Microvascular permeability to water is independent of shear stress, but dependent on flow direction.

Endothelial cells in a cultured monolayer change from a "cobblestone" configuration when grown under static conditions to a more elongated shape, aligned with the direction of flow, after exposure to sustained uniform shear stress. Sustained blood flow acts to protect regions of large arteries from injury. We tested the hypothesis that the stable permeability state of individually perfused microvessels is also characteristic of flow conditioning. In individually perfused rat mesenteric venular microvessels, microvascular permeability, measured as hydraulic conductivity (Lp), was stable [mean 1.0 × 10(-7) cm/(s × cmH2O)] and independent of shear stress (3-14 dyn/cm(2)) for up to 3 h. Vessels perfused opposite to the direction of normal blood flow exhibited a delayed Lp increase [ΔLp was 7.6 × 10(-7) cm/(s × cmH2O)], but the increase was independent of wall shear stress. Addition of chondroitin sulfate and hyaluronic acid to perfusates increased the shear stress range, but did not modify the asymmetry in response to flow direction. Increased Lp in reverse-perfused vessels was associated with numerous discontinuities of VE-cadherin and occludin, while both proteins were continuous around the periphery of forward-perfused vessels. The results are not consistent with a general mechanism for graded shear-dependent permeability increase, but they are consistent with the idea that a stable Lp under normal flow contributes to prevention of edema formation and also enables physiological regulation of shear-dependent small solute permeabilities (e.g., glucose). The responses during reverse flow are consistent with reports that disturbed flows result in a less stable endothelial barrier in venular microvessels.

Microcalcifications increase coronary vulnerable plaque rupture potential: a patient-based micro-CT fluid-structure interaction study.

Asymptomatic vulnerable plaques (VP) in coronary arteries accounts for significant level of morbidity. Their main risk is associated with their rupture which may prompt fatal heart attacks and strokes. The role of microcalcifications (micro-Ca), embedded in the VP fibrous cap, in the plaque rupture mechanics has been recently established. However, their diminutive size offers a major challenge for studying the VP rupture biomechanics on a patient specific basis. In this study, a highly detailed model was reconstructed from a post-mortem coronary specimen of a patient with observed VP, using high resolution micro-CT which captured the microcalcifications embedded in the fibrous cap. Fluid-structure interaction (FSI) simulations were conducted in the reconstructed model to examine the combined effects of micro-Ca, flow phase lag and plaque material properties on plaque burden and vulnerability. This dynamic fibrous cap stress mapping elucidates the contribution of micro-Ca and flow phase lag VP vulnerability independently. Micro-Ca embedded in the fibrous cap produced increased stresses predicted by previously published analytical model, and corroborated our previous studies. The 'micro-CT to FSI' methodology may offer better diagnostic tools for clinicians, while reducing morbidity and mortality rates for patients with vulnerable plaques and ameliorating the ensuing healthcare costs.

Attachment of osteocyte cell processes to the bone matrix.

In order for osteocytes to perceive mechanical information and regulate bone remodeling accordingly they must be anchored to their extracellular matrix (ECM). To date the nature of this attachment is not understood. Osteocytes are embedded in mineralized bone matrix, but maintain a pericellular space (50-80 nm) to facilitate fluid flow and transport of metabolites. This provides a spatial limit for their attachment to bone matrix. Integrins are cell adhesion proteins that may play a role in osteocyte attachment. However, integrin attachments require proximity between the ECM, cell membrane, and cytoskeleton, which conflicts with the osteocytes requirement for a pericellular fluid space. In this study, we hypothesize that the challenge for osteocytes to attach to surrounding bone matrix, while also maintaining fluid-filled pericellular space, requires different "engineering" solutions than in other tissues that are not similarly constrained. Using novel rapid fixation techniques, to improve cell membrane and matrix protein preservation, and transmission electron microscopy, the attachment of osteocyte processes to their canalicular boundaries are quantified. We report that the canalicular wall is wave-like with periodic conical protrusions extending into the pericellular space. By immunohistochemistry we identify that the integrin alphavbeta3 may play a role in attachment at these complexes; a punctate pattern of staining of beta3 along the canalicular wall was consistent with observations of periodic protrusions extending into the pericellular space. We propose that during osteocyte attachment the pericellular space is periodically interrupted by underlying collagen fibrils that attach directly to the cell process membrane via integrin-attachments.

Transient regulation of transport by pericytes in venular microvessels via trapped microdomains.

A phenomenon that has defied explanation for two decades is the time scale for transient reabsorption in the classic experiments of Michel and Phillips on individually perfused frog mesentery microvessels. One finds that transient reabsorption lasts <2 min before a new steady state of low filtration is established when the lumen pressure is abruptly dropped from a high to a low value. Our experiments in frog and rat venular microvessels under a variety of conditions revealed the same time trend for new steady states to be established as in Michel and Phillips' experiments. In contrast, one theoretically predicts herein that the time required for the tissue albumin concentration to increase to values for a new steady state to be achieved through reabsorption is in the order of several hours. In this paper we propose a new hypothesis and theoretical model for this rapid regulation, namely that pericytes covering the interendothelial cleft exits create small trapped microdomains outside the cleft exits which regulate this transient behavior. Our electron microscopy studies on rat mesenteric venular microvessels reveal an average pericyte coverage of approximately 85%. The theoretical model based on this ultrastructural study predicts an equilibration time on the order of 1 min when the lumen pressure is abruptly lowered. The basic concept of a trapped microdomain can also be extended to microdomains in the interstitial space surrounding skeletal muscle capillaries.

Strain amplification and integrin based signaling in osteocytes.

Recent morphological studies have suggested that osteocyte processes are directly attached at discrete locations along the canalicular wall by beta3 integrins at the apex of infrequent, previously unrecognized, canalicular projections. This discovery has led to a new paradigm for the initiation of intracellular signaling, which provides a possible long sought after molecular mechanism for the initiation of intracellular signaling in bone cells. The quantitative feasibility of this hypothesis is explored with a detailed theoretical model, which predicts that axial strains due to the sliding of actin microfilaments about the fixed integrin attachments are in order of magnitude larger than the radial strains in the previously proposed strain amplification theory and two orders of magnitude greater than whole tissue strains.

Oncotic pressures opposing filtration across non-fenestrated rat microvessels.

We hypothesized that ultrafiltrate crossing the luminal endothelial glycocalyx through infrequent discontinuities (gaps) in the tight junction (TJ) strand of endothelial clefts reduces albumin diffusive flux from tissue into the 'protected region' of the cleft on the luminal side of the TJ. Thus, the effective oncotic pressure difference (sigma black triangle down pi) opposing filtration is greater than that measured between lumen and interstitial fluid. To test this we measured sigma black triangle down pi across rat mesenteric microvessels perfused with albumin (50 mg ml(-1)) with and without interstitial albumin at the same concentration within a few micrometres of the endothelium as demonstrated by confocal microscopy. We found sigma black triangle down pi was near 70% of luminal oncotic pressure when the tissue concentration equalled that in the lumen. We determined size and frequency of TJ strand gaps in endothelial clefts using serial section electron microscopy. We found nine gaps in the reconstructed clefts having mean spacing of 3.59 microm and mean length of 315 nm. The mean depth of the TJ strand near gaps was 67 nm and the mean cleft path length from lumen to interstitium was 411 nm. With these parameters our three-dimensional hydrodynamic model confirmed that fluid velocity was high at gaps in the TJ strand so that even at relatively low hydraulic pressures the albumin concentration on the tissue side of the glycocalyx was significantly lower than in the interstitium. The results conform to the hypothesis that colloid osmotic forces opposing filtration across non-fenestrated continuous capillaries are developed across the endothelial glycocalyx and that the oncotic pressure of interstitial fluid does not directly determine fluid balance across microvascular endothelium.

A model for strain amplification in the actin cytoskeleton of osteocytes due to fluid drag on pericellular matrix.

A model is presented that provides a resolution to a fundamental paradox in bone physiology, namely, that the strains applied to whole bone (i.e., tissue level strains) are much smaller (0.04-0.3 percent) than the strains (1-10 percent) that are necessary to cause bone signaling in deformed cell cultures (Rubin and Lanyon, J. Bone Joint Surg. 66A (1984) 397-410; Fritton et al., J. Biomech. 33 (2000) 317-325). The effect of fluid drag forces on the pericellular matrix (PM), its coupling to the intracellular actin cytoskeleton (IAC) and the strain amplification that results from this coupling are examined for the first time. The model leads to two predictions, which could fundamentally change existing views. First, for the loading range 1-20MPa and frequency range 1-20Hz, it is, indeed, possible to produce cellular level strains in bone that are up to 100 fold greater than normal tissue level strains (0.04-0.3 percent). Thus, the strain in the cell process membrane due to the loading can be of the same order as the in vitro strains measured in cell culture studies where intracellular biochemical responses are observed for cells on stretched elastic substrates. Second, it demonstrates that in any cellular system, where cells are subject to fluid flow and tethered to more rigid supporting structures, the tensile forces on the cell due to the drag forces on the tethering fibers may be many times greater than the fluid shear force on the cell membrane.

Effects of irrigation deprivation during the harvest period on leaf persistence and function in mature almond trees.

In nut tree orchards in California, irrigation is typically withheld during the harvest period to reduce the likelihood of bark damage during mechanical shaking of the trees. The ensuing water stress, however, may result in premature defoliation and subsequent yield declines. Our objective was to establish and quantify the water stress resulting from irrigation deprivation and determine its impact on leaf function and persistence in mature almond trees (Prunus dulcis (Mill.) D.A. Webb cv. Nonpareil) during a 3-year field experiment. The severity of the water stress was characterized by measurements of predawn leaf (Psi(pd)) and midday stem (Psi(ms)) water potentials, stomatal conductance (gs), net CO2 assimilation rate (A) and leaf abscission. During 1995, Psi(ms) of fully irrigated (FI) trees was maintained above -1.0 MPa. In trees in the moderate- (MS) and severe-stress (SS) treatments, Psi(ms) was reduced to -1.4 to -2.0 MPa and -2.0 to -2.6 MPa, respectively. After 18 days of irrigation deprivation, A was reduced by 32 and 58% at midday and early afternoon, respectively, compared with morning values. A significant decrease in morning values of A only occurred after 30 days of irrigation deprivation. Water-use efficiency and A declined as evaporative demand increased from morning to afternoon. Assimilation also declined seasonally as leaves aged. Midday stem water potential was highly correlated with A, but less so with gs. The coefficient of determination between Psi(ms) and gs improved considerably when vapor pressure deficit and wind were multiply regressed with Psi(ms). Although A recovered rapidly when MS trees were irrigated, recovery in SS trees was slower and incomplete. Integrating the MS and SS effects for an extended period during 1995 resulted in 14 and 30% declines in A, and 6 and 20% declines in gs, respectively. The apparent Psi(ms) threshold for leaf abscission was -1.8 MPa. Daily canopy light interception declined with decreasing Psi(ms) as a result of premature defoliation (and perhaps altered leaf angles) from 67.9% in FI trees to 61.4 and 60.7% in MS and SS trees, respectively.

Effects of irrigation deprivation during the harvest period on yield determinants in mature almond trees.

Effects of irrigation deprivation during the harvest period on yield determinants in mature almond (Prunus dulcis (Mill.) D.A. Webb cv. Nonpareil) trees were investigated during a 3-year field experiment. Return bloom and fruit set were measured on 2185 individually tagged spurs. Water stress resulting from irrigation deprivation during the harvest period, which purportedly coincides with the time of flower initiation, had no effect on the percentage of spurs that flowered or set fruit during subsequent years. Although water stress had no apparent effect on spur mortality, 66% of the tagged spurs died within 3 years. In addition, many spurs were vegetative by the third year, indicating the importance of spur renewal for sustained fruit production. Reductions in nut yield were evident after two successive years of irrigation deprivation during the harvest period. Regression analysis indicated a loss in yield of 7.7 kg tree(-1) in response to each 1 MPa decrease in stem water potential below -1.2 MPa during the previous seasons. The number of fruiting positions per tree (estimated indirectly for whole trees based on weight of current-year shoots > 5 cm in length) was negatively associated with water stress. Yield reduction in response to water stress during harvest appears to be a compound, multiyear effect, associated with reduced annual growth and renewal of fruiting positions.

Effects of irrigation deprivation during the harvest period on nonstructural carbohydrate and nitrogen contents of dormant, mature almond trees.

Effect of irrigation deprivation during the harvest period on the nonstructural carbohydrate (NC) content of dormant, mature, field-grown almond (Prunus dulcis (Mill.) D.A. Webb cv. Nonpareil) trees was studied. Roots, trunk, branches, spurs and stems of 12 trees were subsampled in February 1997, across a gradient of irrigation treatments (FI = fully irrigated, MS = moderately stressed and SS = severely stressed) to relate NC concentration to the degree of water stress experienced by individual trees during the previous (1996) harvest period. To assess the effect of water stress on whole-tree NC content, three dormant FI trees and three dormant SS trees were excavated on December 10, 1997, and dry weights and NC and N concentrations of the tree components were determined. Whole-tree biomass did not differ significantly between FI and SS trees, although SS trees tended to have less total dry weight. Although roots constituted just 13% of tree biomass, they stored 36 and 44% of tree NC and N contents, respectively. There were negative relationships between the seasonal minimum values of both midday (Psi(ms)) and predawn (Psi(pd)) stem water potentials during the harvest period and root NC content of dormant trees. Severe water stress during the harvest period resulted in a 26% reduction in NC content and a 50% reduction in biomass of current-year stems (> 5 cm in length) per tree. The reduction in NC content is consistent with the previously reported late season reductions in leaf function and persistence. The SS trees exhibited a reduction in NC content but not in N content per tree, indicating that late season accumulation of NC and N were uncoupled in trees subjected to severe harvest-period water stress.

A new technique for studying the stylet tracks of homopteran insects in hand-sectioned plant tissue using light or epifluorescence microscopy.

Homopteran insects, such as aphids, psyllids and scales, inject a proteinaceous salivary sheath into their host plant tissue during feeding. This sheath, also referred to as a stylet track, remains in the tissue after the stylets are withdrawn, and is useful for studying plant resistance to insects and plant virus transmission. We describe a new method for studying stylet tracks. Hand microtome sectioned plant material was fixed and cleared in ethanol. The stylet tracks were stained with acid fuchsin and counterstained with aniline blue or fast green. The acid fuchsin stained stylet tracks were pink to red under light microscopy, and orange under TRITC epifluorescence. Stylet tracks in unstained sections autofluoresced under DAPI epifluorescence. This new technique is significantly faster and less complex than previous techniques, and permitted visualization of stylet tracks with light or epifluorescence microscopy within 1 hr of collecting fresh plant material. The technique was also applicable to a broad range of homopterans and plant taxa and provided excellent photomicrographs.

A new view of mechanotransduction and strain amplification in cells with microvilli and cell processes.

In this paper we shall describe new mechanical models for the deformation of the actin filament bundles in kidney microvilli and osteocytic cell processes to see whether these cellular extensions, like the stereocilia on hair cells in the inner ear, can function as mechanotransducers when subject to physiological flow. In the case of kidney microvilli we show that the hydrodynamic drag forces at the microvilli tip are <0.01 pN, but there is a 38-fold force amplification on the actin filaments at the base of the microvilli due to the resisting moment in its terminal web. This leads to forces that are more than sufficient to deform the terminal web complex of the microvillus where ezrin has been shown to couple the actin cytoskeleton to the Na(+)/H(+) exchanger. In the case of bone cell processes we show that the actin filament bundles have an effective Young's modulus that is 200 times > the measured modulus for the actin gel in the cell body. It is, therefore, unlikely that bone cell processes respond in vivo to fluid shear stress, as proposed in [59]. However, we show that the fluid drag forces on the pericellular matrix which tethers the cell processes to the canalicular wall can produce a 20-100 fold amplification of bone tissue strains in the actin filament bundle of the cell process.

Shear stress induces a time- and position-dependent increase in endothelial cell membrane fluidity.

Blood flow-associated shear stress may modulate cellular processes through its action on the plasma membrane. We quantified the spatial and temporal aspects of the effects of shear stress (tau) on the lipid fluidity of 1,1'-dihexadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate [DiIC(16)(13)]-stained plasma membranes of bovine aortic endothelial cells in a flow chamber. A confocal microscope was used to determine the DiI diffusion coefficient (D) by fluorescence recovery after photobleaching on cells under static conditions, after a step-tau of 10 or 20 dyn/cm(2), and after the cessation of tau. The method allowed the measurements of D on the upstream and downstream sides of the cell taken midway between the respective cell borders and the nucleus. In <10 s after a step-tau of 10 dyn/cm(2), D showed an upstream increase and a downstream decrease, and both changes disappeared rapidly. There was a secondary, larger increase in upstream D, which reached a peak at 7 min and decreased thereafter, despite the maintenance of tau. D returned to near control values within 5 s after cessation of tau. Downstream D showed little secondary changes throughout the 10-min shearing, as well as after its cessation. Further investigations into the early phase, with simultaneous measurements of upstream and downstream D, confirmed that a step-tau of 10 dyn/cm(2) elicited a rapid (5-s) but transient increase in upstream D and a concurrent decrease in downstream D, yielding a significant difference between the two sites. A step-tau of 20 dyn/cm(2) caused D to increase at both sites at 5 s, but by 30 s and 1 min the upstream D became significantly higher than the downstream D. These results demonstrate shear-induced changes in membrane fluidity that are time dependent and spatially heterogeneous. These changes in membrane fluidity may have important implications in shear-induced membrane protein modulation.

Dynamic contact forces on leukocyte microvilli and their penetration of the endothelial glycocalyx.

We develop a theoretical model to examine the combined effect of gravity and microvillus length heterogeneity on tip contact force (F(m)(z)) during free rolling in vitro, including the initiation of L-, P-, and E-selectin tethers and the threshold behavior at low shear. F (m)(z) grows nonlinearly with shear. At shear stress of 1 dyn/cm(2), F(m)(z) is one to two orders of magnitude greater than the 0.1 pN force for gravitational settling without flow. At shear stresses > 0.2 dyn/cm(2) only the longest microvilli contact the substrate; hence at the shear threshold (0.4 dyn/cm(2) for L-selectin), only 5% of microvilli can initiate tethering interaction. The characteristic time for tip contact is surprisingly short, typically 0.1-1 ms. This model is then applied in vivo to explore the free-rolling interaction of leukocyte microvilli with endothelial glycocalyx and the necessary conditions for glycocalyx penetration to initiate cell rolling. The model predicts that for arteriolar capillaries even the longest microvilli cannot initiate rolling, except in regions of low shear or flow reversal. In postcapillary venules, where shear stress is approximately 2 dyn/cm(2), tethering interactions are highly likely, provided that there are some relatively long microvilli. Once tethering is initiated, rolling tends to ensue because F(m)(z) and contact duration will both increase substantially to facilitate glycocalyx penetration by the shorter microvilli.

A hydrodynamic mechanosensory hypothesis for brush border microvilli.

In the proximal tubule of the kidney, Na(+) and HCO(3)(-) reabsorption vary proportionally with changes in axial flow rate. This feature is a critical component of glomerulotubular balance, but the basic mechanism by which the tubule epithelial cells sense axial flow remains unexplained. We propose that the microvilli, which constitute the brush border, are physically suitable to act as a mechanosensor of fluid flow. To examine this hypothesis quantitatively, we have developed an elastohydrodynamic model to predict the forces and torques along each microvillus and its resulting elastic bending deformation. This model indicates that: 1) the spacing of the microvilli is so dense that there is virtually no axial velocity within the brush border and that drag forces on the microvilli are at least 200 times greater than the shear force on the cell's apical membrane at the base of the microvilli; 2) of the total drag on a 2.5-microm microvillus, 74% appears within 0.2 microm from the tip; and 3) assuming that the structural strength of the microvillus derives from its axial actin filaments, then a luminal fluid flow of 30 nl/min produces a deflection of the microvillus tip which varies from about 1 to 5% of its 90-nm diameter, depending on the microvilli length. The microvilli thus appear as a set of stiff bristles, in a configuration in which changes in drag will produce maximal torque.

Starling forces that oppose filtration after tissue oncotic pressure is increased.

We tested the hypothesis that the effective oncotic force that opposes fluid filtration across the microvessel wall is the local oncotic pressure difference across the endothelial surface glycocalyx and not the global difference between the plasma and tissue. In single frog mesenteric microvessels perfused and superfused with solutions containing 50 mg/ml albumin, the effective oncotic pressure exerted across the microvessel wall was not significantly different from that measured when the perfusate alone contained albumin at 50 mg/ml. Measurements were made during transient and steady-state filtration at capillary pressures between 10 and 35 cmH(2)O. A cellular-level model of coupled water and solute flows in the interendothelial cleft showed water flux through small breaks in the junctional strand limited back diffusion of albumin into the protected space on the tissue side of the glycocalyx. Thus oncotic forces opposing filtration are larger than those estimated from blood-to-tissue protein concentration differences, and transcapillary fluid flux is smaller than estimated from global differences in oncotic and hydrostatic pressures.

Endothelium-dependent, shear-induced vasodilation is rate-sensitive.

OBJECTIVES: To quantify the relative contributions of the rate of change and the magnitude of shear stress to endothelium-mediated arteriolar dilation. METHODS: A feedback control system was designed in which shear stress (tau) and the temporal shear gradient (TSG) were prescribed and dynamically controlled in isolated rat cremaster 1A arterioles. The TSG was the quotient of the maximum shear stress and the ramp duration. This system was used to assess the roles of tau and TSG in the initial, transient vasodilations and the secondary, sustained vasodilations in response to steps and ramps in shear stress. RESULTS: Both step- and ramp-shear experiments revealed time-dependent hiphasic vasodilations that we report for the first time. Application of a step-shear stress of 20 dynes/cm2 elicited an initial transient vasodilation that peaked at about 4 min. When the shear stress was applied as a ramp that reached the maximum value of 20 dynes/cm2 over 5 min, a vasodilation was observed over the ramp period, which reached a peak at the end of the ramp period that was much lower than that observed after step shear. After 20 dynes/cm2 was attained, the vessel diameter decreased despite constant maintenance of the maximum shear stress. In both step- and ramp-shear experiments, after the decrease of the initial vasodilation, a second phase of vasodilation began approximately 15 min after the beginning of the shear application. The second phase of vasodilation reached a steady state that was essentially the same for both the step and the ramp shear. By refining the ramping apparatus further, we varied the TSG up to 40 dynes/cm2 per second and showed that the early vasodilation was highly rate sensitive to TSGs greater than 5 dynes/cm2 per second for a given intermediate value of final shear stress (20 dynes/cm2) and was magnitude sensitive when shear was increased gradually (TSG < 5 dynes/cm2 per second). CONCLUSIONS: Our results suggest that two fundamentally different responses to shear stress are mediated by microvascular endothelium: one vasodilation is elicited by shear stress changes on a time scale of a few seconds or less and another is elicited by shear stress changes on a longer time scale. The former response is potent, transient, and rate sensitive; the latter is more modest, sustained, and magnitude sensitive.

Modeling tracer transport in an osteon under cyclic loading.

A mathematical model is developed to explain the fundamental conundrum as to how during cyclic mechanical loading there can be net solute (e.g., nutrient, tracer) transport in bone via the lacunar-canalicular porosity when there is no net fluid movement in the canaliculi over a loading cycle. Our hypothesis is that the fluid space in an osteocytic lacuna facilitates a nearly instantaneous mixing process of bone fluid that creates a difference in tracer concentration between the inward and outward canalicular flow and thus ensures net tracer transport to the osteocytes during cyclic loading, as has been shown experimentally. The sequential spread of the tracer from the osteonal canal to the lacunae is investigated for an osteon experiencing sinusoidal loading. The fluid pressure in the canaliculi is calculated using poroelasticity theory and the mixing process in the lacunae is then simulated computationally. The tracer concentration in lacunae extending radially from the osteonal canal to the cement line is calculated as a function of the loading frequency, loading magnitude, and number of loading cycles as well as the permeability of the lacunar-canalicular porosity. Our results show that net tracer transport to the lacunae does occur for cyclic loading. Tracer transport is found to increase with higher loading magnitude and higher permeability and to decrease with increasing loading frequency. This work will be helpful in designing experimental studies of tracer movement and bone fluid flow, which will enhance our understanding of bone metabolism as well as bone adaptation.

A new view of Starling's hypothesis at the microstructural level.

In this paper we quantitatively investigate the hypothesis proposed by Michel (Exp. Physiol. 82, 1-30, 1997) and Weinbaum (Ann. Biomed. Eng. 26, 1-17, 1998) that the Starling forces are determined by the local difference in the hydrostatic and colloid osmotic pressure across the endothelial surface glycocalyx, which we propose is the primary molecular sieve for plasma proteins, rather than the global difference in the hydrostatic and oncotic pressure across the capillary wall between the plasma and tissue, as has been universally assumed until now. A spatially heterogeneous microstructural model is developed to explain at the cellular level why there is oncotic absorption at low capillary pressures in the short-lived transient experiments of Michel and Phillips (J. Physiol. 388, 421-435, 1987) on frog mesentery capillary, but a small positive filtration once a steady state is achieved. The new model also predicts that the local protein concentration behind the surface glycocalyx can differ greatly from the tissue protein concentration, since the convective flux of proteins through the orifice-like pores in the junction strand will greatly impede the back diffusion of the proteins into the lumen side of the cleft when the local Peclet number at the orifice is >1. The net result is that the filtration in the capillaries is far less than heretofore realized and there may be no need for venous reabsorption.

Fluid pressure relaxation depends upon osteonal microstructure: modeling an oscillatory bending experiment.

When bone is mechanically loaded, bone fluid flow induces shear stresses on bone cells that have been proposed to be involved in bone's mechanosensory system. To investigate bone fluid flow and strain-generated potentials, several theoretical models have been proposed to mimic oscillatory four-point bending experiments performed on thin bone specimens. While these previous models assume that the bone fluid relaxes across the specimen thickness, we hypothesize that the bone fluid relaxes primarily through the vascular porosity (osteonal canals) instead and develop a new poroelastic model that integrates the microstructural details of the lacunar-canalicular porosity, osteonal canals, and the osteonal cement lines. Local fluid pressure profiles are obtained from the model, and we find two different fluid relaxation behaviors in the bone specimen, depending on its microstructure: one associated with the connected osteonal canal system, through which bone fluid relaxes to the nearby osteonal canals; and one associated with the thickness of a homogeneous porous bone specimen (approximately 1 mm in our model), through which bone fluid relaxes between the external surfaces of the bone specimen at relatively lower loading frequencies. Our results suggest that in osteonal bone specimens the fluid pressure response to cyclic loading is not sensitive to the permeability of the osteonal cement lines, while it is sensitive to the applied loading frequency. Our results also reveal that the fluid pressure gradients near the osteonal canals (and thus the fluid shear stresses acting on the nearby osteocytes) are significantly amplified at higher loading frequencies.