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Biomedical subjects

A R Pries

Publications and source records attributed to A R Pries.

17 recordsLinked to original sources
PubMed1992

Blood viscosity in tube flow: dependence on diameter and hematocrit.

Since the original publications by Martini et al. (Dtsch. Arch. Klin. Med. 169: 212-222, 1930) and Fahraeus and Lindqvist (Am. J. Physiol. 96: 562-568, 1931), it has been known that the relative apparent viscosity of blood in tube flow depends on tube diameter. Quantitative descriptions of this effect and of the dependence of blood viscosity on hematocrit in the different diameter tubes are required for the development of hydrodynamic models of blood flow through the microcirculation. The present study provides a comprehensive data base for the description of relative apparent blood viscosity as a function of tube diameter and hematocrit. Data available from the literature are compiled, and new experimental data obtained in a capillary viscometer are presented. The combined data base comprises measurements at high shear rates (u > or = 50 s-1) in tubes with diameters ranging from 3.3 to 1,978 microns at hematocrits of up to 0.9. If corrected for differences in suspending medium viscosity and temperature, the data show remarkable agreement. Empirical fitting equations predicting relative apparent blood viscosity from tube diameter and hematocrit are presented. A pronounced change in the hematocrit dependence of relative viscosity is observed in a range of tube diameters in which viscosity is minimal. While a linear hematocrit-viscosity relationship is found in tubes of < or = 6 microns, an overproportional increase of viscosity with hematocrit prevails in tubes of > or = 9 microns. This is interpreted to reflect the hematocrit-dependent transition from single- to multifile arrangement of cells in flow.

Animals↗
PubMed1992

Redistribution of red blood cell flow in microcirculatory networks by hemodilution.

The effect of isovolemic hemodilution on red blood cell flow distribution was studied in complete self-contained microvessel networks of the rat mesentery. Hematocrit, diameter, and length of all vessel segments as well as the topological structure were determined in control networks (systemic hematocrit, 0.54) and after hemodilution (systemic hematocrit, 0.30). Hemodilution was performed by exchanging blood with hydroxyethyl starch (MW 450,000; 6%) or homologous plasma. With hemodilution, the decrease of microvessel hematocrit exceeded that of systemic hematocrit. The average discharge hematocrit in capillaries was 79% of systemic hematocrit in the control group and 73% with hemodilution (p less than 0.001). The heterogeneity of capillary hematocrit within the network, expressed by the coefficient of variation, increased from 0.4 to 0.7. By using the morphological and topological data of four networks, the distribution of hematocrits was also calculated using a hydrodynamic flow model. The modeling results were found to be in close agreement with the experimental data. This indicates that the observed changes can be deduced from established rheological phenomena, most of all phase separation at arteriolar bifurcations. The changes in hematocrit distribution after hemodilution are accompanied by a redistribution of red blood cell flow within the network: relative to total red blood cell flow, red blood cell flow in the distal capillaries of the network increases by about 40% at the expense of the proximal capillaries that are close to the feeding arteriole and that exhibit the highest red blood cell flow under control conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

A new approach to intravital videomicroscopy of rat spinotrapezius muscle.

A modified preparation of the rat spinotrapezius muscle is described in which optimal conditions for intravital microscopy can be achieved while the supplying blood vessels are left fully intact, and mechanical stress to the muscle during preparation is reduced to an unavoidable minimum. The viability of the preparation is demonstrated using the response of arterial microvessels to endothelium-dependent and -independent dilators and to changes of ambient PO2, the presence of spontaneous vasomotion, and histochemical analysis of pertinent enzyme systems. The preparation is viable for much longer experimental time periods (up to 10 hours) than reported previously, provided the intensity of illumination is kept at a very low level. If the latter prerequisite is met, tissue edema, maximal vasodilation, and the associated loss of responsiveness to vasoactive stimuli of arterioles is reliably avoided.

Animals↗
PubMed1990

Blood flow in microvascular networks. Experiments and simulation.

A theoretical model has been developed to simulate blood flow through large microcirculatory networks. The model takes into account the dependence of apparent viscosity of blood on vessel diameter and hematocrit (the Fahraeus-Lindqvist effect), the reduction of intravascular hematocrit relative to the inflow hematocrit of a vessel (the Fahraeus effect), and the disproportionate distribution of red blood cells and plasma at arteriolar bifurcations (phase separation). The model was used to simulate flow in three microvascular networks in the rat mesentery with 436,583, and 913 vessel segments, respectively, using experimental data (length, diameter, and topological organization) obtained from the same networks. Measurements of hematocrit and flow direction in all vessel segments of these networks tested the validity of model results. These tests demonstrate that the prediction of parameters for individual vessel segments in large networks exhibits a high degree of uncertainty; for example, the squared coefficient of correlation between predicted and measured hematocrit of single vessel segments ranges only between 0.15 and 0.33. In contrast, the simulation of integrated characteristics of the network hemodynamics, such as the mean segment hematocrit or the distribution of blood flow velocities, is very precise. In addition, the following conclusions were derived from the comparison of predicted and measured values: 1) The low capillary hematocrits found in mesenteric microcirculatory networks as well as their heterogeneity can be explained on the basis of the Fahraeus effect and phase-separation phenomena. 2) The apparent viscosity of blood in vessels of the investigated tissue with diameters less than 15 microns is substantially higher than expected compared with measurements in glass tubes with the same diameter.

Animals↗
PubMed1989

Venulo-arteriolar communication and propagated response. A possible mechanism for local control of blood flow.

The effect of microinjection of norepinephrine (10(-5) M) into precapillary microvessels of the rat mesentery was studied using intravital microscopy. Upon application, in 29 out of 40 cases (73%) flow ceased at the site of drug application, although in most cases the precapillary microvessels themselves did not show a diameter change due to a lack of smooth muscle cells as confirmed by transmission electron microscopy. In 17 out of the 29 cases with flow cessation (59%), an intimate contact between the venule draining the site of application and the supplying arteriole was found. Initial constriction was seen at the site where the venule crossed the arteriole. Constriction propagated both up- and downstream along the arteriole, and also across arteriolo-arteriolar arcades. Arteriolar constriction could be abolished by intentionally occluding the venule draining the norepinephrine solution. It is proposed that venuloarteriolar contacts and propagated vasomotor response may contribute to local blood flow regulation by providing a feedback loop between tissue capillaries and resistance arterioles. In three complete mesenteric microvessel networks, the arterioles (n = 34) supplying 273 out of 401 capillaries (68%) were in close proximity to venules draining these same capillaries. Each of these arterioles served, on average, 43 capillaries, showing a bimodal distribution with peaks at 4 to 16 and at 64 to 256 capillaries. On average, 62% of all capillaries drained by a given venule crossing an arteriole originated from this very arteriole, indicating a reasonably effective feedback.

Animals↗
PubMed1989

Red cell distribution at microvascular bifurcations.

The distribution of red cell and blood volume flow was studied at 65 arteriolar bifurcations in the rat mesentery. Hematocrit and flow velocity were measured simultaneously in all three vessel segments constituting a bifurcation. Blood flow distribution was manipulated by irreversibly occluding downstream side branches of one of the daughter vessels. The dependence of fractional red cell volume flow on fractional blood flow was described using a three-parameter (X0, B, A) logit function. The critical volume flow fraction below which only plasma enters a downstream branch (X0), the nonlinearity of the relation between red cell and blood volume flow (B), and the asymmetry of that relation which is described by the parameter A decrease with increasing diameter of the vessel feeding the bifurcation. At diameters above 30 microns, phase separation is very limited. In addition, the nonlinearity parameter B decreases with decreasing hematocrit in the feeding vessel. The asymmetry parameter A strongly depends on the diameter ratio between the two daughter branches: For a given fractional blood flow, the smaller branch receives more red cells than the larger branch. Using a model for plasma skimming based on the assumption of a planar separating surface, the shape of the radial hematocrit profile in the feeding vessel has been calculated. The model predicts a decrease in local hematocrit from the vessel axis toward the wall with a distinct marginal zone free from cell centers. With increasing vessel diameter the hematocrit profile becomes more blunted while the width of the marginal zone increases.

Animals↗
PubMed1989

Time-dependent rheological behaviour of blood flow at low shear in narrow horizontal tubes.

Magnitude and time-dependence of the effects of red cell aggregation and sedimentation on the rheology of human blood were studied during low shear (tau W 2.5 to 92 mPa) flow through horizontal tubes (ID 25 to 105 microns). Immediately following reduction of perfusion pressure to a low value the red cell concentration near the tube walls decreases as a result of red cell aggregation. This is associated with a transient increase of centerline velocity. Simultaneously, sedimentation begins to occur and eventually leads to the formation of a cell-free supernatant plasma layer. Time-course and extent of this sedimentation process are strongly affected by wall shear stress variation, particularly in the larger tubes. At the lower shear stresses, centerline velocity decreases (flow resistance increases) with time following the initial acceleration period, due to sedimentation of red cells. This is followed by a further increase of resistance caused by the elevation of hematocrit occurring because of the reduction of cell/plasma velocity ratio. The time dependence of blood rheological behaviour under these flow conditions is interpreted to reflect the net effect of the partially counteracting phenomena of sedimentation and red cell aggregation.

Blood Flow Velocity↗
PubMed1988

Preferential distribution of leukocytes in rat mesentery microvessel networks.

Distribution of leukocytes in rat mesenteric microvessel networks was studied using intravital fluorescence video microscopy. A digital image analysis system was used to measure vessel diameters, flow velocities and leukocyte fluxes in 306 capillaries of 8 networks. Capillaries were defined as vessel segments connecting divergent to convergent branch points. Their topological position within the network was quantified by a generation number defined as the number of bifurcations between the capillary and the arteriole feeding the network. Proximal capillaries (generation numbers 4 and 5) were slightly but significantly smaller in diameter (8.9 +/- 0.4 micron, mean +/- SEM) than distal ones (generation numbers 20 and 21, 10.1 +/- 0.4 micron). Average capillary flow velocity decreased markedly from 2.0 +/- 1.0 mm.s-1 in proximal to 0.41 +/- 0.06 mm.s-1 in distal capillaries. Average leukocyte concentration was 3.4 +/- 0.5.10(9) 1(-1) and thus significantly below systemic values (6.0.10(9) 1(-1] in proximal capillaries, and above in distal ones (11.7 +/- 2.6.10(9) 1(-1). The analysis of flow and leukocyte flux partition at 138 bifurcations showed preferential distribution of leukocytes to the daughter capillary with higher flow rate. This suggests a tentative explanation for the observed leukocyte accumulation along the microvascular tree: due to their low fractional flow, proximal capillaries draw relatively leukocyte-poor blood from the arteriole feeding the network; this leads to an increased leukocyte concentration in distal capillaries. As a consequence of the concomitant increase of capillary diameter with increasing generation number, leukocytes are preferentially flowing through larger capillaries and are excluded from small ones.

Animals↗
PubMed1988

A versatile video image analysis system for microcirculatory research.

A modular image analysis system is presented consisting of a personal computer equipped with a real time video digitizer, an interactive control unit and a graphic tablet. Together with the corresponding software modules this system can be used for a number of image processing procedures in microcirculatory research including image enhancement, measurement of morphological parameters and image brightness as well as determination of vessel diameter and blood flow velocity.

Animals↗
PubMed1987

Hematocrit fluctuations within capillary tubes and estimation of Fåhraeus effect.

Experimental and theoretical approaches were used to study hematocrit fluctuations in blood flowing along a uniform microvessel. In the experimental studies, human blood cell suspensions were passed along glass tubes with inside diameters 9.8 micron to 16.8 micron. A characteristic pattern of hematocrit fluctuation was observed in the neighborhood of white blood cells, the cell being preceded by a 'plasma gap' with reduced hematocrit and followed by a 'train' of increased hematocrit. The passage times of trains and plasma gaps and the hematocrits within the plasma gaps were determined by microphotometry. From these data, train hematocrits were deduced, expressed as equivalent discharge hematocrits. They ranged from the feed hematocrit to a value of more than 0.8 and were found to vary inversely with white cell velocity at a given flow rate. A theoretical model was developed which relates train formation to the Fåhraeus effect. The Fåhraeus effect is the reduction of tube hematocrit (HT) below discharge hematocrit (HD) which occurs in capillary tubes because the mean velocity of the red blood cells (VRBC) is higher than the mean bulk flow velocity (VB). The ratio of these velocities decreased with increasing hematocrit, and it is shown that train hematocrit is sensitive to this hematocrit-dependence. Increased hematocrit in trains behind slowly moving white cells is associated with reduced red cell velocity in the trains. From the dependence of train hematocrit on white cell velocity, the variation of Fåhraeus effect with hematocrit was deduced. The results were shown to be consistent with a model for the Fåhraeus effect in which VRBC/VB varies linearly with discharge hematocrit HD. In addition, the Fåhraeus effect was found to be approximately independent of vessel diameter over the range examined.

Biomechanical Phenomena↗
PubMed1987

A versatile intravital microscope design.

A new intravital microscope system with a versatile arrangement of optical elements and different photosensitive detection devices is described. The image formed by the microscope modified for telescopic imaging is transferred to an optical platform. Through an arrangement of beam splitters and mirrors, the image can be projected simultaneously into three of four different recording systems, including video- and photocameras and a velocity measuring system. In contrast to earlier approaches, the presented system simultaneously generates images on several devices. These images have the same orientation and field of view as that seen through the oculars. This allows the parallel application of different measuring techniques to a given area of the microcirculation.

Humans↗
PubMed1986

Topological structure of rat mesenteric microvessel networks.

Microvascular lengths, diameters, and flow directions were determined in all vessel segments (n = 1303) between bifurcations in three complete rat mesenteric microvessel networks (25 mm2 each) using intravital video- and photomicroscopy. The classification of vessel segments as arteriolar, venular, or av-segments (all segments connecting the arteriolar to the venular tree) was based on purely topological criteria. The topological structure of the networks was analyzed using the Horton-Strahler technique and a new generation scheme. Generation numbers were assigned to the vessel segments on the basis of the number of upstream (in the arteriolar tree) and downstream (in the venular tree) bifurcations. The mean generation number of the av-segments, a characteristic parameter of the generation scheme, reflects the topological structure of the network more accurately than Horton's branching ratio Rb. Both the arteriolar and venular tree of the mesenteric networks were found to be dichotomous branching structures which were neither strictly symmetric nor strictly asymmetric. The topological information obtained was compared to network models generated by different random branching algorithms. The result of this comparison suggests that the network structure changes at a certain generation level. Distal to this generation level, the mesenteric networks resemble a model network generated by random branching at any segment, while the proximal portion is similar to a model allowing random branching at terminal segments only.

Animals↗
PubMed1986

Generalization of the Fahraeus principle for microvessel networks.

Microvessel hematocrits and diameters were determined in each vessel segment between bifurcations of three complete microvascular networks in rat mesentery. Classification of the segments as arteriolar, venular, or arteriovenular (av) was based on flow direction at branch points. Photographic and videomicroscopic mapping was used to obtain quantitative information on the architecture and topology of the networks. This topological information allowed the analysis of hematocrit distribution within a series of consecutive-flow cross sections, each of which carried the total flow through the network. The observed reduction of mean hematocrit in the more peripheral cross sections is explained by the presence of a "vessel" and a "network" Fahraeus effect. The vessel Fahraeus effect results from velocity difference between red cells and blood within the individual vessel segments due to the existing velocity and cell concentration profiles. The network Fahraeus effect is based on the velocity difference of red cells and blood caused by velocity and hematocrit heterogeneity between the vessels constituting any of the complete-flow cross sections. The network Fahraeus effect is found to account for approximately 20% of the total hematocrit reduction and increases toward the most distal cross sections.

Animals↗
PubMed1985

Radial distribution of white cells during blood flow in small tubes.

The radial distribution of white blood cells (WBC) in blood flowing through glass tubes (i.d. 69 micron) was studied as a function of wall shear stress (range 0.1-2.5 Pa) and suspending medium (plasma, buffered saline, high-molecular-weight dextran solution). It was found that, irrespective of the choice of suspending medium, the highest leukocyte flux at high shear stresses was found in the tube center. WBC redistribution was seen upon lowering the shear stresses: A significant shift of WBC flux toward the marginal fluid layers occurred at the expense of the axial region. After replacement of plasma by other media the flow-dependent redistribution of WBCs was qualitatively unaffected. However, suspension of cells in dextran solution (inducing strong red cell aggregation) resulted in enhanced WBC margination, while in saline (no red cell aggregation) axial accumulation was accentuated. The results support the concept of size-dependent radial distribution of particles in flow of mixed suspensions. If applied to the living microcirculation, the data serve to explain WBC margination in microvessels (the first step in the series of events leading to emigration) in terms of a hydrodynamic phenomenon resulting from red cell/white cell interaction. The pronounced flow dependence of WBC margination results primarily from the effect of shear on red cell aggregation which leads to an alteration of the effective particle size distribution in the flowing blood.

Blood↗