Apparent Reflection Coefficient Research Articles

Ultrasound monitoring of a deformable tongue-food gel system during uniaxial compression–an in vitro study

This study presents a novel ultrasound method for exploring the mechanical deformability of artificial tongue models (ATMs) topped with agar and/or gelatin food gels during uniaxial compression. The ATMs were made from polyvinyl alcohol and displayed different levels of rigidity and surface roughness. We quantified deformation of the ATM-food system as it underwent compression induced by a texture analyzer. We collected real-time measurements via an ultrasound transducer (1 MHz) placed beneath the ATM, enabling to monitor non-invasively the ultrasound time of flight (ToF) and the apparent reflection coefficient (R*) of the ATM-food interface. The results on ToF reflected the tie between the level of force and ATM deformation; they were also precise enough to identify gel fracture. R* was a crucial parameter influenced by both ATM properties (rigidity, surface roughness) and food gel properties (syneresis, ability to mold to surface asperities).

Real time ultrasound assessment of contact progress between food gels and tongue mimicking surfaces during a compression

An in vitro study was conducted to further explore the potential of an emerging ultrasound method for monitoring in real time the progress of the contact between gels with different physical properties and tongue mimicking surfaces (TMSs) during uniaxial compressions. A 1-MHz ultrasound transducer was used to measure the apparent reflection coefficient (R*) of the interface between TMSs (of varied roughness) and gels (agar and/or gelatin). The patterns of R* variations clearly depended on the gels capacity to deform and mold the asperities of the TMSs during compression. Rough TMSs induced a significant decline of R* values, demonstrating an increase of the fraction of the TMSs in direct contact with the gels. Rigidity, fracture properties or adhesion energy of gels influenced the kinetics of contact progress and associated patterns of R* variations. Moreover, the presence of a thick lubrication film between the gels and the TMSs promoted the transmission of ultrasound waves (acting as a coupling agent) and led to decreasing values of R*. Such phenomena were observed both when depositing a water lubrication film of controlled thickness on the TMSs (to mimic salivary film) and when considering gels with high water release capacities. The study confirms the potential of ultrasound methods for exploring physical phenomena related to interactions between food and tongue surface. Such developments could better contribute to unravel the determinants of texture perceptions during food oral processing.

Using ultrasound to characterize the tongue-food interface: An in vitro study examining the impact of surface roughness and lubrication

We measured the apparent reflection coefficient of a 1-MHz ultrasound compressional wave at the interface between rough and lubricated tongue mimicking surfaces and various food gels, composed of agar or gelatin. For the smoothest mimicking surface, when a lubricating layer was present, the apparent reflection coefficient was fairly similar for the different food gels (33.6% on average). The apparent reflection coefficient was significantly larger in the following situations: (i) tongue asperities were high and dense; (ii) lubrication levels were low; and (iii) gels were less rigid (range for the different gels—45.9–84.3%). The apparent reflection coefficient conveys the ability of food gels to mold themselves to surface asperities or to form a coupling film of liquid at the interface. This study demonstrates that ultrasound methods can and should be used to explore the physical phenomena that underlie the texture perceptions resulting from tongue-palate interactions.

Easily fabricated 60 GHz WR15/MSL transition to measure apparent reflection coefficient of differentially fed patch antennas

A new procedure to measure the apparent reflection coefficient of differential patch antennas at 60 GHz is described. The measurement method makes use of two WR15/microstrip-line (MSL) transitions which are simple to realise. The experimental results are compared with simulated results. This comparison emphasises the limited effect of transition losses on measurement accuracy.

A comparison of laboratory reflection coefficient data with predictions from a time domain model of sea floor scatter using the small slope approximation (BoRIS-SSA)

Reflection coefficient data has the potential for sediment model verification and remote sensing of sediment parameters. However, a significant component of the bistatic acoustic response is scattering from the rough interface. For example, in typical ocean conditions, surface roughness can alter the apparent normal reflection coefficient by 15 dB or more over a frequency range from 10 to 100 kHz. Therefore, it is important to quantify the contribution from scattering to determine the smooth surface reflection coefficient from bistatic data. The Bottom Response from Inhomogeneities and Surface (BoRIS-SSA) code uses the small slope approximation to calculate bistatic response from rough surfaces. BoRIS-SSA can incorporate both stochastic realizations of the ocean bottom or data from stereo-photographic equipment. In this study, bistatic laboratory data taken at 50–150 kHz from a smoothed and roughened interfaces will be compared to BoRIS-SSA calculations. The model will be considered for its ability to predict both the average value of the bistatic response and the distribution of the data.

Free water and seismic reflectivity in the lower continental crust

Many deep seismic reflection data sets show bright reflectivity filling much of the lower continental crust. Where these reflections are well developed, they have apparent reflection coefficients of at least 0.1. It has been proposed that this reflectivity could be explained by the existence of free water in the lower crust with stratified porosity or pore pressure, and that this free water might also explain observations of high electrical conductivity. If this explanation is correct then it follows that seismic observations of apparent heterogeneity in the lower crust reveal only heterogeneities in fluid distribution, and are not necessarily related to changes in rock type. That hypothesis is tested here. Water in the lower crust has a density and velocity comparable with liquid water at the Earth's surface. Numerical modelling of the seismic properties of rocks containing free water is used to delineate the range of porosity and pore geometry required to produce bright reflections. Porosities of several per cent or thin sheet-like pores with high crack density are required. Rocks in textural equilibrium do not contain thin pores and require porosities in excess of 5% to explain the reflection coefficients. The lower crust will compact, water will be expelled, and sharp boundaries in porosity and pore pressure will diffuse in geologically short times unless the water is distributed in isolated equi-dimensional pores. It is concluded that free water cannot explain the seismic reflections seen from the lower crust in stable areas, and that there are good geophysical and petrological reasons for supposing that the lower crust in the stable continental interior and on stable continental shelves typically contains little or no free water. The widespread observations of seismic reflectivity in the lower continental crust do indeed therefore imply significant heterogeneities in rock type.

Seismic structure of 15 Ma oceanic crust formed at an ultrafast spreading East Pacific Rise: Evidence for kilometer‐scale fracturing from dipping reflectors

A grid of seismic reflection data from 15 Ma Cocos Plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading (∼200 mm/yr, full rate) East Pacific Rise (EPR). The data contain an unexpected quantity of bright reflectors throughout the crust, generally <5 km in length. Significant reflection dip angles (∼10–30°) are seen only on the isochron profiles, and several reflectors have a confirmed isochron dip orientation. Certain upper crustal reflectors (UCRs) appear to form a single network with isochron dip. The most prominent example (apparent reflection coefficient ∼0.2) projects toward a ∼70 m disruption in the igneous seafloor, and waveform analysis demonstrates that the reflector is likely the product of a low‐velocity zone on the order of tens of meters thick. Our observations suggest that the isochron‐dipping UCRs represent small‐slip, kilometer‐scale faults/fractures, and none are primarily related to the seismic 2/3 boundary or to the sheeted dike/gabbro transition. Lower crustal reflectors (LCRs) have no preferred orientation or regular pattern on the 30 km scale of this survey. Certain LCRs may be similar to the UCRs in origin, while others may originate from near‐axis ductile flow, lithologic banding, off‐axis magmatism, or some combination thereof. Overall, reflectors in this area are abundant and complex, consistent with previous off‐axis studies, but at odds with observations of young crust (<5 Ma) near the EPR. This dichotomy may be resolved by a delay in reflector formation, through some combination of postaccretionary tectonics, magmatism and hydrothermal alteration.

NEM and filipin increase albumin transport in lung microvessels.

This study was undertaken to evaluate the role of transcytosis as a bulk transfer mechanism for the passage of albumin from blood to tissue. Isolated rat lungs were continuously weighed and perfused with an albumin-serum buffer solution under strictly controlled hemodynamic conditions, which allowed measurements of microvascular pressure and of the capillary filtration coefficient (L(p)S). With the use of a tissue uptake technique, it was possible to determine lung albumin clearance under isogravimetric conditions (Cl(iso)), or at elevated filtration rates, to obtain an "apparent albumin reflection coefficient" (sigma(alb)). Experiments were performed during control and after reducing lung temperature from 35 degrees to 22 degrees C and after infusions of the transcytosis inhibitors N-ethylmaleimide (NEM) or filipin. Cooling moderately increased vascular resistance and reduced L(p)S and Cl(iso) largely in proportion to the induced increases in viscosity. At 35 degrees C, NEM (0.13 mM) caused a marked increase in L(p)5 and in Cl(150) and also caused a reduction in sigma(alb.) Furthermore, Cl(iso) increased for the highest dose of filipin tested (1.8 microg/ml). The demonstrated relative cooling insensitivity of the transfer of albumin across the endothelium in rat lungs does not support the contention of transcytosis of proteins across the endothelium. Furthermore, neither NEM nor filipin inhibited lung microvascular albumin transport, but actually increased lung endothelial permeability.

Use of Collocated Sensors to Measure Coastal Wave Reflection

Two different methods for estimating frequency-dependent reflection coefficients for waves incident on a coastline using collocated measurements of elevation (or pressure) and horizontal current are investigated by simulating time series with known true reflection coefficients and added uncorrelated noise. The methods are applicable to measurements made in the nearshore zone where waves propagate essentially shore-normally. A time domain method is shown to introduce a significant positive bias in the estimated reflection coefficient. A contour plot is calculated giving the bias as a function of the estimated reflection coefficient and the coherence between the estimated incoming and outgoing wave time series, which can be used to provide corrections for the bias. A new principal component analysis technique is found to be essentially bias-free. For this method, 95% confidence levels on zero reflection coefficient are computed for a range of numbers of degrees of freedom. Spatial separation between the sensors equivalent to a time delay, at the wave phase velocity, of τ\i\dx produces a spurious peak with a reflection coefficient of one at frequency 1/(4τ\i\dx). The peak is very sensitive to small errors in estimating the time delay, which result in apparent reflection coefficients even higher than one. The conclusion is that the time delay must be made as small as possible in order to push the spurious peak to a high frequency. Application of these results is demonstrated using field data taken on a natural beach.

Water uptake by roots of maize and sunflower affects the radial transport of abscisic acid and its concentration in the xylem

The radial movement of cis-abscisic acid (ABA) has been investigated in young excised roots of Zea mays L. and Helianthus annuus L. which were grown hydroponically. In addition to the symplastic path, ABA was largely translocated across the root apoplast by solvent drag with the water in the transpiration stream. On the apoplastic path ABA may even cross the endodermis. Depending on the ABA concentration of the medium (range: 5–500 nM) and in the root apoplast, the solvent-drag component of the flow of ABA counteracted the dilution of ABA in the xylem caused by transpirational water flow. Acidification of the rhizosphere and of the root apoplast increased the apoplastic transport component. In sunflower, the apoplastic flow of ABA was significantly weaker than in maize roots. This was also indicated by the larger apparent reflection coefficient (σABA) of sunflower roots for ABA (sunflower: σABA = 0.97 ± 0.02, n = 6 roots; maize: σABA = 0.68 ± 0.06, n = 6 roots; ±SD). For both species, σABA was smaller than unity. Root reflection coefficients were affected by factors such as pH, ABA concentration of the medium, and by the suction force applied to excised root systems. Due to the complex composite structure of the permeation barrier in the root, the reflection coefficient estimated from solvent drag is also complex. Since unstirred layers affected the absolute value of the reflection coefficient, σABA has been termed `apparent'. It is concluded that the pH and ABA concentration of the soil solution as well as the transpiration rate (suction force) modify the intensity of the root-to-shoot signal which is influenced by an apoplastic bypass flow of ABA. The latter may be substantially affected by the existence of Casparian bands in the exodermis, which were lacking in the roots studied in this paper.

Sound dispersion and attenuation in the thickness direction of paper materials

A through-transmission, dry-contact ultrasonic technique aimed at investigating sound propagation in the thickness direction of paper materials is presented. The method uses piezoelectric ceramic transducers immersed in fluid-filled rubber wheels. Thickness and longitudinal velocity are determined from time domain measurements for several commercial paper specimens between 40 and 1750 μm. Dispersion and attenuation are reported at frequencies up to 1.5 MHz. Dispersion is less than 10% between 0.25 and 1.25 MHz. Amplitude calculations are used to evaluate the attenuation coefficient versus the frequency. Using a nearly local form of the Kramers–Kronig relationships, the attenuation coefficient is also obtained from the phase information. By comparing the amplitude- and phase-determined attenuation coefficients, an apparent reflection coefficient is predicted. It is shown that the Kramers–Kronig relationships can be applied to papers thicker than 200 μm under dry coupling conditions. The attenuation coefficient is inversely related to the elastic stiffness for homogeneous specimens. A linear relationship is observed between the attenuation loss and the thickness. The apparent reflection coefficient correlates to the apparent surface roughness of paper.

Shallow structure of oceanic crust in the western North Atlantic from seismic waveform inversion and modeling

Seismic reflection data from Mesozoic oceanic crust in the western North Atlantic, in the vicinity of the Blake Spur Fracture Zone, have imaged a number of short, subhorizontal events in the upper 1 km of the crystalline crust. Wide‐angle expanding spread profile (ESP) data from the same region indicate the presence of first‐order velocity discontinuities in this depth interval. Using ESP data reprocessed to enhance precritical reflection events, we investigate the relationship between the precritical and postcritical reflections with three different approaches: amplitude analysis, forward modeling, and waveform inversion. The precritical reflections have high apparent reflection coefficients (∼ 0.1) and may be divided into two types: those which can be traced out to critical ranges (5–6 km), representing velocity steps, and those which fade out at ranges of 3–4 km, representing shorter‐wavelength velocity variations. Synthetic seismogram modeling indicates that the velocity steps occur within a 50 m depth interval, while the shorter‐wavelength features may correspond to high‐ or low‐velocity zones with a thickness of 20 m (10% of the dominant seismic wavelength) or less. We investigate the velocity structure in detail by full waveform inversion of a selected portion of ESP data, transformed into the intercept time‐slowness domain. The inversion method minimizes the sample‐by‐sample misfit between the data and reflectivity synthetics in a least squares sense. A conjugate gradient algorithm was used, starting from a velocity structure derived from forward modeling of wide‐angle ESP data. The inversion tightly constrains short‐wavelength components of the velocity structure, and indicates that intracrustal reflections result both from major velocity steps and from zones of alternating high and low velocity. Synthetic seismograms generated from sonic log data from Deep Sea Drilling Project hole 418A show reflections of similar amplitude and character to those seen in the ESP data. These reflections probably originate from rather subtle features of the velocity‐depth profile, related primarily to changes in porosity in the pillow lava sequence. A deeper reflector, 800 ms below oceanic basement in the Blake Spur Fracture Zone, may correspond to the top of a zone of serpentinized upper mantle.

Water Transport across Maize Roots

A double pressure probe technique was used to measure simultaneously water flows and hydraulic parameters of individual cells and of excised roots of young seedlings of maize (Zea mays L.) in osmotic experiments. By following initial flows of water at the cell and root level and by estimating the profiles of driving forces (water potentials) across the root, the hydraulic conductivity of individual cell layers was evaluated. Since the hydraulic conductivity of the cell-to-cell path was determined separately, the hydraulic conductivity of the cell wall material could be evaluated as well (Lp(cw) = 0.3 to 6.10(-9) per meter per second per megapascal). Although, for radial water flow across the cortex and rhizodermis, the apoplasmic path was predominant, the contribution of the hydraulic conductance of the cell-to-cell path to the overall conductance increased significantly from the first layer of the cortex toward the inner layers from 2% to 23%. This change was mainly due to an increase of the hydraulic conductivity of the cell membranes which was Lp = 1.9.10(-7) per meter per second per megapascal in the first layer and Lp = 14 to 9.10(-7) per meter per second per megapascal in the inner layers of the cortex. The hydraulic conductivity of entire roots depended on whether hydrostatic or osmotic forces were used to induce water flows. Hydrostatic Lp(r) was 1.2 to 2.3.10(-7) per meter per second per megapascal and osmotic Lp(r) = 1.6 to 2.8.10(-8) per meter per second per megapascal. The apparent reflection coefficients of root cells (sigma(s)) of nonpermeating solutes (KCI, PEG 6000) decreased from values close to unity in the rhizodermis to about 0.7 to 0.8 in the cortex. In all cases, however, sigma(s) was significantly larger than the reflection coefficient of entire roots (sigma(sr)). For KCI and PEG 6000, sigma(sr) was 0.53 and 0.64, respectively. The results are discussed in terms of a composite membrane model of the root.

Electromagnetic scattering by pyramidal and wedge absorber

Electromagnetic scattering from pyramidal and wedge absorbers used to line the walls of modern anechoic chambers is measured and compared with theoretically predicted values. The theoretical performance for various angles of incidence is studied. It is shown that a pyramidal absorber scatters electromagnetic energy more as a random rough surface does. The apparent reflection coefficient from an absorber wall illuminated by a plane wave can be much less than the normal absorber specifications quoted by the manufacturer. For angles near grazing incidence, pyramidal absorbers give a large backscattered field from the pyramid side-faces or edges. The wedge absorber was found to give small backscattered fields for nuclear-grazing incidence. Based on this study, some new guidelines for the design of anechoic chambers are advocated because the specular scattering models used at present do not appear valid for pyramids that are large compared to the wavelength. >

Seismic reflection coefficients from mantle fault zones

Summary. Several bright reflections from structures within the mantle can be seen on BIRPS' deep seismic reflection profiles. We have calculated apparent reflection coefficients for the brightest of these events and obtain values around 0.1. It is not possible to produce such large reflections by either compositional layering or seismic anisotropy if olivine and pyroxene are the only significant minerals in the mantle. These large reflections can be produced by a mafic layer or a partially hydrated layer within normal peridotite. The brightest reflections seem to be best explained as major faults or shear zones within the mantle.

The permeability of the secretin stimulated exocrine pancreas to non-electrolytes.

1. A method of measuring the permeability of the pancreas by determining the apparent reflexion coefficient (sigmaA) is described, in the isolated pancreas secreting maximally under the influence of secretin. The principle is to add a non-electrolyte to the perfusate which will create an osmotic gradient (RTsigmadeltaC) counter to that of active transport and reduce the secretion rate. This is compared with the effect of an equal concentration (0.1 M) of sucrose (RTdeltaC; sigma = 1). The apparent reflection coefficient is obtained by dividing the percentage reduction in the secretion rate due to the test molecule with that due to sucrose. 2. Sucrose when added to the perfusate inhibits pancreatic secretion. For every 10 mM increase in sucrose concentration, the secretion rate was inhibited by 7.1%. It has been estimated that an osmotic gradient of 131 m-osmole/kg water will cause zero flow rate. This is a measure of the pressure required to counteract the local osmotic gradient set up by active transport, it is equivalent to about 3.4 atm. 3. Non-electrolytes with molecular volumes greater than about 85 cm3 mole-1 are relatively impermeable, below this value they enter the pancreatic juice with increasing ease as the molecular volume decreases. 4. SigmaA for a number of compounds has been measured: urea 0.17; ethanediol 0.27; thiourea 0.51; glycerol 0.69; creatinine 0.81; erythritol 0.91; arabinose 0.96; xylose 0.98; sorbitol 0.98. 5. The addition of non-electrolytes to the perfusate had effects on pancreatic secretion which were a function of sigmaA. For molecules with sigmaA lying between 0.81 and 1.0 an osmotic load of 0.1 M increased both the concentration of sodium plus potassium and the concentration of chloride plus bicarbonate by about 50 m-mole/l. Whereas the cation change is almost exclusively one of sodium that of the anions was preferentially an increase in chloride. For compounds with sigmaA lying between 0 and 0.81 the concentration of sodium plus potassium was proportional to sigmaA. 6. A number of compounds have been described which inhibit pancreatic secretion, other than by an osmotic effect. These include acetaldehyde, thioglycerol, nicotinamide, ribose, dihydroxyacetone, and glyceraldehyde. 7. It is concluded that the pancreas is more permeable than the gall-bladder of rabbit, fish and bullfrog, the proximal tubule of the kidney of rat and the small intestine of bullfrog, but is probably similar to that of small intestine of guinea-pig and man.

Physical properties of isolated perfused basement membranes from rabbit loop of Henle.

Isolated, perfused segments of late proximal straight tubule, descending thin limb of Henle, and ascending thick limb of Henle from the rabbit were studied before and after removal of the epithelium with sodium deoxycholate. The relationship between transmural hydrostatic pressure and outer tubule diameter was similar in paired intact tubules and basement membranes, indicating that basement membrane is the principal determinent of tubule distensibility. As calculated from teh measured perfusate flow at several different transmembrane hydrostatic pressures, the hydraulic conductivity of the basement membranes was 6-8 X 10(-3) cm3/cm2.min.cmH2O. With use of these LP values and the calculated oncotic pressure required experimentally to reduce transmembrane hydrostatic pressure transiently to zero, the apparent reflection coefficient of the basement membranes for serum albumin was estimated to be 0.05-0.16. It is concluded that basement membranes of the loop and of other previously studied segments of rabbit nephron provide very strong and elastic mechanical support to the epithelium while having minimal resistance to flow of water and of solutes as large as serum albumin.

Sodium chloride reflection coefficient in rabbit gall bladder

By means of the Kedem-Katchalsky thermodynamic description of active transport, a relationship has been derived between the apparent reflection coefficient and the Staverman reflection coefficient for passive transport of a solute which is both actively and passively transported. The relationship between volumetric flow and its driving forces, containing the Staverman reflection coefficient, was tested for sodium chloride in rabbit gall bladder and the reflection coefficient was evaluated.

Permeability of pea chloroplasts to alcohols and aldoses as measured by reflection coefficients

1. 1. Intact chloroplasts were isolated in 2 min from Pisum sativum and then placed in external solutions varying in composition and in osmotic pressure. The relative osmotic changes in chloroplast volume caused by the impermeable solute sucrose compared with equal concentrations of various alcohols and aldoses were determined. From such osmotic responses the reflection coefficients ( σ x ) of the latter compounds were calculated using a version of the Boyle Van 't Hoff relation derived from irreversible thermodynamics. 2. 2. The reflection coefficients of pea chloroplasts for alcohols became progressively larger as hydroxymethyl groups were added to the molecule, viz. methanol ( σ x = 0.00 ), ethylene glycol (0.40), glycerol (0.63), meso-erythritol (0.90), and adonitol (1.00). This increase in σ x (decrease in permeability) with increasing molecular size parallels the decreasing lipid-water partition coefficients of the solutes. The reflection coefficient of d-mannitol was 1.01, of d-sorbitol was 1.02, and of meso-inositol was 1.00, suggesting that these six-carbon polyhydroxy alcohols are impermeant and hence would be suitable osmotica in which to suspend chloroplasts. 3. 3. Chloroplasts are more permeable to certain aldopentoses, viz. d-xylose ( σ x = 0.43 ), d-lyxose (0.47) and l-arabinose (0.53), than they are to the optical isomers of the same compounds, i.e. l-xylose (0.87), l-lyxose (0.93), and d-arabinose (0.85). Although all of these compounds predominantly occur in a chair form of the pyranose ring, the first three are in the C1 conformation, while the latter three are in the 1C conformation. The greater permeability of the C1 aldopentoses was even more apparent for d-ribose, which had a reflection coefficient of 0.00. 4. 4. Based on osmotic responses from 14 to 50 mM, the aldohexoses tended to have higher reflection coefficients for the limiting membranes of pea chloroplasts than did the aldopentoses: α- d-glucose ( σ x = 0.98 ), β- d-glucose (0.99), 2-deoxy- d-glucose (1.00), d-galactose (0.92), 2-deoxy- d-galactose (0.91), 6-deoxy- d-galactose (0.89), d-mannose (0.85), l-mannose (1.00), and 6-deoxy- l-mannose (1.00). 5. 5. Based on the correlation between the reflection coefficient and the particular sugar stereoisomer, it is proposed that carriers in the limiting membranes of pea chloroplasts are specific for aldopentoses in the C1 form of the chair conformation of the pyranose ring and, furthermore, are particularly efficient in shuttling d-ribose into pea chloroplasts. At concentrations above 50 mM, the apparent reflection coefficient of the C1 aldopentoses increased, suggesting that such carriers were becoming saturated. Moreover, the osmotic responses when two sugars were present together indicated that d-xylose, d-lyxose, l-arabinose, and d-ribose all competed for the same carrier.

Osmotic volume flow in the proximal tubule of Necturus kidney.

Volume changes due to osmotic flow in the distal portion of proximal tubules of Necturi were measured by the split oil drop technique. In agreement with previous findings no volume flow was induced by NaCl concentrations close to 60 mM. The tubule wall was found to be permeable to plasma electrolytes, which have an apparent reflection coefficient of 0.69. The mean apparent hydraulic conductivity was 0.33 x 10(-11) cm(3)/dyne sec, comparable with other epithelia. A number of lipid-insoluble nonelectrolytes of widely varying molecular size had apparent reflection coefficients of about 0.5. In view of the insensitivity to molecular size it seems likely that apparent reflection coefficients determined from tubular volume changes depend primarily on the porosity of the intercellular barrier closest to the lumen and give little information about the subsequent fate of the test substances.