Hydrostatic Pressure Difference Research Articles

Fluid transport across human fetal membranes affected by human amniotic fluid prolactin: an in vitro study.

The aim of the study was to investigate the effects of human amniotic fluid (AF) prolactin (PRL) on the transfer of fluids across human fetal membranes, detected in vitro by weight change. We have developed a system, consisting of an inner compartment (4 ml), suspended in an outer compartment (40 ml), and separated from it by a 2 cm2 circular piece of fresh human fetal membrane (amnion, chorion and adhering decidua). The weight of the inner compartment was continuously registered with an electronic precision balance. Osmolality changes or hydrostatic pressure differences did not affect the rate of mass transfer through the membranes. Potassium cyanide significantly influenced the mass transfer in favour of the applied hydrostatic pressure difference (+2 cm H2O to the amniotic side) or osmotic pressure difference (amniotic side 265 mosm/kg/decidual side 285 mosm/kg). The PRL preparations used were human and ovine pituitary PRL, as well as three fractions isolated from human AF by fractionated ammonium sulphate and ethanol precipitations, followed by Sephacryl chromatography. All PRL preparations were tested in physiologic concentrations (0.5 to 5 micrograms/ml). Only the two largest AF-PRL variants significantly disturbed the balance of fluid transfer across the membranes when added to the fetal (amniotic) side of the membrane. This resulted in a net increase in fetomaternal transfer of 120 to 180 microliters. This effect could be repeated and lasted for at least 25 minutes. Using an antibody against hPRL the effect was completely blocked. Neither AF-PRL added to the maternal (decidual) side of the membrane nor oPRL or human pituitary PRL added to the amniotic or decidual side changed the rate of mass transfer across the membranes.

Glomerular hemodynamic adaptations in remnant nephrons: effects of verapamil.

Chronic verapamil administration has been shown to reduce the renal dysfunction, glomerular sclerosis, and mortality in partially nephrectomized rats; the mechanism is unknown. Therefore, the present micropuncture study examines the effect of verapamil on glomerular hemodynamics of Munich-Wistar rats 4 wk after five-sixths nephrectomy. Whole-kidney function was similar in nephrectomized rats treated with verapamil (Nx-VER) and saline (Nx-SAL), and glomerular filtration rate (GFR) was one-third of control. Both Nx-VER and Nx-SAL rats exhibited a similar degree of glomerular hyperperfusion and hyperfiltration. However, mean glomerular capillary hydrostatic pressure difference (delta P) was lower in Nx-VER than Nx-SAL rats (42.4 +/- 1.2 vs. 47.2 +/- 1.0 mmHg, P less than 0.005) because of an increase in Bowman's space hydrostatic pressure (PBS) in Nx-VER rats (15.5 +/- 0.8 vs. Nx-SAL 11.9 +/- 0.7 mmHg, P less than 0.005). Glomerular ultrafiltration coefficient (LpA) was significantly lower in Nx-SAL (P less than 0.005) than in Nx-VER or control rats. Urinary protein excretion and the magnitude of glomerular sclerosis in Nx-SAL and Nx-VER rats were not different. In conclusion, chronic VER administration normalizes LpA and reduces delta P, by increasing PBS, in Nx rats, alterations that neutralize each other, leading to the constancy of single-nephron GFR.

Renal adrenergic effector mechanisms: glomerular sites for prostaglandin interaction.

Micropuncture experiments were performed in Munich-Wistar rats to ascertain the renal microcirculatory sites at which prostaglandins interact with the renal nerve and angiotensin II. Renal nerve stimulation (RNS) of 3 Hz alone decreased single-nephron glomerular filtration rate (SNGFR) by 30%, the consequence of 10 and 35% reductions in the glomerular capillary hydrostatic pressure difference (delta P) and the single-nephron plasma flow (SNPF), respectively. Pre- and postglomerular vascular resistances increased. RNS during prostaglandin inhibition (indomethacin) resulted in a 70% reduction in SNGFR, secondary to 1) a further diminution in delta P and in SNPF, via heightened pre- and postglomerular vasoconstriction and 2) a marked decline in the glomerular ultrafiltration coefficient (LpA), from 0.058 +/- 0.006 to 0.027 +/- 0.002 nl.s-1.mmHg-1.g kidney wt-1 (P less than 0.005). Acute angiotensin II inhibition (MK-421 and [Sar1,Ala8]angiotensin II) in rats pretreated with indomethacin partially attenuated the effects of RNS on vascular resistances and therefore on delta P, SNPF, and SNGFR and prevented the reduction in LpA. Thus vasodilatory prostaglandins act as local modulators of both renal nerve and angiotensin II constrictive actions on glomeruli and renal microcirculation.

The relationship between renal metabolism and proximal tubule transport during ontogeny.

The proximal tubules of newborn and adult animals reabsorb a similar fraction of the filtered load of Na+ and H2O (65%-70%). In tubules from adult animals, transcellular, active Na+ reabsorption accounts for one-third of the total, while two-thirds occur passively through the paracellular pathway, driven by hydrostatic and oncotic forces (one-third) and by cell-generated effective osmotic and ionic gradients (one-third). Since two-thirds of the Na+ is reabsorbed passively and does not require energy, the mature proximal tubule has a high Na+/O2 molar ratio (48 Eq of Na+/mol of O2). Measurements of ouabain-sensitive oxygen consumption in suspensions of proximal tubules indicate that in newborn, aerobic metabolism can support about 50% of the net Na+ transport rate compared with the 33% in tubules from adult animals. Independent confirmation of the direct and proportional relationship between active Na+ transport and ouabain-sensitive O2 consumption exists for the adult but not for the newborn. However, measurements of epithelial conductances and of transepithelial hydrostatic and oncotic pressure differences indicate that passive paracellular fluxes can account for the remaining 50% of the proximal Na+ reabsorption in newborn. The high permeability of the proximal tubules of newborn animals to small molecular weight solutes suggests that cell-generated osmotic and ionic transepithelial gradients are minimal in the tubules of newborn animals. Yet in the newborn, the osmolality of the end proximal tubule fluid was found to exceed that in plasma. This indicates that osmotic gradients due to differences in reflection coefficients for preferentially reabsorbed solutes and Cl- do exist across the proximal tubules of the newborn and suggests that these gradients may contribute to Na+ and H2O reabsorption. If this is indeed the case, then the contribution of active and of hydrostatic and oncotic pressure-driven flows to the overall reabsorption of Na+ and fluid has been overestimated. Resolution of this discrepancy requires measurements of the reflection coefficients for HCO3- and Cl- in the proximal tubule of the newborn. The metabolic processes by which energy is supplied to renal proximal cells during development are also incompletely characterized. There is evidence that maturation of aerobic metabolism, Krebs cycle enzymes activity, and of the mitochondrial membrane surface area precede the development of net reabsorptive transport (Na+, H2O, HCO3, glucose). By contrast, maturation of Na(+)-K(+)-ATPase activity at the basolateral cell membrane follows that in reabsorptive transport and does not limit its development.(ABSTRACT TRUNCATED AT 400 WORDS)

Marine acoustic gradient sensor

A pressure gradient sensor is disclosed for optically sensing the direction and magnitude of an acoustic wave propagating through a fluid. Each sensor consists generally of at least three fiber-optic displacement sensors disposed within a circular array oriented perpendicular to the longitudinal axis of a streamer. Each sensor phase-modulates a coherent light signal indicative of the water pressure sensed outside the streamer. The output of the signals include a component due to transient pressure waves and a component due to hydrostatic pressure differences between the sensors.

Embolized Air Collection in the Superior Vena Cava of ???Upright??? Sheep

In 34 studies on six "upright" anesthetized sheep central venous pressure from sites in the upper and lower superior vena cava (SVC) was recorded before, during, and after the injection into a neck vein of 1-ml increments of air every minute for 15 min. After the injection of air, the pressure recorded from the upper site increased significantly. This increase was the result of air collecting in the SVC with a consequent loss or partial loss of the SVC hydrostatic pressure difference. The time of onset and the duration of collection were not influenced by hyper- or hypovolemia nor by 1 kPa positive end-expiratory pressure. The readily identifiable change in the venous pressure in the upper SVC provided an earlier and more reliable warning of these small-volume air emboli than did a Doppler probe directed at the right side of the heart.

A model of osmotic and hydrostatic pressure effects on volume absorption in the proximal tubule.

A computer model of the proximal tubule of the rabbit is described in which the tubule is treated as a single cylindrical barrier to the flow of solute and water between lumen and bath, and volume absorption is assumed to be driven exclusively by hydrostatic and osmotic pressure differences across this barrier. The model mimics the function of the tubule in two in vitro preparations: in simulations of the isolated tubule perfused under oil, the model correctly describes the solute concentration gradients that exist between the perfusate and absorbate and predicts differences in solute concentrations among absorbate droplets on the same tubule if luminal concentration becomes limiting. This prediction was tested experimentally with glucose and found to be correct. In simulations of the tubule perfused in an aqueous bath, the role of transmural hydrostatic pressure was explored; it is predicted that, at normal rates of in vitro perfusion (approximately 10 nl/min), increases in pressure have very little effect on volume absorption but can greatly alter the osmotic differences present across the wall of the tubule, especially with high values of osmotic water permeability. At high rates of perfusion, the ability of the tubule to produce a lumen hypotonic to the bath is reduced, but the direct effects of pressure on volume absorption become more apparent, resulting in relatively little effect of perfusion rate on volume absorption if the osmotic water permeability is sufficiently high. A similar relationship was seen experimentally. In all, this simple model provides a good prediction of function in isolated perfused tubules without any assumptions of hypertonic compartments within the epithelium.

Isolated perfused Ambystoma proximal tubule: hydrodynamics modulates ultrastructure.

A method using a pressure-sensing servo-pipette is described for measuring downstream transepithelial pressure within isolated renal tubules perfused at flow rates designed to keep luminal solution composition constant. The hydrodynamics of in vitro microperfusion of isolated proximal tubules of Ambystoma tigrinum was varied and different states of transepithelial hydrostatic pressure difference, axial tubule flow, and transepithelial transport were correlated with epithelial ultrastructure. Tubules analyzed by ultrastructural morphometry were as follows: unperfused with and without ouabain, perfused single-end cannulated with and without ouabain, and perfused double-end cannulated tubules incubated in substrate Ringer. The results indicate that proximal tubule fine structure is well preserved for more than 3 h in unperfused and perfused tubules. Small transepithelial hydrostatic pressure gradients (less than 162 Pa) increase tubule diameters and decrease cell height without changing volumes of the cells, lateral intercellular spaces (LIS), or the basal extracellular labyrinth (BEL). Pressure gradients of 271 Pa have no further effect on tubule diameters or cell height, but significantly reduce volumes of LIS and BEL. Transport inhibition and axial flow changes have minor structural effects. This study demonstrates a close dependence of tubule ultrastructure on hydrodynamic conditions and provides guidelines for optimizing the latter during perfusion of isolated renal tubules.

Evaluation of the Starling Fluid Flux Equation

It is commonly thought that fluid is filtered in the arterial and is absorbed in the venous end of the capillary, cuased by the considerable hydrostatic pressure difference between the arterial and the venous end, while the transcapillary colloid osmotic pressure difference remains nearly constant. We now know that extravascular forces, i.e., tissue fluid pressure, tissue colloid osmotic pressure, and lymph flow, are dynamic factors that change to oppose transcapillary fluid movement. Therefore, the filtration-absorption theory will apply only transiently until the tissue forces readjust.

Modulation of renal adrenergic effector mechanisms by calcium entry blockers.

Micropuncture studies in anesthetized Munich-Wistar rats were undertaken to investigate the effects of calcium channel blockade on the glomerular hemodynamic responses to 3-Hz renal nerve stimulation. Stimulation alone increased afferent and efferent arteriolar resistances by 85 and 35%, respectively. Because of these increases both single nephron plasma flow and glomerular capillary hydrostatic pressure difference fell to levels significantly below control, leading to a 26% reduction in single nephron filtration rate (P less than 0.005). These changes, however, were largely attenuated during calcium channel blockade (verapamil, nifedipine). Single nephron filtration rate was only decreased by 14% (P less than 0.05) due to a reduction in single nephron plasma flow. The role of angiotensin II on the residual vasoconstrictive effect of stimulation was also investigated. Pretreatment with an angiotensin-converting enzyme inhibitor (MK 421) of rats infused with verapamil abolished the residual vasoconstriction. The data suggest that calcium influx is an important step for the vasoconstrictive effects of the renal nerves. Additionally, angiotensin II contributes to increased vascular resistance during renal nerve stimulation via a separate, calcium channel mechanism.

Spatial heterogeneity and microvascular fluid exchange: A simple macroscopic equation

The effect of the spatial distribution of capillary membrane “pores” on the estimate for the hydraulic conductivity ( L p ) and the reflection coefficient ( σ s ) in the presence of an axial gradient in the hydrostatic pressure difference ( ΔP) is examined. The differential fluid balance is integrated along the length of the capillary and a simple analytic solution is obtained for a capillary membrane of arbitrary structure. The effective hydrostatic pressure difference, ( ΔP) eff, is not simply the average of the arterial [( ΔP) a] and venous [( ΔP) v] pressure differences but is instead found to be (ΔP) eff ≡ [(ΔP) a + (ΔP) v ] 2 + κ [(ΔP) a − (ΔP) v] 2 , where κ ≡ 1 – 2 ( L I p L p ). The two macroscopic coefficients, L p and L I p , are related to integrals of the pore density along the length of the capillary. Failure to account for spatial heterogeneity may lead to an incorrect interpretation of the experimental data, e.g., the estimate for σ s could be >1 or <0 depending on the pore distribution and pressure gradient along the capillary. The single capillary hydraulic conductivity measurements of Gore 1982, Amer. J. Physiol. 242, H268–H287 and data on the microvascular pressure distributions (Bohlen and Gore 1977), Microvasc. Res. 14, 251–264) are used to estimate the magnitude of these effects.

Determinants of capillary permeability: a review of mechanisms based on single capillary studies in the frog.

THE objective of this article is to examine two complementary hypotheses that, together, describe the role of the intercellular junction in the regulation of capillary permeability. One hypothesis states that the permeability of capillaries with continuous endothelium is modulated by the area available for the diffusion of small solutes in the spaces between adjacent endothelial cells. The second hypothesis states that the entry of solute into the junctional pathway and the diffusion of solute within the junctional pathway are regulated by the size and distribution of a network of fibrous molecules within the wide part of the junction. Transport through the junction may also be modulated by charge and specific chemical interactions between the solute and side chains on the fibrous network. When evaluating recent research designed to test the two hypotheses, I will draw mainly on experimental data obtained on individually perfused capillaries in frog mesentery and frog muscle. For these capillaries, methods have been developed to measure the permeability properties of the capillary wall under conditions where the area for transcapillary exchange and the forces determining exchange across the capillary wall are directly measured. Many of the problems to be discussed in this review were identified first from experiments on the microvascular bed in whole organs. Studies based on single capillaries enable the mechanisms of exchange across the capillary wall to be studied directly, removing some of the uncertainties that arise when 1) microvessels, having a range of permeability properties, contribute to exchange in a whole microvascular bed, 2) it is not possible to obtain a precise measurement of exchange area, and 3) the forces determining exchange at the capillary wall (hydrostatic pressure difference, solute concentration difference) are estimated indirectly from measurements in large arteries and veins. For a review of the advantages and limitations of single capillary studies see

On direct measurement of pumping rates in the mussel Mytilus edulis

Pumping rates in Mytilus edulis L. were measured by means of a “constant-level-tank” method, in which hydrostatic pressure differences between inhalant and exhalant water levels were recorded by means of a laser beam reflected from a tethered mirror floating on the water surface. Hydrostatic pressure gradients were determined to ±0.05 mm H2O or better. The developed technique of directly measuring pumping rates in mussels is not subject to the artefacts of other methods. The pumping rates measured in M. edulis were substantially higher than those previously determined by means of direct techniques, but similar to the maximum filtration rates, as obtained by means of two indirect techniques, i.e. about 50 ml min-1 for a 0.15 g dry weight mussel. Positive hydrostatic pressures drastically affected water pumping. The pumping rate decreased linearly with increasing hydrostatic pressures towards a maximum pump pressure between 3 and 5 mm H2O. Negative pressures only affected the pumping rate slightly or insignificantly, except when the mussels were exposed to rapidly increasing negative pressures. Under this condition a shunt was presumably established between the inner demibranchs, allowing water to bypass the gills.

An in vitro model of erythroid egress in bone marrow

An in vitro system has been developed that mimics the passage of erythrocytes from the bone marrow to the circulation. Bone marrow egress and its proper regulation are vital physiologic processes. However, because of the inaccessibility of the marrow, it is difficult to evaluate the various factors important in controlling these processes or even to define the precise mechanism by which egress occurs. The in vitro system has been designed to evaluate the importance of different physical parameters in regulating egress. It consists of a thin silicon wafer (thickness approximately equal to 1.0 micron) cemented over the tip of a large (15.0 micron ID) micropipette. The wafer contains a single circular pore. Cells were observed under the microscope as they passed through the pore under controlled pressures. The rate and duration of passage were obtained from videorecordings of the experiment. The measured passage times agreed well with the predictions of a simple analytical model of a cell passing through a thin aperture. The experimental results confirm the conclusion reached from the analysis that the pressures needed to drive a cell through the pore are well within the physiologic range, and the time needed to complete egress is typically less than 1.0 seconds. These results support the hypothesis that erythrocyte egress may be driven by a hydrostatic pressure difference across the pore.

An In Vitro Model of Erythroid Egress in Bone Marrow

An in vitro system has been developed that mimics the passage of erythrocytes from the bone marrow to the circulation. Bone marrow egress and its proper regulation are vital physiologic processes. However, because of the inaccessibility of the marrow, it is difficult to evaluate the various factors important in controlling these processes or even to define the precise mechanism by which egress occurs. The in vitro system has been designed to evaluate the importance of different physical parameters in regulating egress. It consists of a thin silicon wafer (thickness approximately equal to 1.0 micron) cemented over the tip of a large (15.0 micron ID) micropipette. The wafer contains a single circular pore. Cells were observed under the microscope as they passed through the pore under controlled pressures. The rate and duration of passage were obtained from videorecordings of the experiment. The measured passage times agreed well with the predictions of a simple analytical model of a cell passing through a thin aperture. The experimental results confirm the conclusion reached from the analysis that the pressures needed to drive a cell through the pore are well within the physiologic range, and the time needed to complete egress is typically less than 1.0 seconds. These results support the hypothesis that erythrocyte egress may be driven by a hydrostatic pressure difference across the pore.

A theoretical model for the annular jet instability

The instability of an annular gas-core liquid jet is modeled theoretically by treating the liquid layer as a membrane moving under the influences of its own inertia, surface tension, and the gaseous hydrostatic pressure difference between its two sides. Essential physical mechanisms are reconstructed without making any attempt to fit experimental data with model constants. The results compare favorably with those of experiment.

Marine seismic sensor

A hydrophone streamer that includes several arrays of optical fiber pressure sensors. Each array consists of at least three sensors symmetrically disposed around the inside of the streamer skin to form a vertically-disposed array. Each sensor modulates a coherent light beam in accordance with the instantaneous ambient water pressure. The output signals of the sensors include an AC component due to seismic waves and a DC component due to hydrostatic pressure difference between the sensors of an array. Means are provided to resolve the AC and DC components to determine the arrival direction of the received seismic waves.

Observation of electro-kinetic phenomena by imposing oscillating pressure and voltage gradients across some epithelial membranes.

When hydrostatic pressure across epithelia fixed between a Ussing-type chamber was oscillated mechanically, the membrane potential was found to oscillate synchronously. When the transepithelial potential was oscillated artificially, the hydrostatic pressure difference also oscillated synchronously. Under such oscillating gradient conditions, concentration changes in the unstirred layer can be minimized. Accordingly, the method using such oscillatory gradients seem to be appropriate for observation of the true streaming potential and electro-osmosis, respectively.

Permeability of Soil-Geotextile Systems

A specially designed laboratory-type permeameter was used to characterize liquid flow behavior through various combinations of soils and geotextile fabrics subject to different flow rates and hydrostatic pressures of water. Spunbonded geotextiles made from two different fiber types ( e.g., polyester and polypropylene) were selected for the study. Volume flow rates were varied up to 21 cm/second by adjusting hydrostatic pressure differences from 2.5 to 76.2 cm. The results suggest single layer barriers of the geotextiles alone have small effects on flow rates of water, but sand-geotextile combinations exhibited strikingly different behavior. In prewashed soil-geotextile combinations, the permeability decreased non linearly with increasing depth of soil above the fabrics, independent of the supporting fabric. For soils that were not prewashed, however, fine particles in suspension settled at the soil-water interface to produce an almost impervious layer or cake structure. This caking was a function of time and became the controlling factor of flow throughout the composite. High clogging in even thin fabrics was observed when sand was replaced by fine material such as Kaolite. The behavior was different than that of the fine material in unwashed sand. In addition, geotextile fabrics showed low resiliency to recovery from cyclic compression. The practical interpretation of this is that the porosity of the fabric may decrease significantly with time if incorporated into structures that are subject to compressive forces.

Are axoplasmic microtubules necessary for membrane excitation?

The excitability of the squid giant axon was studied as a function of transmembrane hydrostatic pressure differences, the latter being altered by the technique of intracellular perfusion. When a KF solution was used as the internal medium, a pressure difference of about 15 cm water had very little effect on either the membrane potential or excitability. However, within a few minutes after introducing either a KCl-containing, a KBr-containing, or a colchicine-containing solution as the internal medium, with the same pressure difference across the membrane, the axon excitability was suppressed. In these cases, removal of the pressure difference restored the excitability, indicating that the structure of membrane was not irreversibly damaged. Electron-microscopic observations of these axons revealed that the perfusion with a KF solution or colchicine-containing solution preserves the submembranous cytoskeletal layer, whereas perfusion with a KCl or KBr solution dissolves it. These results suggest that the submembranous cytoskeletons including microtubules provide an important mechanical support to the excitable membrane but are not essential elements in channel activities.