Hydrostatic Pressure Variation Research Articles

He activated loading device for low temperature uniaxial and anvil cell pressure experiments

We report on a compact loading cell for the continuous variation of uniaxial forces up to a few tons in low temperature investigations. The cell is based on a He activated bellow and a mechanical force amplification system. Due to its modular design, it may be used in studies of the effect of uniaxial pressure on solids or the in situ variation of hydrostatic pressure in an anvil cell. High pressure He is supplied by a fine capillary, so that bellow, force amplifier, and experiment may be cooled uniformly to mK temperatures even at high magnetic fields. The pressure in the bellow is controlled by a simple proportional-integral regulator at room temperature. Characteristic features of a piezoelectric sensor, used to measure the load at low temperature, are presented in detail. With the method described here, we observe long-term stability to better than ±0.2 N in sweeps over a wide range of temperatures and fields. The performance of the loading device is illustrated by means of measurements into the superconducting state of URu2Si2.

Pressure dependence of so‐called transtrophoblastic channels during fetal perfusion of human placental villi

To test the influence of perfusion pressures on structural preservation of human placental villi and on the dilatation of the so-called transtrophoblastic channels, cotyledons of 32 term human placentas have been perfused in vitro. Periods of perfusion with isotonic Ringer solution under various arterial and venous hydrostatic pressures were followed by perfusion fixation. In some experiments, lanthanum hydroxide as an extracellular marker was added to the fixative. Distention of the fetal vascular system, stromal edema and continuity, as well as trophoblastic vacuolization were studied via electron microscopy with subsequent morphometry. The findings suggest that arterial hydrostatic pressures in the perfusion system of about 80 cm H2O are needed to guarantee homogeneous perfusion of the fetal vascular system. To avoid stromal edema and trophoblastic vacuolization, venous hydrostatic pressures of 4 cm H2O and arterial hydrostatic pressures of 80 cm H2O should not be exceeded. It is concluded that the trophoblastic vacuoles are dilated segments of the so-called transtrophoblastic channels. The functional importance of in vivo variations of fetal intravascular hydrostatic pressure for the dilatation of transtrophoblastic channels and for fetal water balance is discussed. Microsc. Res. Tech. 38:52–62, 1997. © 1997 Wiley-Liss, Inc.

Pressure dependence of so-called transtrophoblastic channels during fetal perfusion of human placental villi.

To test the influence of perfusion pressures on structural preservation of human placental villi and on the dilatation of the so-called transtrophoblastic channels, cotyledons of 32 term human placentas have been perfused in vitro. Periods of perfusion with isotonic Ringer solution under various arterial and venous hydrostatic pressures were followed by perfusion fixation. In some experiments, lanthanum hydroxide as an extracellular marker was added to the fixative. Distention of the fetal vascular system, stromal edema and continuity, as well as trophoblastic vacuolization were studied via electron microscopy with subsequent morphometry. The findings suggest that arterial hydrostatic pressures in the perfusion system of about 80 cm H2O are needed to guarantee homogeneous perfusion of the fetal vascular system. To avoid stromal edema and trophoblastic vacuolization, venous hydrostatic pressures of 4 cm H2O and arterial hydrostatic pressures of 80 cm H2O should not be exceeded. It is concluded that the trophoblastic vacuoles are dilated segments of the so-called transtrophoblastic channels. The functional importance of in vivo variations of fetal intravascular hydrostatic pressure for the dilatation of transtrophoblastic channels and for fetal water balance is discussed.

In vivo effects of tumor necrosis factor-alpha on capillary permeability and vascular tone in a skeletal muscle.

The present study aims at analysing short-term effects of TNF alpha on capillary permeability, on transcapillary fluid fluxes, and on vascular tone in a whole organ cat skeletal muscle in vivo preparation. The denervated muscle was isolated from the body but with intact vascular supply. The experimental setup allowed continuous recording of vascular tone, of hydrostatic capillary pressure and tissue volume variations. The capillary filtration coefficient (CFC), which represents the net effect for transcapillary fluid exchange of capillary permeability and number of open capillaries, was calculated before and during intra-arterial TNF infusion. We found that TNF had a minor effect on vascular tone in terms of a small vasodilation, and that no effect on hydrostatic capillary pressure could be recorded. CFC increased by 64% during the TNF infusion and this increase must be attributed to an increase in capillary permeability rather than an increase in the number of capillaries available for fluid exchange since the TNF effect on vascular tone is small. This was also supported by TNF-induced transcapillary filtration. TNF is a potent drug for increasing capillary permeability causing transcapillary filtration in vivo. Its release, e.g. during sepsis, may therefore contribute to the capillary leakage often seen in this clinical situation.

Mechanisms of water exchanges: the Starling equation

This review considers the main mathematical models used in the understanding of transvascular fluid exchanges and oedema formation. Models by Starling, Kedem-Katchalsky and Wiederhielm are briefly described. The main factors which determine the value of the physical parameters involved in these models are the osmotic and interstitial pressures in plasma and tissue. With regard to the regulation of interstitial osmotic pressure, the importance of the presence of mucopolysaccharidic gel in the interstitial compartment creating an exclusion volume for proteins diffusing from plasma is underlined, as well as the importance of lymphatic flow, a parameter neglected in several models of transcapillary exchanges. Consequences of variations in venous hydrostatic pressure, plasma oncotic pressure, endothelial permeability are discussed using the described models. In case of changes combining an increase in venous pressure and permeability and a decrease in plasma oncotic pressure, the importance of correcting venous pressure relatively to protein supply is discussed.

Activity patterns inThalamita crenataPortunidae, decapoda): A shaping by the tidal cycles

The activity rhythms of the swimming Portunid Thalamita crenata have been studied on the basis of crab sightings in surveys along three transects of a mangrove swamp in Kenya. Activity is well matched to the environmental regime and is prevalent during the high tides. However, activity does not occur over the whole of the high tide, but is confined to “temporal windows”; corresponding to a water level between 10 and 30 cm. Our findings suggest that activity in Thalamita crenata is shaped by the tidal cycles and we propose that variation in hydrostatic pressure as the tide changes is the cue which elicits activity.

Deformation of a thin-walled cylindrical tube submerged in a liquid

A long, thin, horizontal elastic cylindrical tube either fully or partially submerged in a liquid is considered. The tube supports a centered line load along its bottom and the load-carrying capacity of the tube is analysed. A nonlinear boundary value problem that takes into account the variable hydrostatic pressure is formulated. Perturbation solutions are obtained for the cases of small pressure gradients, especially in the neighborhood of the critical buckling pressure. Numerical solutions based on Newton's iteration and shooting methods are also obtained for general pressure gradients and for both the fully and partially submerged cases. The results show that, given a pressure gradient, there is an upper limit for the maximum load of the partially submerged tube. This maximum load decreases with depth when the shell is fully submerged and thus the corresponding equilibrium solution is unstable.

Pressure Stratification Effects on Multiphase Transport Across a Vertical Slot Porous Insulation

In this work, thermal stratification effects on heat and mass transfer in a porous insulation are analyzed. The vertical boundaries of the porous system are partially permeable for simulating holes or cracks in wall. Hydrostatic pressure variations are considered on the vertical boundaries and a set of realistic boundary conditions is imposed on the system under consideration. The transient intercoupled equations governing the complicated transport process along with the convective boundary conditions are solved by an efficient numerical scheme. The dependence of the Nusselt number and the field variables on several important parameters is investigated systematically. The results clearly show that infiltration can have a major effect on the overall heat transfer even for small pressure gradients across the insulation slab. Furthermore, it has been found that the opening locations have a pronounced effect on the heat transfer rate across the slab and the corresponding condensation rate. The qualitative information extracted from this investigation can be used to diminish the infiltration and condensation problems in the design of building insulation.

Effects of thiopental on resistance vessels in cat skeletal muscle

Barbiturates are used clinically as anaesthetics and to reduce raised intracranial pressure. One side effect is hypotension, usually ascribed to a depression of cardiac contractility, while their effects on the resistance vessels are more controversial: both vasodilation and vasoconstriction have been described. This study analyzes the effects of thiopental on basal vascular tone in the cat skeletal muscle. We found that total resistance increased by almost 20% at low (50 mumol/l) and decreased down to about 50% of control at high (350 mumol/l) plasma concentrations of thiopental. The vasoconstriction dominated in the large arterioles (i.d. greater than 25 microns) and the vasodilation in the small arterioles (i.d. less than 25 microns). A dose-dependent inhibition of myogenic vascular reactivity (here defined as the maximum resistance increase to a transient rise in transmural pressure) coincided with the vasodilation. Autoregulation of blood flow was depressed by thiopental. During vasoconstriction there was a net transcapillary fluid absorption and during vasodilation a net fluid filtration. The fluid movements could be ascribed to variations in capillary hydrostatic pressure. If applicable to the cerebral circulation these results suggest that thiopental at high plasma concentrations might induce, instead of reduce, interstitial brain oedema.

Mixed convection-radiation interaction in boundary-layer flow over horizontal surfaces

The effect of buoyancy forces and thermal radiation on the steady laminar plane flow over an isothermal horizontal flat plate is investigated within the framework of first-order boundary-layer theory, taking into account the hydrostatic pressure variation normal to the plate. The fluid considered is a gray, absorbing-emitting but nonscattering medium, and the Rosseland approximation is used to describe the radiative heat flux in the energy equation. Both a hot surface facing upward and a cold surface facing downward are considered in the analysis. Numerical results for the local Nusselt number, the local wall shear stress, the local surface heat flux, as well as the velocity and temperature distributions are presented for gases with a Prandtl number of 0.7 for various values of the radiation-conduction parameter, the buoyancy parameter, and the temperature ratio parameter.

Gas transport from methane‐saturated, tidal freshwater and wetland sediments

Variations in hydrostatic pressure controlled by diurnal tides triggered ebullition from subtidal freshwater sediments dominated by methanogenesis in the White Oak River estuary, North Carolina. Pulses of gas consisting of 50–80% methane were released when the tidal cycle reached its nadir. In August, site‐to‐site variations in these fluxes ranged from 60 to 650 ml (39–425 mg) CH4 m−2 d−1. At a single site, ebullition made up 50% of the total CH4 flux out of the sediments; the remainder was transported across the sediment‐water interface by molecular diffusion.The sedimentary gas bubble reservoir varied seasonally between 2.6 and 14.8 liters m−3 at two White Oak sites. These quantities represented 10–30% of the total sedimentary CH4 inventory, the balance of which was dissolved in pore waters. Methane shifted between the two pools with seasonal changes in temperature as bubble methane partial pressure maintained equilibrium with dissolved CH4.Factors controlling the composition of sedimentary gas bubbles were investigated by collecting samples from several environments. These bubbles consisted primarily of CH4, N2, and CO2. The ratio of CH4 to N2 was found to be a useful indicator of mechanisms transporting gases from sediments and was controlled by both the ebullition rate and the presence of rooted emergent macrophytes.

Similarity solution for free and forced convection hydromagnetic flow over a horizontal semi-infinite plate through a non-homogeneous porous medium

A similarity analysis for the free and forced convection hydromagnetic flow over a horizontal semi-infinite flat plate through a non-homogeneous porous medium is presented, taking into account the hydrostatic pressure variation normal to the flat plate. The similarity solution of the problem under consideration is obtained under certain valid simplifying assumptions when, (i) the plate temperature is inversely proportional to the square root of the distance from the leading edge, (ii) the intensity of the applied magnetic field, normal to the plate, changes with the inverse square root of the distance from the leading edge, and (iii) the permeability of the porous medium, occupying a semi-infinite region of the space bounded by the flat plate, is proportional to the distance measured in the direction of the flow. A numerical solution of the resulting system of ordinary differential equations of motion and energy is obtained, depending on the Prandtl number Pr, the magnetic parameterMn,the bouyancy parameter λ, and the permeability parameterPm.The variations of the fundamental quantities of the problem are shown graphically followed by a quantitative discussion.

Computer aided positioning of manometric probe in the upper digestive tract.

A method not needing fluoroscopic control for positioning a manometric probe in the gastroduodenal tract is described. It is based on hydrostatic pressure variations occurring at each recording port of a multichannel perfused probe after changes in patient's position. The data are fed into a computer and the schematic representation of the probe showing its position and direction appear on the CRT screen. The results were visually and mathematically compared with those obtained with fluoroscopy and showed good correspondence between the two techniques in identification of the position of the probe.

Cardiopulmonary responses to continuous administration of endotoxin.

The cardiopulmonary response to continuous administration of lipopolysaccharide (LPS) was studied in chronically instrumented sheep. LPS was administered in doses of 0 (sham), 6, 9, 12, and 24 ng.kg-1.h-1 for 24 h. No significant changes in the measured variables occurred in the sham group and in the 6 ng.kg-1.h-1-LPS group. With 9, 12, and 24 ng.kg-1.h-1-LPS, cardiac index rose and peripheral resistance fell to the same extent in all three groups. Lung lymph flow (QL) increased with increasing concentration of LPS. These elevations in QL occurred in the presence of only minor increases in the pulmonary artery pressure, which rose to the same extent in the 9, 12, and 24 ng.kg-1.h-1 groups. Consequently, the changes in QL were attributable to changes in fluid conductance of the pulmonary microvasculature rather than variations in hydrostatic pressure. The increase in QL correlated with a decrease in prekallikrein levels (r = 0.97), indicating that the changes in fluid conductance might have been kinin mediated.

Breakdown of the boundary-layer approximation for mixed convection above a horizontal plate

The mixed convection flow over a horizontal plate is investigated. Boundary-layer equations, modified to account for the hydrostatic pressure variation across the boundary layer, are solved numerically by a finite-difference scheme. If the (normal) fluid is cooled from below (or heated from above), the numerical results indicate that the derivative of the wall shear stress tends to infinity as a critical distance from the leading edge is approached. This occurs when the wall shear stress is till positive, i.e. before the classical separation criterion is satisfied. A mechanism of self-amplification is shown to be responsible for the breakdown.

Regulation of proximal tubular fluid reabsorption in the rat kidney

Epithelia such as the renal proximal tubule reasorb fluid "isotonically" by osmotic coupling to active salt transport. This coupling has a rectifying character which means that solute accumulation on the basolateral side induces higher flow rates than the same accumulation on the apical side (for example, see [9]) and that small rises of hydrostatic pressure on the basolateral side reduce the fluid transport to almost zero but have very little effect from the apical side (for example, see [14]). These phenomena are usually related to the "leakiness" of the basolateral spaces [4], which are believed to be rendered hyperosmotic by the active salt transport and thus be the site where the coupling of salt and fluxes occurs [2]. It has been argued that a pressure gradient opposing the outflow of resorbate from these spaces to the interstitium may result in trapping of solutes and fluid so that the backleak of resorbate through the tight junctions rises and a higher fraction of the accumulated solutes is lost by diffusion from the basolateral spaces into the interstitium [19, 201. For the renal proximal tubule this concept has not only been used to explain rectification and isotonicity of the tubular fluid transport but also the dependency of fluid transport rate on the rate of glomerular filtration [17]. It has been argued that variation of glomerular l~tration rate with its concomitant variations of peritubular blood flow are reflected in appropriate variations of the renal interstitial hydrostatic pressure, which may finally adjust the tubular fluid transport rate to the rate of glomerular filtration by the above mechanism. However, apart from the fact that variations of glomerlar filtration do not induce significant alterations of the peritubular hydrostatic pressure [11], the original concept itself has come under challange through a variety of recent observations. For instance, it was found that water pemeates leaky epithelia predominantly through the cells [18]. In addition, a reinvestigation of the reflection coefficients for Na + and C1in isolated rabbit tubules in vitro found values a good deal closer to I than was originally the case [2, 6, 10]. Furthermore, many obervations on tight epithelia suggest that osmotic or hydrostatic pressure gradients across the epithelium may alter the cellular transport activity directly by unknown mechanisms localized within the cells themselves (for literature see [12]). This seems also to be true for leaky epithelia since, for example, when hydrostatic pressure on the basolateral side of gallbladders is raised, the undirectional sodium flux from the apical to the basolateral side is diminished whereas the opposing flux is only minimally increased [7]. Similarly, the addition of

Effect of Intra-arterial Sodium Nitroprusside on Intracranial Pressure and Cerebral Autoregulation

Sodium nitroprusside (SNP) is employed to reduce systemic blood pressure in cerebrovascular surgery. Its application has also been suggested to improve cerebral vasospasm when administered intravenously or intra-arterially in combination with a-adrenergic agents. The purpose of this study is to investigate the vascular response of normal and elevated intracranial pressure (ICP). Twelve adult cats were anesthetized and intubated. Each animal was paralysed and mechanically normoventilated. ICP was measured through a 22G needle into the left lateral ventricle and connected to a strain gauge transducer. Different levels of ICP were achieved using an artificial cerebro-spinal fluid, which exerted a variable hydrostatic pressure via a 21G cannula introduced into the cisterna magna. The effects of SNP were studied in each animal at three ICP levels: normal (control), 20 mmHg, and 40 mmHg. A catheter was inserted into the left internal carotid through the lingual artery, and SNP (40 μg/kg/min) was infused intra-arterially through this catheter. Administration of SNP always caused an increase of ICP. This increment degree of ICP caused by SNP (ΔICP=ICPSNP-ICPinitial) was more marked in the control group with normal ICP, amounting to 5.27±2.51 mmHg (P<0.01). In the groups with ICP levels of 20 and 40 mmHg, ΔICP were 2.21±0.66 and 0.21±0.40 mmHg, respectively (not significant). Comparison of the three groups revealed that the increase of ICP of the groups with mild or high ICP was significantly lower than that of the control group (t-test, P<0.01 for 20 mmHg and P<0.001 for 40 mmHg group). As the initial level of ICP was raised, ΔICPs decreased. The results confirmed the observations made by other investigators that SNP has an influence on ICP. Because cerebral autoregulation should be intact at the ICP levels as such induced in the present study (20 and 40 mmHg), the high-dosage (40 μg/kg/min) and the route of administration of SNP in this series (internal carotid) might exert a direct influence on the cerebral vessel walls. SNP seems to cause cerebral vasoparalysis and disturb pressure autoregulation.

Transepithelial and cell membrane electrical resistances of the rabbit proximal convoluted tubule.

A technique using double-barreled perfusion pipettes and intracellular microelectrodes was developed to measure transepithelial, apical, and basolateral membrane electrical resistances in isolated rabbit proximal convoluted tubules (PCT). This technique has been tested successfully with respect to cable analysis: the transepithelial resistance (RT) did not change with tubule length and the measured core resistance of the lumen (RC) varied according to prediction with lumen diameter and perfusate resistivity. In control solutions, a linear I-V relationship was observed at the entry of the tubule for current varying from -300 to +300 nA. The mean RT was 1,050 +/- 70 omega X cm (n = 33) (a specific resistance of 8.2 omega X cm2). Bath proteins and large variations in transtubular hydrostatic pressure had no significant effect on RT, whereas RT was not systematically related to transepithelial PD or to the sodium-to-chloride permeability ratio (n = 22). Perfusate substitution of 50 mM NaCl by mannitol increased RT by 21% (n = 7) but the same maneuver in the peritubular solution had no significant effect after a 5-min equilibration period. The ratio of apical to basolateral cell membrane resistance (RA/RBL) determined with intracellular microelectrodes was 3.1 +/- 0.3 (n = 27) in control solutions and increased within 1 min by 36% (n = 8) when glucose and alanine were replaced by mannitol in the perfusate solution. Using simultaneous initial changes in transepithelial and basolateral potential differences when glucose and alanine were removed, the individual values of RA and RBL were determined. Mean RBL was 4,900 +/- 990 omega X cm (39 +/- 1.3 omega X cm2) and mean RA was 15,000 +/- 4,300 omega X cm (118 +/- 33 omega X cm2).

Effect of alternating hydrostatic pressure on the platelet-vascular wall system

The effect of alternating hydrostatic pressure on the function of the platelets--vascular wall system has been studied. It has been shown that exposure to pressure leads to the increased capacity of platelets to aggregation, adhesion and release. This is accompanied by the lowering of antiaggregation activity of the vascular wall and increase in the latter of thromboplastin factors. It has been ascertained that acetylsalicylic acid is capable to reduce the pressure treatment-induced effects. The importance of the discovered changes in the mechanisms of hemostasiological disorders is discussed.

Regional edema formation in isolated perfused dog lungs.

Studies using gravimetric analysis of lungs of frozen animals have suggested that the differences in pulmonary microvascular pressure between non-dependent and dependent lung do not influence the formation of regional pulmonary edema. We wondered if the inability to detect variation in regional extravascular lung water (EVLW) was due to the slow freezing process and, therefore, reassessed the distribution of EVLW in vertically suspended isolated perfused dog lungs with a radioisotopic technique that does not require freezing. Total lung water (TLW), blood or intravascular lung water (IVLW), and EVLW were measured in absolute quantities using a positron camera and the positron-emitting isotopes C15O as a blood label and H2(15)O as a total lung water label. Mean isotopic TLW in 17 lungs that were normal or moderately edematous (wet:dry ratio < 7) was 142 +/- 9 (SE) ml compared to the gravimetric estimate of 148 +/- 7 ml (r = 0.92) and isotopic EVLW was 64 +/- 6 ml compared to the gravimetric estimate of 70 +/- 6 ml (r = 0.8). Analysis of the distribution of regional isotopically measured EVLW in the 17 lungs in various states of spontaneous edema formation revealed a small non-dependent to dependent, gravity-related increase in percent regional EVLW compared to percent regional TLW, which did not vary with the degree of edema in the lung. Serial measurements of absolute regional EVLW in four lungs during spontaneously developing edema also failed to show a disproportionate increase in accumulation of EVLW in any lung zone. Thus, despite the wide variation in microvascular hydrostatic pressure between top and bottom of the vertical isolated lung, edema formation seems to be uniform.