Model-independent relationships between hematocrit,blood viscosity,and yield stress derived from Couette viscometry data

This paper describes a procedure, based on Tikhonov regularization, for obtaining the shear rate function or equivalently the viscosity function of blood from Couette viscometry data. For data sets that include points where the sample in the annulus is partially sheared the yield stress of blood will also be obtained. For data sets that do not contain partially sheared points, provided the shear stress is sufficiently low, a different method of estimating the yield stress is proposed. Both the shear rate function and yield stress obtained in this investigation are independent of any rheological model of blood. This procedure is applied to a large set of Couette viscometer data taken from the literature. Results in the form of shear rate and viscosity functions and yield stress are presented for a wide range of hematocrits and are compared against those reported by the originators of the data and against independently measured shear properties of blood.     

Detection of red cell aggregation by low shear rate viscometry in whole blood with elevated plasma viscosity

The viscosity of whole blood measured at low shear rates is determined partly by shear resistance of the red cell aggregates present, stronger aggregation increasing the viscosity in the absence of other changes. Effects of cell deformability can confound interpretation and comparison in terms of aggregation, however, particularly when the plasma viscosity is high. We illustrate the problem with a comparison of hematocrit-adjusted blood from type 1 diabetes patients and controls in which it is found the apparent and relative viscosities at a true shear rate of 0.20 s-1 are lower in the patient samples than age matched controls, in spite of reports that aggregation is increased in such populations. Because the plasma viscosities of the patients were higher on average than controls, we performed a series of experiments to examine the effect of plasma protein concentration and viscosity on normal blood viscosity. Dilution or concentration by ultrafiltration of autologous plasma and viscosity measurements at low shear on constant hematocrit red cell suspensions showed (a) suspension viscosity at 0.25 and 3 s-1 increased monotonically with plasma protein concentration and viscosity but (b) the relative viscosity increased, in concert with the microscopic aggregation grade, up to a viscosity of approximately 1.25 mPa-s but above this the value the relative viscosity no longer increased as the degree of aggregation increased in concentrated plasmas. It is suggested that in order to reduce cell deformation effects in hyperviscous pathological plasmas, patient and control plasmas should be systematically diluted before hematocrit is adjusted and rheological measurements are made. True shear rates should be calculated. Comparison of relative viscosities at low true shear rates appears to allow the effects of red cell aggregation to be distinguished by variable shear rate viscometry in clinical blood samples.     

Red blood cell aggregation quantitated via Myrenne aggregometer and yield shear stress

Although the study of red blood cell (RBC) aggregation continues to be of basic science and clinical interest, aggregation standards for calibration do not exist, and most aggregation studies report data in terms of arbitrary units: quantitative comparisons between studies are thus essentially precluded. However, use of low shear viscometry plus the Casson equation provides a yield shear stress that has defined units and is known to reflect RBC aggregation. Employing human RBC-plasma suspensions exhibiting a wide range of aggregation, the present study examined relations between yield shear stress values and aggregation indices obtained using the Myrenne aggregometer: the latter approach uses a light-transmission technique and provides an "M" index at stasis and an "M1" at very low shear. Our results for normal controls and for angina patients without coronary artery disease indicate highly significant correlations (p<0.001) between the yield stress and both M and M1. Thus, within the range of aggregation studied, these findings lend support to the rheological validity of the Myrenne approach; extension of our findings to intensely aggregating RBC suspensions may require additional validation studies.     

Control of the shape of a thrombus-neointima-like structure by blood shear stress

Fluid mechanical factors are thought to influence vascular morphogenesis. Here we show how blood shear stress regulates the shape of a thrombus-neointima-like tissue on a polymer micro-cylinder implanted in the center of the rat vena cava with the micro-cylinder perpendicular to blood flow. In this model, the micro-cylinder is exposed to a laminarflow with a known shear stress field in the leading region and a vortexflow in the trailing region. At 1, 5, 10, 20, and 30 days after implantation, it was found that the micro-cylinder was encapsulated by a thrombus-neointima-like tissue with a streamlined body profile. The highest growth rate of the thrombus-neointima-like tissue was found along the trailing and leading stagnation edges of the micro-cylinder. Blood shear stress in the laminar flow region was inversely correlated with the rate of thrombus formation and cell proliferation, and the percentage of smooth muscle a actin-positive cells. These biological changes were also found in the trailing vortex flow region, which was associated with lowered shear stress. These results suggest that blood shear stress regulates the rate of thrombus and neointimal formation and, thus, influences the shape of the thrombus-neointima-like structure in the present model.     

Negative regulation of vascular smooth muscle cell migration by blood shear stress

Urocortin (UCN) protects hearts against ischemia and reperfusion injury whether given before ischemia or at reperfusion. Here we investigate the roles of PKC, reactive oxygen species, and the mitochondrial permeability transition pore (MPTP) in mediating these effects. In Langendorff-perfused rat hearts, acute UCN treatment improved hemodynamic recovery during reperfusion after 30 min of global ischemia; this was accompanied by less necrosis (lactate dehydrogenase release) and MPTP opening (mitochondrial entrapment of 2-[(3)H]deoxyglucose). UCN pretreatment protected mitochondria against calcium-induced MPTP opening, but only if the mitochondria had been isolated from hearts after reperfusion. These mitochondria also exhibited less protein carbonylation, suggesting that UCN decreases levels of oxidative stress. In isolated adult and neonatal rat cardiac myocytes, both acute (60 min) and chronic (16 h) treatment with UCN reduced cell death following simulated ischemia and re-oxygenation. This was accompanied by less MPTP opening as measured using tetramethylrhodamine methyl ester. The level of oxidative stress during reperfusion was reduced in cells that had been pretreated with UCN, suggesting that this is the mechanism by which UCN desensitizes the MPTP to reperfusion injury. Despite the fact that we could find no evidence that either PKC-epsilon or PKC-alpha translocate to the mitochondria following acute UCN treatment, inhibition of PKC with chelerythrine eliminated the effect of UCN on oxidative stress. Our data suggest that acute UCN treatment protects the heart by inhibiting MPTP opening. However, the mechanism appears to be indirect, involving a PKC-mediated reduction in oxidative stress.     

Influence of cardiac flow rate on turbulent shear stress from a prosthetic heart valve

Elevated turbulent shear stresses associated with sufficient exposure times are potentially damaging to blood constituents. Since these conditions can be induced by mechanical heart valves, the objectives of this study were to locate the maximum turbulent shear stress in both space and time and to determine how the maximum turbulent shear stress depends on the cardiac flow rate in a pulsatile flow downstream of a tilting disk valve. Two-component, simultaneous, correlated laser velocimeter measurements were recorded at four different axial locations and three different flow rates in a straight tube model of the aorta. All velocity data were ensemble averaged within a 15 ms time window located at approximately peak systolic flow over more than 300 cycles. Shear stresses as high as 992 dynes/cm2 were found 0.92 tube diameters downstream of the monostrut, disk valve. The maximum turbulent shear stress was found to scale with flow rate to the 0.72 power. A repeatable starting vortex was shed from the disk at the beginning of each cycle.     

The influence of fluid shear stress on the remodeling of the embryonic primary capillary plexus

The primary capillary plexus in early yolk sacs is remodeled into matured vitelline vessels aligned in the direction of blood flow at the onset of cardiac contraction. We hypothesized that the influence of fluid shear stress on cellular behaviors may be an underlying mechanism by which some existing capillary channels remain open while others are closed during remodeling. Using a recently developed E-Tmod knock-out/lacZ knock-in mouse model, we showed that erythroblasts exhibited rheological properties similar to those of a viscous cell suspension. In contrast, the non-erythroblast (NE) cells, which attach among themselves within the yolk sac, are capable of lamellipodia extension and cell migration. Isolated NE cells in a parallel-plate flow chamber exposed to fluid shear stress, however, ceased lamellipodia extension. Such response may minimize NE cell migration into domains exposed to fluid shear stress. A two-dimensional mathematical model incorporating these cellular behaviors demonstrated that shear stress created by the blood flow initiated by the embryonic heart contraction might be needed for the remodeling of primary capillary plexus.     

Shear stress and shear rate differentially affect the multi-step process of leukocyte-facilitated melanoma adhesion

Previous studies have shown that neutrophils (PMNs) facilitate melanoma cell extravasation [M.J. Slattery, C. Dong, Neutrophils influence melanoma adhesion and migration under flow conditions, Intl. J. Cancer 106 (2003) 713–722] Little is known, however, about the specific interactions between PMNs, melanoma and the endothelium (EC) or the molecular mechanism involved under flow conditions. The aim of this study is to investigate a “two-step adhesion” hypothesis that involves initial PMN tethering on the EC and subsequent melanoma cells being captured by tethered PMNs. Different effects of hydrodynamic shear stress and shear rate were analyzed using a parallel-plate flow chamber. Results indicate a novel finding that PMN-facilitated melanoma cell arrest on the EC is modulated by shear rate, which is inversely-proportional to cell–cell contact time, rather than by the shear stress, which is proportional to the force exerted on formed bonds. β₂ integrins/ICAM-1 adhesion mechanisms were examined and the results indicate LFA-1 and Mac-1 cooperate to mediate the PMN–EC–melanoma interactions under shear conditions. In addition, endogenously produced IL-8 contributes to PMN-facilitated melanoma arrest on the EC through the CXC chemokine receptors 1 and 2 (CXCR1 and CXCR2) on PMN. These results provide new evidence for the complex role of hemodynamic forces, secreted chemokines and PMN–melanoma adhesion in the recruitment of metastatic cancer cells to the EC.     

Evaluation of the hypomagnetic environment effects on capillary blood circulation,blood pressure,and heart rate

The impact of attenuated magnetic field (МF) on human health is a burning issue of present-day cosmonautics. A series of experiments with animals exposed to attenuated MF revealed violent disorders in the development of the cardiovascular system. The purpose of this study was to estimate the effects of hypomagnetic environment (HME) on capillary blood circulation, blood pressure (BP), and heart rate (HR) in healthy humans. Participants (n = 34) were 24 men and 10 women without cardiovascular symptoms. The mean age was 43.3 ± 15.4 years. Thirteen participants, eight men and five women, were randomly selected for a repeated investigation under natural conditions (sham exposure). The mean age in this group was 47.9 ± 18 years. Cardiac rhythm and heart rate were recorded using an Astrocard cardiac monitor (Russia). BP was measured by means of a Tonocard automatic blood pressure monitor (Russia). Capillary circulation was determined using a digital capillaroscope (Russia) with a high-speed CMOS camera (100 frames/s). The duration of HME exposure was 60 min. It has been demonstrated that HME increases capillary circulation rate by 22.4% in healthy humans without cardiovascular symptoms as compared to the records made under natural conditions. There was a significant HR reduction by the end of HME exposure as compared to the measurements taken at the beginning. At the end of the exposure, diastolic BP dropped considerably relative to mid-exposure values; on the contrary, systolic BP significantly increased by the end.     

Prediction of microbial growth rate versus biomass yield by a metabolic network with kinetic parameters

Identifying the factors that determine microbial growth rate under various environmental and genetic conditions is a major challenge of systems biology. While current genome-scale metabolic modeling approaches enable us to successfully predict a variety of metabolic phenotypes, including maximal biomass yield, the prediction of actual growth rate is a long standing goal. This gap stems from strictly relying on data regarding reaction stoichiometry and directionality, without accounting for enzyme kinetic considerations. Here we present a novel metabolic network-based approach, MetabOlic Modeling with ENzyme kineTics (MOMENT), which predicts metabolic flux rate and growth rate by utilizing prior data on enzyme turnover rates and enzyme molecular weights, without requiring measurements of nutrient uptake rates. The method is based on an identified design principle of metabolism in which enzymes catalyzing high flux reactions across different media tend to be more efficient in terms of having higher turnover numbers. Extending upon previous attempts to utilize kinetic data in genome-scale metabolic modeling, our approach takes into account the requirement for specific enzyme concentrations for catalyzing predicted metabolic flux rates, considering isozymes, protein complexes, and multi-functional enzymes. MOMENT is shown to significantly improve the prediction accuracy of various metabolic phenotypes in E. coli, including intracellular flux rates and changes in gene expression levels under different growth rates. Most importantly, MOMENT is shown to predict growth rates of E. coli under a diverse set of media that are correlated with experimental measurements, markedly improving upon existing state-of-the art stoichiometric modeling approaches. These results support the view that a physiological bound on cellular enzyme concentrations is a key factor that determines microbial growth rate.     

Change in properties of the glycocalyx affects the shear rate and stress distribution on endothelial cells

The endothelial glycocalyx mediates interactions between the blood flow and the endothelium. This study aims to evaluate, quantitatively, effects of structural change of the glycocalyx on stress distribution and shear rate on endothelial cells. In the study, the endothelial glycocalyx is modeled as a surface layer of fiber matrix and when exposed to laminar shear flow, the matrix deforms. Fluid velocity and stress distribution inside the matrix and on cell membranes are studied based on a binary mixture theory. Parameters, such as the height and porosity of the matrix and the drag coefficient between fluid and matrix fibrils, are based on available data and estimation from experiments. Simple theoretical solutions are achieved for fluid velocity and stress distribution in the surface matrix. Degradation of the matrix, e.g., by enzyme digestion, is represented by reductions in the volume fraction of fibrils, height, and drag coefficient. From a force balance, total stress on endothelial surface remains constant regardless of structural alteration of the glycocalyx. However, the stress that is transmitted to endothelial cells by direct "pulling" of fiber branches of the glycocalyx is reduced significantly. Fluid shear rate at the cell membrane, on the other hand, increases. The study gives quantitative insight into the effect of the structural change of the glycocalyx on the shear rate and pulling stress on the endothelium. Results can be used to interpret experiments on effects of the glycocalyx in shear induced endothelial responses.     

Effects of shear rate and suspending viscosity on deformation and frequency of red blood cells tank-treading in shear flows

The tank-treading rotation of red blood cells (RBCs) in shear flows has been studied extensively with experimental, analytical, and numerical methods. Even for this relatively simple system, complicated motion and deformation behaviors have been observed, and some of the underlying mechanisms are still not well understood. In this study, we attempt to advance our knowledge of the relationship among cell motion, deformation, and flow situations with a numerical model. Our simulation results agree well with experimental data, and confirm the experimental finding of the decrease in frequency/shear-rate ratio with shear rate and the increase of frequency with suspending viscosity. Moreover, based on the detailed information from our simulations, we are able to interpret the frequency dependency on shear rate and suspending viscosity using a simple two-fluid shear model. The information obtained in this study thus is useful for understanding experimental observations of RBCs in shear and other flow situations; the good agreement to experimental measurements also shows the potential usefulness of our model for providing reliable results for microscopic blood flows.     

The relationship between wall shear stress distributions and intimal thickening in the human abdominal aorta

Purpose

The goal of this work was to determine wall shear stress (WSS) patterns in the human abdominal aorta and to compare these patterns to measurements of intimal thickness (IT) from autopsy samples.

Methods

The WSS was experimentally measured using the laser photochromic dye tracer technique in an anatomically faithful in vitro model based on CT scans of the abdominal aorta in a healthy 35-year-old subject. IT was quantified as a function of circumferential and axial position using light microscopy in ten human autopsy specimens.

Results

The histomorphometric analysis suggests that IT increases with age and that the distribution of intimal thickening changes with age. The lowest WSS in the flow model was found on the posterior wall inferior to the inferior mesenteric artery, and coincided with the region of most prominent IT in the autopsy samples. Local geometrical features in the flow model, such as the expansion at the inferior mesenteric artery (common in younger individuals), strongly influenced WSS patterns. The WSS was found to correlate negatively with IT (r² = 0.3099; P = 0.0047).

Conclusion

Low WSS in the abdominal aorta is co-localized with IT and may be related to atherogenesis. Also, rates of IT in the abdominal aorta are possibly influenced by age-related geometrical changes.