constant gravity. This is in contrast with the existing studies on thermal convection stability in a magnetic field, which show that marginal stability is independent of Pm and always increases with increasing applied field.

Journal Article↗

PubMed1996

Optomotor behaviour in Xenopus laevis tadpoles as a measure of the effect of gravity on visual and vestibular neural integration.

The ability of aquatic vertebrates to maintain their position requires integration of visual and vestibular sensory information. To understand better how aquatic animals integrate such information, we measured the optomotor behaviour of Xenopus laevis tadpoles raised in growth chambers in microgravity (< 10(-3)g), normal gravity (1 g), hypergravity (3 g) and on a slowly rotating clinostat (simulated microgravity). The goal of this research was to determine how development in an altered gravitational force field affects the visual- and vestibular-dependent behaviour of tadpoles. This research represents the first time that the optomotor behaviour of an organism raised from fertilization in microgravity has been tested. Significant differences were observed in the optomotor behaviour among the four gravity treatments. When first exposed to normal gravity, the microgravity-raised tadpoles exhibited the strongest (or most positive) optomotor behaviour, while the 3 g centrifuge tadpoles showed no optomotor response. Some abnormal behaviours (such as erratic swimming, lying motionless and abnormal swimming posture) were observed in the tadpoles raised in altered gravity on the initial day of testing. One day later, the tadpoles raised in hypergravity did not differ significantly in their optomotor behaviour from control tadpoles raised in normal gravity. However, tadpoles raised in microgravity still displayed an exaggerated optomotor response. One week after the tadpoles had been introduced to normal gravity, there was no longer a significant difference in optomotor behaviour among the different gravity treatments. This convergence of optomotor behaviour by tadpoles from the different treatment reflects the acclimation of their vestibular systems to normal gravity.

Animals↗

PubMed1993

[Measurement of the center of gravity using the link model for controlling functional electrical stimulation].

The purpose of this study is to better understand the reliability of using the link model when determining the center of gravity. We measured the centers of gravity using both the link model and the floor reaction force in 11 healthy subjects. Dempster's and Clauser's anthropometric data were used in measuring the center of gravity by the link model, and these results were compared with those measured by the floor reaction force. The motion area of the center of gravity during quiet standing was determined by experimental data, using the link model calculation. Joint torque was measured using Cybex II in one paraplegic patient who was treated by functional electrical stimulation (FES). We discuss whether the paraplegic patient can stand safely in a determined center of gravity motion area. Displacements of the center of gravity measured by the link model were very close to those obtained by the floor reaction force. The parameters of body segments proposed by Dempster were more reasonable than those of Clauser. The maximum differences of the center of gravity between those measured by the link model using Dempster's data and those measured by the floor reaction force, were 1.47 cm for the fore-aft direction, 1.85 cm for the lateral direction, and 2.34 cm for the vertical direction. The torque caused by electrical stimulation was stronger than that calculated by the link model during quiet standing. Our results suggest that the link model system that measures displacement of the center of gravity using Dempster's data, is clinically applicable for the closed-loop control system of FES.

Adult↗

PubMed1991

Measurement of brain tissue specific gravity using pycnometry.

In this paper we introduce and characterize pycnometry, a method used to measure fluid density, for determining a tissue's specific gravity. It uses a 2-ml glass pycnometer filled with distilled water to determine a tissue sample's displacement volume. The tissue's density is determined when it's weight is divided by this volume and specific gravity is computed by dividing the tissue density by the density of water. Pycnometry was validated using pre-calibrated glass, specific gravity standards over the range 1.03-1.26, and compared to the density gradient method using rat brain tissue. We observed that the specific gravity values obtained using pycnometry were highly correlated with the specific gravity standards (slope = 1.0107, r = 0.996) and with the density gradient column when tissue volumes larger than 0.120 ml were used with the pycnometer (slope = 1.0707, r = 0.9826). Good correlation was also observed between percent water content values computed using the Nelson equation with pycnometry or density gradient specific gravity values versus the measured percent water content values obtained with the wet weight/dry weight method. Pycnometry is an accurate, reproducible technique to measure tissue specific gravity and brain edema and is best suited for use in a laboratory that engages sporadically in brain edema measurement.

Animals↗

PubMed2001

Microtubule self-organisation depends upon gravity.

The molecular processes by which gravity is transduced into biological systems are poorly, if at all, understood. Under equilibrium conditions, chemical and biochemical structures do not depend upon gravity. It has been proposed that biological systems might show a gravity dependence by way of the bifurcation properties of certain types of non-linear chemical reactions that are far-from-equilibrium. We have found that in-vitro preparations of microtubules, an important element of the cellular cytoskeleton, show this type of behaviour. On earth, the solutions show macroscopic self-ordering, and the morphology of the structures that form depend upon the orientation of the sample with respect to gravity at a critical moment at an early stage in the development of the self-organised state. An experiment carried out in a sounding rocket, showed that as predicted by theories of this type, no self-organisation occurs when the microtubules are assembled under low gravity conditions. This is an experimental demonstration of how a very simple biochemical system, containing only two molecules, can be gravity sensitive. At a molecular level this behaviour results from an interaction of gravity with macroscopic concentration and density fluctuations that arise from the processes of microtubule contraction and elongation.

Centrifugation↗

PubMed1998

Improving the specific gravity adjustment method for assessing urinary concentrations of toxic substances.

Changes in urinary flow induce changes in urinary concentrations of toxic substances. The authors modified the conventional specific gravity adjustment method for measuring urinary concentration of toxic substances to compensate for the dilution effects from varying degrees of hydration. The conventional specific gravity adjustment method is a special case to the more general method proposed in this article. The conventional method generally does not correct the urinary concentration to the mean specific gravity of urine. It requires the assumption that a change in urine flow preserves the relative ratio between the mass of the xenobiotic and the mass of total dissolved solids. The derivation of the modified specific gravity adjustment method shows that a change in urine flow does not necessarily preserve this ratio. An experimental slope between urinary flow and urinary specific gravity was linear on a log scale. A ratio was formed between the experimental slope and Araki's "b" slopes for various substances to predict changes in urine concentrations due to changes in the specific gravity of a spot sample. Since excretion rates typically vary for different substances with changes in urinary flow, an appropriately weighted exponential adjustment factor is required for each substance to normalize its concentration to the standard specific gravity of urine.

Adult↗

PubMed2002

The relationship between urine osmolality and specific gravity.

BACKGROUND: In general, there is a good correlation between the specific gravity and osmolality of a urine sample. In certain clinical conditions, such as uncontrolled diabetes mellitus, nephrotic syndrome, after the administration of intravenous radiocontrast material or saline diuresis, dependence upon specific gravity for determining the concentrating ability will result in over- or underestimation. METHODS: We studied the relationship between specific gravity and osmolality in vitro with simulated urines of varying composition. Urine samples from patients with different clinical conditions were also analyzed. RESULTS: The in vitro curves for sodium chloride, urea, creatinine, glucose, contrast dye, and albumin were plotted (specific gravity versus osmolality). We found a linear correlation between the specific gravity and osmolality of the 6 substances that were studied and for their combinations. The urine samples obtained from patients with different clinical conditions documented that reliance on specific gravity could over- or underestimate the urine osmolality. CONCLUSIONS: We concluded that in those clinical conditions, urine osmolality should always be determined and it should not be estimated based on specific gravity.

Humans↗

PubMed2002

Mutations in the gravity persistence signal loci in Arabidopsis disrupt the perception and/or signal transduction of gravitropic stimuli.

Gravity plays a fundamental role in plant growth and development, yet little is understood about the early events of gravitropism. To identify genes affected in the signal perception and/or transduction phase of the gravity response, a mutant screen was devised using cold treatment to delay the gravity response of inflorescence stems of Arabidopsis. Inflorescence stems of Arabidopsis show no response to gravistimulation at 4 degrees C for up to 3 h. However, when gravistimulated at 4 degrees C and then returned to vertical at room temperature (RT), stems bend in response to the previous, horizontal gravistimulation (H. Fukaki, H. Fujisawa, M. Tasaka [1996] Plant Physiology 110: 933-943). This indicates that gravity perception, but not the gravitropic response, occurs at 4 degrees C. Recessive mutations were identified at three loci using this cold effect on gravitropism to screen for gravity persistence signal (gps) mutants. All three mutants had an altered response after gravistimulation at 4 degrees C, yet had phenotypically normal responses to stimulations at RT. gps1-1 did not bend in response to the 4 degrees C gravity stimulus upon return to RT. gps2-1 responded to the 4 degrees C stimulus but bent in the opposite direction. gps3-1 over-responded after return to RT, continuing to bend to an angle greater than wild-type plants. At 4 degrees C, starch-containing statoliths sedimented normally in both wild-type and the gps mutants, but auxin transport was abolished at 4 degrees C. These results are consistent with GPS loci affecting an aspect of the gravity signal perception/transduction pathway that occurs after statolith sedimentation, but before auxin transport.

Arabidopsis↗

PubMedDate not supplied

Determination of the specific gravity of human pineal.

Specific gravity values for the pineal gland in any species are lacking. These data are necessary for calculating and interrelating pineal weight and volume. This report deals with the specific gravity of human pineal gland. The specific gravity of fresh, unfixed human pineal has been determined to be 1.197 +/- 0.036 SEM at 20 degrees C (N = 11) using water as the measuring medium. By contrast, human brain has been reported in the literature to have a specific gravity of 1.036. Specific gravities of human pineals fixed in Bouin's fluid, Bouin's fluid followed by 80% ethanol, 80% ethanol, and embalming fluid were found to be 1.113 +/- 0.025 (N = 5), 0.941 +/- 0.009 (N = 5), 0.990 +/- 0.022 (N = 3), and 1.149 +/- 0.021 (N = 7), respectively. These are corrected values accounting for the specific gravities of the respective measuring fluids. It is shown that the specific gravity of human pineal as derived in this study is directly applicable to calculations of pineal volumes and weights not only in humans, but in other species as well.

Aged↗

PubMed1991

Possible mechanisms of indirect gravity sensing by cells.

We have to distinguish between (a) direct gravisensing, in which specialized cells function as parts of a gravisensing organ and (b) indirect gravisensing, in which other cells that have no specialized gravity detectors are nevertheless affected by the inertial acceleration. In both cases, cells may detect (a) the direction of gravity ("up" versus "down"), and /or (b) the amplitude (0 - 1 g) of gravity. This chapter argues that the weight of single normal-sized cells (approximately 10 microns in diameter) is too small compared with other cellular forces to allow them the distinction between up and down. However, the weight of the surrounding medium is much larger. Cells may be able to sense certain environmental changes caused by gravity and thus may sense indirectly at least the amplitude of gravitational forces. In particular, the fluid environment of the cell can be expected at normal gravity to support microconvective currents that cease to flow at microgravity. Thus, the absence of gravity may be transduced into the accumulation of metabolites and ions from the cells and depletion of fresh nutrients. These changes, in turn, can affect the contacts of cells, their membrane potential, their cytoskeleton, and thus, ultimately, their behavior. As to ground-based simulations of microgravity, the above considerations suggest that the averaging of the vectorial force of gravity in clinorotation is inadequate for simulation because it may actually increase rather than suppress convective mixing above the normal levels.

Animals↗

PubMed2004

Perception mechanism of gravistimuli in gravity resistance responses of plants.

Gravity resistance is a response that enables plants to develop against the gravitational force. Hypergravity conditions produced by centrifugation have been used to analyze the mechanisms of gravity resistance responses. Under hypergravity conditions, plants construct short and thick shoots and increase cell wall rigidity for resisting the gravitational force. Hypergravity caused a decrease in the percentage of cells with transverse microtubules, and an increase in that with longitudinal microtubules. Such a prompt reorientation of cortical microtubules is involved in the changes in morphology of shoots by gravity. Hypergravity increased cell wall rigidity by increasing the molecular mass of xyloglucans via suppression of xyloglucan breakdown as well as by the thickening of cell walls. Blocker reagents of mechanoreceptors nullified the above-mentioned changes induced by hypergravity. Gravity resistance responses were brought about normally in mutants deprived of gravitropism. This result indicates that the graviperception mechanism in gravity resistance is independent of that in gravitropism. Gravity resistance responses were brought about independently of the direction of gravistimuli, but the responses disappeared in the presence of blockers of mechanoreceptors. Thus, in gravity responses, plants may perceive the gravitational force independently of the direction of stimuli by mechanoreceptors on the plasma membrane, and may utilize the signal to construct a tough body.

Arabidopsis↗

PubMedDate not supplied

A comparison of reagent strips and the refractometer for measurement of urine specific gravity in hospitalized children.

Pediatric nurses in acute care settings routinely test urine for specific gravity, pH, glucose, protein, and other substances. In one tertiary care facility, nurses used the refractometer to test urine specific gravity and the reagent strip to test for other substances. This study was designed to provide data to determine if the reagent strip and the refractometer were interchangeable for measuring urine specific gravity in pediatric clients. Nurses obtained urine for specific gravity testing in 157 pediatric patients ranging in age from 1 day to 16 years. Each urine specimen was tested twice, once using the refractometer and once with the reagent strip. A Bland-Altman plot was used to determine the extent of agreement between the two measurement methods. The plot showed strong agreement between the two methods across a wide range of values for urine specific gravity. As a result of this study, staff nurses decided to use the reagent strip for urine specific gravity when other urine tests are needed and to use the refractometer when only a specific gravity is needed. This decision has resulted in a time savings for nurses who now do not have to repeat a reagent strip measurement. The decision also resulted in a savings of approximately $1200 in purchase of new refractometers for a newly constructed unit.

Adolescent↗

PubMed1992

Identification of specific gravity sensitive signal transduction pathways in human A431 carcinoma cells.

Epidermal growth factor (EGF) activates a well characterized signal transduction cascade in human A431 epidermoid carcinoma cells. The influence of gravity on EGF-induced EGF-receptor clustering and early gene expression as well as on actin polymerization and actin organization have been investigated. Different signalling pathways induced by the agents TPA, forskolin and A23187 that activate gene expression were tested for sensitivity to gravity. EGF-induced c-fos and c-jun expression were decreased in microgravity. However, constitutive beta-2 microglobulin expression remained unaltered. Under simulated weightlessness conditions EGF- and TPA-induced c-fos expression was decreased, while forskolin- and A23187-induced c-fos expression was independent of the gravity conditions. These results suggest that gravity affects specific signalling pathways. Preliminary results indicate the EGF-induced EGF-receptor clustering remained unaltered irrespective of the gravity conditions. Furthermore, the relative filamentous actin content of steady state A431 cells was enhanced under microgravity conditions and actin filament organization was altered. Under simulated weightlessness actin filament organization in steady state cells as well as in EGF-treated cells was altered as compared to the 1 G reference experiment. Interestingly the microtubule and keratin organization in untreated cells showed no difference with the normal gravity samples. This indicates that gravity may affect specific components of the signal transduction circuitry.

Calcimycin↗

PubMed2001

Effect of gravity field on the nonequilibrium/nonlinear chemical oscillation reactions.

Biological systems have evolved for a long time under the normal gravity. The Belousov-Zhabotinsky (BZ) reaction is a nonlinear chemical system far from the equilibrium that may be considered as a simplified chemical model of the biological systems so as to study the effect of gravity. The reaction solution is comprised of bromate in sulfuric acid as an oxidizing agent, 1,4-cyclohexanedione as an organic substrate, and ferroin as a metal catalyst. Chemical waves in the BZ reaction-diffusion system are visualized as blue and red patterns of ferriin and ferroin, respectively. After an improvement to the tubular reaction vessels in the experimental setup, the traveling velocity of chemical waves in aqueous solutions was measured in time series under normal gravity, microgravity, hyper-gravity, and normal gravity using the free-fall facility of JAMIC (Japan Microgravity Center), Hokkaido, Japan. Chemical patterns were collected as image data via CCD camera and analyzed by the software of NIH image after digitization. The estimated traveling velocity increased with increasing gravity as expected. It was clear experimentally that the traveling velocity of target patterns in reaction diffusion system was influenced by the effect of convection and correlated closely with the gravity field.

Bromates↗

PubMedDate not supplied

[The effect of halo-gravity traction in the preoperative treatment of neuromuscular scoliosis].

The purpose of this study is to evaluate the influence of halo-gravity-traction on paralytic scoliosis in various neurologic diseases. Between 1980 and 1993 preoperative halo-gravity-traction was applied in 32 patients with paralytic scoliosis (23 patients with myelomeningocele, 6 patients with poliomyelitis, 3 patients with cerebral palsy). In the myelomeningocele group the average curvature before treatment was 97.8 degrees, after surgery 45.1 degrees; which is an improvement of 53.9%. Halo-gravity-traction accounted for 12.8% improvement. In the poliomyelitis group the average curvature before treatment was 104.3 degrees, after surgery 58.0 degrees; which is an improvement of 44.4%. Halo-gravity-traction accounted for 16.9% improvement. In the cerebral palsy group the average curvature before treatment was 75.0 degrees, after surgery 39.0 degrees; which is an improvement of 48.0%. Halo-gravity-traction accounted for -2.7% improvement. If there is an effect the question remains, whether this will have consequences for the surgical outcome. The comparison between good and bad responders with the surgical result shows, that this result is independent of the halo-gravity-traction. From this results we draw the conclusion, that preoperative halo-gravity-traction can not be recommended in paralytic scoliosis.

Adolescent↗

PubMed2005

Center of gravity and radiographic posture analysis: a preliminary review of adult volunteers and adult patients affected by scoliosis.

STUDY DESIGN: This is a prospective radiographic and force plate analysis involving adult volunteer and patients with scoliosis. OBJECTIVE: To assess accurately the center of pressure in standing volunteers and patients with scoliosis, and correlate these finding with radiographic data. SUMMARY OF BACKGROUND DATA: A simple and commonly applied parameter of global balance is the plumbline offset. This radiographic measurement refers to the center of C2 (or C7) drawn vertically downward. Although this measurement is simple, it may not accurately reflect the balance of the spine. METHODS: This study included adult volunteers (n = 41) and patients with scoliosis (n = 45). Full-length, freestanding spine radiographs were obtained with subjects on a force plate. Simultaneous assessment of the radiologic spinal posture and the floor projection of the center of pressure (gravity line) was possible. The latter was projected on the full spine images and correlated to common radiographic parameters. RESULTS: The position of the gravity line differed significantly from the plumbline in frontal and sagittal planes (P < 0.001). This difference was maintained in both study populations. The mean frontal plane alignment of the gravity line was consistently to the right of the plumbline. The mean sagittal plane alignment of the gravity in relation to the plumbline revealed an offset anteriorly. CONCLUSIONS: The data analysis of offsets between the gravity line and radiographic parameters revealed a frontal plane mean displacement of the gravity line to the right. In the sagittal plane, a highly significant lack of correlation between the gravity line and plumbline was noted. The plumbline represents a common and convenient visual display of apparent sagittal plane imbalance, but its value as a marker of true postural balance must be questioned.

Adult↗

PubMed1992

Molecular cloning and characterization of gravity specific cDNA in rice (Oryza sativa L.) suspension callus.

Rice (Oryza sativa L. var. Nipponbare) suspension callus was exposed to gravity stress at 450,000 g for 2 hours, after which poly(A)+RNA was isolated and a cDNA library was constructed. Three different gravity specific cDNAs, namely, GSC 128, GSC 233 and GSC 381 of 0.67, 0.60 and 0.68 kilobase pairs and transcripts of 1.9, 1.6 and 2.0 kb, respectively, were isolated by differential screening and Northern hybridization. The maximum level of transcript was achieved after 4 hours of exposure to gravity at 450,000 g for GSC 128, 2 hours for GSC 233 and 8 hours for GSC 381 followed by a gradual decrease to undetectable levels with the extension of gravitation time. Callus (GSC 128), shoot and callus (GSC 381) and root and callus (GSC 233) specific expression of transcripts was identified. Although the protection of callus by treatment with ABA, kinetin and sucrose extended the period of expression of mRNA in suspension callus after gravity exposure, the expression of gravity-inducible mRNA was exclusively regulated by the degree of callus viability or survival after the stress. In addition, we demonstrated that the level of GSC 381 transcript was markedly increased by exposing the cell to periodical gravity stress, suggesting that this mRNA is expressed and translated into special proteins which are closely related to the survival of the cell against gravity stress. The sequence of GSC 233 and GSC 381, consisting of 417 and 531 base pairs of the longest open reading frames, encode polypeptides with calculated molecular weights of 15.29 and 19.47 kDa, respectively. A sequence homology search against a data bank revealed that GSC 233 and GSC 381 differed from other stress inducible genes in terms of the coding sequence and expression characteristics.

Adaptation, Physiological↗