Changes In Hydrostatic Pressure Research Articles

Correlation of silica glass properties with the infrared spectra

Infrared (IR) spectroscopy has been used to interpret structural changes in silica glasses. Specifically, a shift of the Si–O stretching band in IR spectra is used to monitor changes in average Si–O–Si bond angle in the glass structure. A similar structural change is induced by the change of fictive temperature, hydrostatic pressure or compressive stress, with the average Si–O–Si bond angle decreasing with the increase of these parameters. It is anticipated that these similar structural changes would produce a similar change in glass properties. In order to confirm this expectation, HF etch rates of silica glasses were measured as a function of fictive temperature and stress. The experimental results on HF etch rates, together with changes in other glass properties in the literature, were compared with the change in glass structure revealed by IR spectroscopy. It was found that the similar structural change is accompanied by the consistent changes in a variety of glass properties. Monitoring the IR spectra of a silica glass sample, therefore, can be used to deduce changes in glass properties.

Asymmetry of tubuloglomerular feedback effector mechanism with respect to ambient tubular flow.

The contribution of preglomerular resistance changes to the tubuloglomerular feedback (TGF) effector response was evaluated by micropuncture in hydropenic rats. Studies were performed in free-flowing nephrons to compare the fractional compensation (C) for perturbations in late proximal flow (VLP) with associated changes in glomerular capillary hydrostatic pressure (PGC). VLP was monitored by a noninvasive optical technique, and PGC was monitored by direct capillary micropuncture. Data were employed in conjunction with a model of glomerular filtration to analyze the TGF effector mechanism. C varied with the applied perturbation (VH), showing a single peak near the natural operating point. In contrast, the effect of TGF on PGC was asymmetric about the operating point, such that PGC was sensitive only to large positive perturbations. The model predicts that changes in preglomerular resistance account for only a small fraction of the integrated TGF response to small disturbances in the ambient state of the nephron but account for a greater share of the response to larger increments in flow. The TGF response in hydropenic rats is mediated by both pre- and postglomerular vascular elements.

Climatic changes, streamflow, and long-term forecasting of intraplate seismicity

Regions of intraplate seismicity in the eastern United States are spatially isolated areas of persistent, diffuse earthquake activity. There is no widely accepted explanation for the origin of these earthquakes. We have suggested that climate plays a key role in triggering such intraplate seismicity. Long-term increases and decreases in rainfall cause periodic regional and temporal variations in the elevation of the water table that, by pore pressure diffusion, result in small changes in fluid pressure at any given depth in the crust. In a fractured, hydraulically permeable crust, the depth of penetration of this pore pressure diffusion can be as deep as the brittle-ductile transition (15–18 km). In a seismogenic crust, stress corrosion and fatigue of rock asperities might be more important than purely mechanical effects due to small changes in hydrostatic fluid pressure; however, because any chemical effects are quasi-static, the temporal characteristics of the triggering process might ultimately be determined by the mechanical process, resulting in a “hydraulically induced” seismicity trigger that acts somewhere along paths of pore pressure diffusion. We called this model of intraplate earthquake generation “hydroseismicity”. Because streamflow is related to the regional and temporal morphology of the water table, we searched for and report here several attempts to find links between climatic changes, streamflow, and intraplate seismicity. Our results fall broadly into four categories: (1) temporal correlations of streamflow with the earthquake strain factor; (2) spectral analyses of flow and seismicity and the identification of common spectral peaks; (3) numerical modeling to estimate fluctuations in pore pressure at hypocentral depths; and (4) climatic driving mechanisms (e.g., sunspot cycles) that might substantiate a climate-earthquake link. We observe a statistically significant peak in the Fourier spectrum of surface streamflow for the seismic zones bisected by the Mississippi River, Illinois, and James River, Virginia, in the period range of 11–13 years that might be associated with sunspot activity. In addition, there is positive correlation between periods of above average values of the standard deviation of streamflow time series and periods of seismicity in the central Virginia seismic zone. Many aspects of the weather appear to be modulated by a 20-year cycle. We observe a similar periodicity (18–20 years) in seismicity in the central Virginia seismic zone. A good agreement is observed when a streamflow time series is superimposed on the record of the earthquake strain factor if a value of 50 km 2/year is assumed for crustal hydraulic diffusivity. In the central Virginia seismic zone, it is found that the number of earthquakes versus depth, ψ, is directly proportional to pressure fluctuations at the depth ψ. In addition, the fractal dimension determined from downward-continued streamflow is approximately the same as the fractal dimension of intraplate seismicity. Furthermore, using the Gutenberg-Richter relation and assuming that the earthquake data sets in the New Madrid and central Virginia seismic zones are complete for all magnitudes m ⩾ 2, the ratio of the number of earthquakes occurring per year in the New Madrid zone to the central Virginia zone is about 40. The ratio of the standard deviations of downward-continued Mississippi River streamflow (at Thebes, Illinois) to the James River streamflow is also about 40. One interpretation of this common ratio is that the number of intraplate earthquakes generated in a seismogenic crust is directly proportional to the standard deviation of vertical variations in the elevation of the water table. If the hydroseismicity hypothesis is correct, then long-term variations in streamflow can be used to forecast long-term statistical variations in intraplate seismic activity.

Otoacoustic emissions: a new tool for monitoring intracranial pressure changes through stapes displacements

Changes in hydrostatic intracranial pressure (ICP) are thought to be transmitted to cochlear liquids, thereby altering the mechanical load on the stapes footplate. Hence the stiffness of stapes' annular ligament is expected to reflect ICP changes. Corresponding middle-ear transmission changes have been assessed using click-evoked otoacoustic emissions (EOAE) in two experiments. The fast one was performed in 22 normal human subjects submitted to posture changes. In the second one, controlled ICP increases were applied to 18 patients in neurosurgery operating theater. EOAEs were monitored in these subjects throughout the experiments and their phases and amplitudes were analyzed as a function of frequency. ICP affected EOAEs in a systematic manner in both experiments, i.e., the main effect of ICP increase was a phase lead of EOAE components below 2 kHz without any significant amplitude or frequency shift. Their variations ere much more evident than the modifications in middle-ear impedance assessed by standard impedance or admittance measurements. The second experiment led to a quantitative linear relationship between ICP and EOAE phases. It demonstrated that the sensitivity of EOAE phase monitoring technique proves to be quite high, i.e., 55 daPa increases in ICP are readily detected in all subjects. The results are consistent with the predictions of classical middle-ear models as to the transmission alterations induced by ICP-related changes in the stiffness of the annular ligament of the stapes. Non-invasive monitoring of ICP in patients with hydrocephalus treated with ventriculo-peritoneal shunts might be attempted with this technique, although it is restricted to patients with detectable EOAEs (i.e., about 50% in this series).

Stoneley and flexural modes in pressurized boreholes

The propagation of Stoneley and flexural waves in a fluid‐filled borehole is adequately described by the linear equations of elasticity. However, when the borehole fluid is pressurized either due to the hydrostatic head at a given depth or with the aid of packers at the wellhead, both the fluid and the surrounding formation are subjected to biasing stresses. Under this situation, wave propagation along the borehole is described by the equations of motion for small dynamic fields superposed on a bias. The resulting formulation allows us to study the influence of a change in the fluid pressure on the Stoneley and flexural mode dispersion curves. Since the biasing stresses in the surrounding formation exhibit a radial decay away from the borehole, it is expedient to employ a perturbation technique to calculate changes in the borehole Stoneley and flexural wave dispersion curves as a function of hydrostatic pressure change in the fluid. A key advantage of this perturbation technique is that it separates contributions of the acoustoelastic effect due to the borehole fluid and that due to the formation. Insofar as the fluid nonlinear properties at a given pressure and temperature are known, the model provides a procedure for estimating the acoustoelastic coefficient of the formation for the borehole Stoneley and flexural wave velocities for a given change in the fluid pressure. The formation acoustoelastic coefficient can be expressed as a fractional change in the acoustic wave velocity caused by a unit change in the borehole pressure. Computational results show that acoustoelastic coefficients for the Stoneley and flexural waves are larger for formations with higher degree of nonlinearity which is typically associated with poorly consolidated rocks.

Effects of Surface Roughness and Rainfall Impact on Overland Flow

Bed roughness and rainfall impact affect hydraulic friction in overland flow. The relationship between friction, velocity, discharge, and channel slope was examined for the case of shallow, spatially varied flow on rough surfaces. Rainfall was simulated on artificially roughened surfaces in a laboratory. Measured flow velocities were compared to theoretical velocities computed from a dynamic equation for spatially varied flow and from the Darcy-Weisbach uniform flow equation. The measurements showed that the constant that relates the Darcy-Weisbach friction coefficient to Reynolds number can become larger than the theoretical value for smooth laminar flow. An analysis of the dynamic equation showed that frictional forces were much larger than forces resulting from changes in hydrostatic pressure and momentum. Water-surface profiles based on the Darcy-Weisbach friction equation nearly matched those from the dynamic equation for spatially varied flow. A regression was developed from the Darcy-Weisbach equation. The regression accurately predicted flow velocity when absolute roughness, rainfall rate, channel discharge, slope, slope length, and viscosity were known.

Postural vascular response vs. sympathetic vasoconstriction in human skin during orthostasis

Sympathetic activation and local vascular smooth muscle reactions to vessel distension contribute to the increase in vascular resistance in the skin during orthostasis. The relative contribution of these two mechanisms to the changes of skin blood flow along the body axis on standing was investigated in healthy male subjects by laser-Doppler (LD) fluxmetry. Compared with recumbency, LD flux (LDF) in the standing subjects was reduced by -19.6 +/- 7.2% at the forehead and by -69.6 +/- 9.6% in the leg. In the absence of hydrostatic pressure changes, the LDF changes on standing averaged -29 +/- 13%, independent of skin region, reflecting the effect of vasoconstriction due to sympathetic activation. The postural vascular response, elicited by lowering the arm or the leg from heart level, was significantly attenuated in orthostasis compared with recumbency. The vessel reaction to local alteration of transmural pressure was studied in the skin of the forehead and lower leg by application of external pressure in supine subjects. No difference in vessel responsiveness to changes of transmural pressure was found between these skin sites. The findings suggest that the changes of skin perfusion in orthostasis result from a nonadditive interaction of height-dependent and -independent mechanisms.

F248. Process and device for determining the viscosity of liquids

A process and a device for determining the viscosities of liquids are described, which work quickly, reliably and accurately. The process according to the invention makes use of the fact that disturbing effects are eliminated when measuring the change in the rate of flow of the liquid from the viscometer, while utilizing the change in the hydrostatic pressure during viscosity measurement and that the viscosities can be reliably determined from the measured data by the Hagen-Poiseuille law. The viscometer according to the invention is designed so that the hydrostatic pressure change during measurement is maximal.

Relationship Between Shape Memory Material Properties and Applications

The shape memory effect has been observed in several material systems: metals, ceramics and polymers. Mainly metallic systems in which a strain-induced or thermally-induced martensitic transformation occurs, have been explored. Also, recent discoveries have shown that shape memory effects in specific systems can also be driven by changes in hydrostatic pressure or magnetic fields. This indicates that not all effects related to the martensitic transformation have been explored yet for use in application. But even in classic systems designers are challenged to better analyse what functions the material properties can really bring about in applications such as actuators, smart materials or medical devices

Spontaneous changes in arterial blood pressure and renal interstitial hydrostatic pressure in conscious rats.

1. Previous work has demonstrated a positive relationship between experimentally induced changes in arterial pressure (AP) and renal interstitial hydrostatic pressure (RIHP). The purpose of the present study was to test the hypothesis that RIHP is positively correlated with the normal changes in AP that occur spontaneously in conscious rats. 2. Rats were chronically instrumented for the recording of AP (via an aortic catheter) and RIHP. RIHP was measured by implanting a Millar microtransducer, whose tip had been encapsulated in a 35 microns pore polyethylene matrix (5 mm long, 2 mm o.d.), approximately 5 mm below the renal cortical surface. 3. A total of 56 h of simultaneous analog recording of AP and RIHP was obtained from ten rats. Each 1 h segment was digitized and evaluated at frequencies of 1, 0.1, 0.02 and 0.01 Hz. 4. In forty-nine out of fifty-six of these 1 h recordings taken at 1 Hz, there were significant positive linear correlations between AP and RIHP (mean r = 0.32) with a mean slope of 0.11 mmHg RIHP/1 mmHg AP. Low-pass filtering to 0.01 Hz significantly increased the r value to 0.48. 5. These results demonstrate that spontaneous changes in AP and RIHP are positively correlated. The spontaneous coupling of AP and RIHP may be of importance in the regulation of salt and water excretion by the pressure diuresis mechanism.

Early leg blood flow adjustment during dynamic foot plantarflexions in upright and supine body position.

The time courses of leg blood flow, systolic peak velocity, heart rate and blood pressure have been studied in nine health volunteers during dynamic exercise in upright sitting and in a tilted sitting body position. In both positions the subjects performed single and repeated foot plantarflexions against light and moderate forces corresponding to 5%-10% and 25%-30% of maximal voluntary contraction. The following measurement techniques were used: Doppler ultrasound method (blood flow parameters), FINAPRES device (arterial blood pressure) and standard ECG chest leads (heart rate). At rest the supine blood flow parameters measured in the arteria femoralis were significantly higher than in the upright sitting position. In both positions, even one single plantarflexion at the light exercise intensity caused significant increases in blood flow for almost 20s. The major part of the blood flow response to repeated contractions always occurred within the first 10s at virtually unchanged blood pressures. During this initial phase upright leg blood flow increased by factors of 2.5 (light exercise) and 3.1 (moderate exercise). The corresponding values in the tilted sitting posture were 1.7 and 1.9, respectively. The initial increases in the upright position were too large to be attributed only to the increase of the perfusion pressure caused by the withdrawal of the hydrostatic pressure on the venous side ("muscle pump"). Additional, fast decreases in local resistance have to be considered. In the supine posture effects on local resistance have to be taken into account for the early increases in blood flow since hydrostatic effects on arterio-venous pressure differences are too small. The present findings indicate that the effects of repeated contraction-relaxation cycles on the early adjustment of muscle blood flow are not sufficiently described by a "muscle pump" that induces only venous volume shifts and hydrostatic pressure changes. Additional fast effects on local resistance have to be taken into account.

Mechanosensitivity of NMDA receptors in cultured mouse central neurons

Changes in osmotic and hydrostatic pressure were found to modulate NMDA responses of cultured embryonic mouse neurons recorded in various patch-clamp configurations. In nucleated patches, NMDA currents were potentiated by reductions in external osmolarity and were reduced in hyper-osmotic solutions. These changes, which were greater for low concentrations of NMDA, were not observed for responses to kainate, glycine, or GABA. They could be mimicked by directly changing the pipette pressure in nucleated, outside-out, inside-out, and cell-attached patches. Osmosensitivity of NMDA responses was also observed in the whole-cell mode, but only after prolonged dialysis. Mechanosensitivity of NMDA receptors could have an important role in neuronal regions experiencing changes in membrane tension, such as spines or growth cones.

Differential Effects of Agents on Bronchial and Vascular Tone and Lung Weight in the Rat Isolated Perfused Lung

Summary: A ventilated perfused model of rat lung is described enabling continuous simultaneous measurement of pulmonary perfusion pressure (PPP), pulmonary inflation pressure (PIP) and lung weight. The preparation was stable for 2 h before oedema formation commenced. Phenylephrine injected into the pulmonary artery produced selective increases in PPP (ED50 4.0±0.7 nmol, n=5) without affecting airway tone or lung weight. Carbachol under basal perfusion conditions caused bronchoconstriction (ED50 4.25±0.4 nmol, n=7) without affecting PPP and lung weight. Bradykinin (BK) caused increases in PPP (ED50 22±3 nmol, n=8), lung weight and PIP (ED50 35.6±3.6 nmol, n=8). Low doses of BK (25-50 nmol) caused increases in lung weight which were reversible. Higher doses (100-200 pmol) caused irreversible increases in lung weight which albumin-bound dye infusion showed were associated with albumin accumulation. Manipulation of venous outflow pressure revealed that the changes in lung weight induced by BK were potentially explicable as sequelae of vascular hydrostatic pressure changes possibly resulting from venoconstriction. It is concluded that this model is useful for studying drug actions on bronchial tone, pulmonary vasculature tone and vascular permeability.

Control of cytoskeletal mechanics by extracellular matrix, cell shape, and mechanical tension

We have investigated how extracellular matrix (ECM) alters the mechanical properties of the cytoskeleton (CSK). Mechanical stresses were applied to integrin receptors on the apical surfaces of adherent endothelial cells using RGD-coated ferromagnetic microbeads (5.5-microns diameter) in conjunction with a magnetic twisting device. Increasing the number of basal cell-ECM contacts by raising the fibronectin (FN) coating density from 10 to 500 ng/cm2 promoted cell spreading by fivefold and increased CSK stiffness, apparent viscosity, and permanent deformation all by more than twofold, as measured in response to maximal stress (40 dyne/cm2). When the applied stress was increased from 7 to 40 dyne/cm2, the stiffness and apparent viscosity of the CSK increased in parallel, although cell shape, ECM contacts, nor permanent deformation was altered. Application of the same stresses over a lower number ECM contacts using smaller beads (1.4-microns diameter) resulted in decreased CSK stiffness and apparent viscosity, confirming that this technique probes into the depth of the CSK and not just the cortical membrane. When magnetic measurements were carried out using cells whose membranes were disrupted and ATP stores depleted using saponin, CSK stiffness and apparent viscosity were found to rise by approximately 20%, whereas permanent deformation decreased by more than half. Addition of ATP (250 microM) under conditions that promote CSK tension generation in membrane-permeabilized cells resulted in decreases in CSK stiffness and apparent viscosity that could be detected within 2 min after ATP addition, before any measurable change in cell size. Permanent deformation only decreased after 20 min, once the CSK lattice had physically contracted. Importantly, regardless of cell shape or membrane continuity, CSK stiffness increased in direct proportion to the applied stress, as predicted by tensegrity (tensional integrity) cell models. These results suggest that the effects of ECM on CSK mechanics are not due to changes in osmotic or hydrostatic pressures. Rather, ECM alters CSKstiffness and apparent viscosity by binding integrins, promoting formation of molecular links with the CSK, transmitting mechanical stresses across these linkages, and inducing structural rearrangements within a continuous, tensionally integrated CSK lattice. In contrast, permanent deformation in the CSK appears to be more tightly coupled to cell extension and depends on both passive plasticity and dynamic remodeling events.

Mechanical Regulation of Cartilage Biosynthetic Behavior: Physical Stimuli

The biosynthetic response of calf cartilage disk explants to small-amplitude dynamic compression was studied in radially unconfined compression over a wide range of frequencies. The relative importance of oscillatory fluid flow, hydrostatic pressure, streaming potential, and cell deformation in modulating chondrocyte metabolism was explored by quantifying the frequency dependence and the spatial (radial) distribution of the biosynthetic response within the 3-mm-diameter explant disks. At frequencies greater than 0.001 Hz (cycle/s), dynamic compression increased biosynthesis of proteoglycans and proteins. While compression at frequencies between 0.002 and 0.01 Hz caused a stimulation of biosynthesis that was distributed throughout the disk, compression at 0.1 Hz caused a stimulation that was confined mainly to the outer radial periphery. These distributions were compared to previous estimates of the radial distribution of physical forces and flows within the matrix. The results suggest that the stimulation of chondrocyte biosynthesis by dynamic mechanical compression at amplitudes up to 10% (stresses up to 0.5 MPa) is related to changes in fluid flow and/or cell shape rather than changes in hydrostatic pressure. Since static compression to the original cut thickness caused a slight decrease in biosynthesis in the center of the disks, we also studied the possible role of limited diffusive transport in the marked inhibition of synthesis seen during large displacement static compression. Experiments in which the surface area-to-volume ratio of disks or the concentration of labeling substrate or serum were varied provided no evidence that limited diffusive transport was responsible for the inhibition of biosynthesis by large displacement static compression. Recovery of biosynthesis from static compression and histological analyses of compressed tissue suggested that there was no significant cell damage even during 12 h of 50% static compression.

Anatomy and mechanics of the telescopic ovipositor system of Scelio latreille (Hymenoptera: Scelionidae) and related genera

The telescopic ovipositor system possessed by the genus Scelio Latreille (Hymentoptera : Scelionidae) and related genera is described in detail for the first time, and found to be anatomically and mechanically unique amongst the parasitic Hymenoptera. Its basic plan is similar to the Ceratobaeus-type ovipositor system described previously for other scelionids, in that the ovipositor is invaginated entirely into the body cavity when at rest and attached to the terminal metasomal segment only by a lightly sclerotized collapsible membranous tube. There are, however, significant anatomical and mechanical modifications that permit the Scelio-type system greater extendability. Telescopic extension of multiple sections of greatly elongated intersegmental membrane, operated by changes in hydrostatic pressure, allows the entire ovipositor system, including the terminal metasomal tergite (T7 + T8), to be exserted from the body cavity during oviposition, thus extending the range of the ovipositor by as much as 3.5 times its length. These changes are accompanied by the incoporation of the lateral apodemes into the wall of the most distal segment of telescopic tube, and the loss of their associated musculature. Similar to other scelionids, orientation of the ovipositor in Scelio is controlled by contraction of muscles connecting the proximal head of the ovipositor with the fused terminal metasomal tergite (T7 + T8). A model for the mechanics of extension and retraction of the Scelio-type ovipositor system is proposed, and is supported by anatomical evidence, behavioural observations and direct manipulation of the system. The evolution of this system in relation to the exploitation of a particular host group, the eggs of Orthoptera, and its significance in resolving phylogenetic problems within the Scelionidae, are also discussed.

Vertical movements and distribution of planktonic larvae of the serpulid polychaete Spirobranchus polycerus (Schmarda); effects of changes in hydrostatic pressure

Vertical distribution, in a 60 cm water column, was recorded for 1, 2, 3, 4 and 6 day larvae of the tropical serpulid Spirobranchus polycerus (Schmarda), in the dark and under low level overhead illumination. All age classes of larvae were distributed unevenly under both light conditions. One-day larvae concentrated at both the bottom and the top of the water column; 2- and 3-day larvae concentrated at the top, as did 4-day larvae in the dark. Four-day larvae in the light and 6-day larvae under both light regimes were equally concentrated at the top and the bottom. Upward and downward vertical displacement rates were measured for swimming 1-day larvae; sinking speed was measured for anaesthetized larvae. One day larvae swim up or down more frequently than horizontally. One-day larvae respond to an increase in hydrostatic pressure with an increase in the percent of larvae moving downward; they respond to a decrease in pressure with an increase in the percent moving upward. The same overall change in pressure was applied at three rates; in all cases the change in the percent of larvae moving up or down was significant at the p<0.05 level. Maximum calculated rates of change in pressure experienced by larvae swimming up and down are close to the lower end of the range of experimentally applied rates of change of pressure. The results are discussed in terms of earlier models for the effects of changes in hydrostatic pressure on decapod larvae.

Failure by fracture or yielding in strain hardening materials according to the t-criterion

An attempt is described to develop an improved theoretical approach for the failure of materials, being initially either precracked or uncracked. As a guide for this an idealized approach to the fundamental reaction of solids during state transitions is employed. By setting an upper limit T v,0 of mechanical energy for volume change and another one T D,0 for distortion, beyond which the material transits to either brittle fracture or general rupture by yielding, a closed limiting failure surface is obtained. The necessity of considering the constitutive relation between hydrostatic pressure, p, and unit volume change, θ, in addition to the equivalent stress-strain relation ( \\ ̄ gs- \\ ̄ ge) is emphasized. Finally conclusions are made concerning the experimental evaluation of T v,0 and T D,0 and the application of the present failure criterion in the case of precracked materials. Predictions obtained from this criterion agree well those of McClintock's criterion based on void coalescence, and are clearly supported by the classical experiments performed by Bridgman.

Reduced permselectivity in isolated perfused rat kidneys following small elevations of glomerular capillary pressure.

A modified rat kidney preparation was used to explore how changes in hydrostatic pressure affect the permselective properties of the glomerular capillary bed. The maximally vasodilated kidneys of 18 rats were perfused with albumin solutions (16.7 g l-1) at different flow rates and hence arterial pressures (PA). One kidney in each rat was exposed to pressure elevations with the other kidney serving as a control perfused at constant PA of about 100 mmHg. Both the vascular resistance to flow and the glomerular filtration rate (GFR 34.6 +/- 2.9 ml min-1 100 g-1) were similar in the two kidneys at equal PA and remained constant throughout the experiment. The ratio of albumin clearance over GFR (theta) was initially around 0.4% at constant PA and gradually increased during 1.5 h to reach 0.7% at the end of the experiment. A direct increase of PA from 100 to 200 mmHg for 15 min resulted in a calculated increase of the effective glomerular filtration pressure gradient of 10-15 mmHg and in a two-fold increase of theta when measured at an identical PA of 100 mmHg. Albumin clearance was almost fully normalized within 20 min similar to that observed in e.g. skeletal muscle. However, the glomerular capillary barrier seemed to be far more sensitive to elevations of hydrostatic pressure than other capillary walls which require capillary pressure increments of 60 mmHg in order to induce similar reversible changes in permeability. Therefore, we conclude that an elevated PGC per se induces changes of glomerular permselectivity, which may have important pathophysiological implications during conditions of proteinuria.

The chondrocyte: a cell under pressure.

Mechanical load is an important regulator of chondrocyte metabolic activity. Changes in amplitude or frequency of the load can have a significant effect on the production of matrix macromolecules and of agents leading to cartilage breakdown. The composition of cartilage reflects the net response of the chondrocytes to the prevailing loading pattern, with cartilage proteoglycan content highest in heavily loaded regions and removal of load leading to cartilage thinning and proteoglycan loss. The mechanism of mechanotransduction is poorly understood; the chondrocytes appear to react to cartilage deformation and to the changes in hydrostatic pressure, extracellular ionic composition and streaming potentials induced by the load.