rheological factors influencing platelet interaction with vessel surfaces

Rheological Factors Influencing Platelet Interaction with Vessel SurfacesVincent T. Turitto, Harvey J. Weiss, and Hans R. Baumgartner Citation: J. Rheol. 23, 735 (1979); doi: 10.1122/1.549542 View online: http://dx.doi.org/10.1122/1.549542 View Table of Contents: http://www.journalofrheology.org/resource/1/JORHD2/v23/i6 Published by the The Society of Rheology Related ArticlesDNA configurations and concentration in shearing flow near a glass surface in a microchannel J. Rheol. 49, 127 (2005) Human blood shear yield stress and its hematocrit dependence J. Rheol. 42, 1 (1998) A constitutive equation for the viscosity of stored red cell suspensions: Effect of hematocrit, shear rate, andsuspending phase J. Rheol. 35, 1639 (1991) Rheology of Fibrin Clots. VI. Stress Relaxation, Creep, and Differential Dynamic Modulus of Fine Clots in LargeShearing Deformations J. Rheol. 27, 135 (1983) A Technique for Studying the Shearing of Biological Cells and Hemolysis under Conditions of HighHydrodynamic Stress J. Rheol. 25, 357 (1981) Additional information on J. Rheol.Journal Homepage: http://www.journalofrheology.org/ Journal Information: http://www.journalofrheology.org/about Top downloads: http://www.journalofrheology.org/most_downloaded Information for Authors: http://www.journalofrheology.org/author_information

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Rheological Factors Influencing Platelet

Interaction with Vessel Surfaces

VINCENT T. TURITTO and HARVEY J. WEISS, Departmentof Medicine, The Roosevelt Hospital, and Columbia University,

New York, New York; and HANS R. BAUMGARTNER, PharmaResearch Department, F. Hoffmann La Roche & Co. Ltd., Basel,

Switzerland

Synopsis

An in vitro perfusion system was used to investigate platelet interaction with subendothelium from rabbit aorta exposed to blood under controlled flowconditions. Amorphological technique was used to measure platelet adhesion and thrombus formation. Classical mass transport theory modified to account for the dependence ofplatelet diffusivity on wall shear rate was used to analyze the results.

Platelet adhesion increased with wallshear rate (10-650 sec-I), red cell concentration(10-70%),and platelet concentration (50-300 nl-I) and decreased with axial distance(0-20 mm) from the leading edge. Under these flowconditions platelet adhesion ratewas determined predominantly by diffusional transport of platelets to the vessel surface. As shear rate increased to 10,000 sec:", a transition from diffusion to a morekinetic rate limiting adhesion was observed. Few thrombi wereobserved at lowvaluesof platelet concentration «150/ml), red cell concentration «25%), or wall shear rate«200 sec-1) . The formation of thrombi increased continuously with increasing wallshear rate to 10,000sec"! even in the region where values of platelet adhesion becamerelatively independent of shear rate. Thrombus formation wasenhanced by an increasein red cell or platelet concentration and was significantly greater on the upstreamportions of vessel segments.

INTRODUCTION

The application of rheological principles to the study of blood iswell established and has led to characterization of blood viscosity!and to models for describing flow and mass transport rates in the circulation and in various conduits.2-4 However, the role that fluiddynamical factors play in influencing cell reactivity and the interaction of blood cells with both vascular and artificial surfaces are lesswell understood.

© 1979 by The Society of Rheology, Inc. Published by John Wiley & Sons, Inc.Journal of Rheology, 23(6), 735-749 (1979) 0148-6055/79/0023-0735$01.00


736 TURITTO, WEISS, AND BAUMGARTNER

Under normal flow conditions, the fluid properties ofthe blood aredetermined by the red blood cells which constitute 40%of the bloodvolume. The platelets, less than 10%of the red cell concentration,normally circulate in the blood stream as discoid cells which arenonadherent to each other and to the endothelium of the vessel wall.However, disruption of the vessel wall or exposure of blood to an artificial surface leads to rapid platelet attachment, release of cellularconstituents, and growth of platelet thrombi. These platelet masses,although only a small fraction of the blood volume «1%), can significantly alter blood flow in the small vessels or flow passageways.

In this present work, we will discuss how various physical factors,such as shear rate, red cell concentration, and platelet concentrationaffect platelet adhesion and thrombus formation on vessel surfaces.Parts of this work have been presented previously in journals moreappropriate to biological considerations. This paper is a summarywhich attempts to emphasize the basic role of flowand mass transportin the understanding of cell-surface interactions.

THEORETICAL CONSIDERATIONS

The simplest approach to a treatment of blood flow is to considerthe blood as a homogeneous, Newtonian fluid. Considerable controversy has existed in the literature concerning the cell-free layer nearwalls due, presumably, to the migration of cells toward the tube axis.A peripheral plasma layer exists in tubular flowwhose width increaseswith decreasing hematocrit (% red cell volume in blood) and is relatively independent of flowoonditions.f However in tubes greater than100 j.l.m it is unlikely that the thickness of the cell-free layer exceedsthe dimensions of the red cell itself at hematocrits of 40-45%.6 Exceptat low hematocrits, any inward migration of cells is apparentlycounterbalanced by the crowding of cells and the outward displacements of cells caused by cellular collisions.f Above shear rates of 100sec-1, blood behaves as a Newtonian fluid at normal hematocrits(40-45%).1 Below 100 sec-lor at higher hematocrits, blood can besatisfactorily treated as a power-law fluid."

Our approach to analyzing platelet adhesion with surfaces has beento use the Leveque equation, modified to predict the platelet flux tothe surface in blood flowing laminarly through an annular perfusionchamber's'';

. Co) = 11K+ 1/h'wD2wI6.43x)1/3'

(1)


PLATELET INTERACTION WITH SURFACES 737

where j is the mass flux, Co the bulk platelet concentration, K theplatelet-surface reactivity, 'Yw the wall shear rate, Dw the plateletdiffusivity at the wall, and x the axial distance from the leadingedge.

Shear-Dependent Platelet Diffusivity

In flowing blood, red cell rotation and collision cause cells to undergo erratic radial displacements from streamline flow. It has beendemonstrated that such excursions increase with hematocrit and tuberadius.6,10,1l Asa first approximation, these motions can be modeledas an enhanced platelet diffusion coefficientwhich varies with a powerfunction of shear rate12,13:

(2)

where A and n are constant.Substitution into Eq. (1) results in a mass flux which is basically

dependent on platelet-surface reactivity, wall shear rate, plateletconcentration, and axial dependence.

. CoJ = 11K + I/(A'Yw2n+1/6.43x)l/3·

The limiting cases for the mass flux are (1) reaction-controlled:

11K» 1/(A'Yw 2n+1/6.43x)l/3,

which results in

j=KC o

(3)

(4)

or a mass flux independent of shear conditions and physical parameters in general-it is under these conditions that the effect ofchemical factors influencing platelet-surface reactivity should bestudied; or (2) diffusion-controlled:

11K« l/(A'Yw2n+l/6.43x)l/3,

which results in

(5)

where mass flux is maximally dependent on shear rate and physicalparameters and independent of platelet-surface reactivity. Thiscondition should be maintained when investigating the effects ofphysical factors on mass flux.



Theoretically, it is possible to shift from diffusion-controlled toreaction-controlled flux by increasing shear rate or decreasingplatelet-surface reactivity.

Shear Rate Consideration

Wall shear rates in the annular chamber were calculated from theflow equations for a Newtonian fluid.l'' The correction in wall shearrate due to the non-Newtonian velocity profile observed between 20and 100 sec-I can be shown to have a negligible effect on mass fluxin tubular flow7,12 and analogously in annular flow. The power-lawcoefficients for blood as a function of hematocrit are given by Bugliarello et a1.7 Using a slit flow approximation to annular flow,I5 it canbe estimated from the non-Newtonian velocity profile in slit flow thatthe wall shear rates will be at most 20%greater than predicted by theNewtonian calculation at an hematocrit of 70%. The deviation willbe proportionally less as hematocrit decreases.

EXPERIMENTAL METHODS

We have used an annular perfusion chamber to expose vessel segments to blood under controlled flow conditions. Everted vesselsegments completely denuded of their endothelial lining-" aremounted on the core of an annular chamber so that blood passesthrough the annular space formed by the subendothelial surface andthe outer cylinder wall. Chambers of different annular spacing wereused to investigate the full physiological range of shear rate undermoderate flow rates.9,14,17 Experimentally measured rates of dissolution of benzoic acid correlated excellently with those predictedtheoretically in these chambers.V

Blood was collected from the rabbit'? or human'? into sodium citrate anticoagulant and recirculated by a roller pump through thechamber at 37°C for a predetermined flow rate and time. After exposure to blood, the subendothelial segments were fixed, embeddedin Epon, and sectioned for evaluation.l"

Platelet interaction was evaluated around the perimeter of thevessel segment at a selected axial distance by light microscopy of thestained semi-thin sections. The morphological distinctions are described by Baumgartner et a1.20 In this present work, we are concerned with the percentage of the surface covered with (1) platelets



(adhesion) and (2) platelet masses greater than 5 J.!m in height(thrombi).

Variation of Red Cell Concentration

Platelets suspended in plasma (PRP), plasma free of platelets(PPP), and packed red cells were prepared by centrifugation of wholeblood and were recombined in such a manner that hematocrit variedfrom 10 to 70%, whereas platelet concentrations remained constant."

Variation of Platelet Concentration

Five 30-ml aliquots of whole blood were centrifuged at 1500 g for2 min 45 sec. From each tube 0,6,9,12, and 15 ml ofPRP were removed, pooled, and centrifuged at 2500g for 20 min to obtain PPP.Amounts of PPP equivalent to PRP removed were returned to eachtube and the blood mixed and left at room temperature for 30 min.Prior to exposure the blood was incubated for 8 min at 37°C.

RESULTS AND DISCUSSION

Exposure of subendothelium from everted vessel segments to bloodunder controlled flow conditions simulating arterial shear rates in aperfusion chamber produces the initial sequence of events which occurs when subendothelium is exposed in vivo to native arterialblood.16,22 As the naturally occurring substrate for platelet-vesselwall interaction, the subendothelial model is particularly relevant forstudying physical, chemical, and surface-associated factors whichinfluence platelet diffusive transport and reactivity.

As indicated by Eq. (1), the physical parameters which influencethe flux of platelets to the surface are 'Yw, Dw, Co, and x. We haveinvestigated certain of these parameters with rabbit blood, but morerecently using human subjects. The original results using rabbitblood (previously published) willbediscussed and preliminary resultswith human blood (unpublished) presented for comparison. Resultswith human blood are necessary (1) both as a baseline for studyingpatients who exhibit defects in platelet funetion'f which may arisefrom physical or chemical mechanisms and (2) because evidence ofspecies differences in platelet-surface interaction has recently beenreported on artificial surfaces.P



Shear Rate

As indicated in Eq. (1), shear rate is the flow parameter which influences platelet flux to the surface. We have investigated the effectof wall shear rates ranging from 10 to 10,000 sec"! on the attachmentof rabbit platelets to subendothelium.I'-l? As shown in Fig. 1, threeshear rate zones are apparent: (1) a low range, 1'w = 10-200 sec",where the dependence of platelet adhesion is maximum; (2) a highrange, l'w = 1300-10,000 sec" 1, within which the adhesion rate is fairlyconstant and even begins to decrease with shear rate; and (3) an intermediate zone, 1'w = 200-1300 sec", in which a transition frombeing maximally dependent to being relatively independent of shearrate occurs. Values of platelet adhesion in human blood are shownin Table I for various shear rates; rates of adhesion (as determinedby normalization with time) are indicated in Fig. 1. These values arecomparable to those obtained with rabbit blood except in the highrange of shear rate, where the human values appear less than thosefor the rabbit and are relatively independent of shear rate.

The low, intermediate, and high range of shear rate correspond tothose regions predicted by theoretical considerations, namely diffu-

102

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101 104102 103

WALL SHEAR RATE (SEC-I)

Fig. 1. Average initial rates of platelet adhesion with standard error obtained onsubendothelium exposed to rabbit (.) Dr human (0) citrated blood flowingat variouswall shear rates. Dashed line indicates hypothetical rates of adhesion for diffusioncontrolled reaction at high shear rates.


PLATELET INTERACTION WITH SURFACES

TABLE I

Shear Dependence of Platelet Interaction

741

WallShear Rate

(sec-I)

50200650800

260010,000

PlateletAdhesion

(% coverage)

8.7(1.3)b23.3(2.0)40.5(6.3)39.2(1.7)28.8(5.1)31.6(7.1)

ThrombusFormation

(% coverage)

0.0(0.0)1.0(0.2)4.9(2.6)7.6(1.3)5.8(1.9)

11.8(4.8)

a Subendothelial segments were exposed for 3 min in all experiments.b Numbers in parentheses are standard error of the mean.

sion-, intermediate, and reaction-controlled rates of mass flux, respectively. In the diffusion-controlled (low shear) range, Eq. (5)predicts the dependence of mass flux on shear rate and the slope ofthe dashed regression line in Fig. 1, m = 0.63 ± 0.04, indicates thedependence of platelet diffusivity on shear rate, n = 0.45 ± 0.06 [Eq.(2»).

The nature of the adhesion at high shear rate is not entirely clear.Although a reaction-controlled adhesion rate is consistent with theoretical considerations, the situation is complicated by the formationof platelet thrombi which continue to increase as shear rate increases(Table II). Thus, the total mass flux (platelets) on the subendothelium would appear to be increasing with shear rate, and the lack ofincrease in adhesion might be due to either physical exclusion ofplatelets from the vessel surface or their incorporation by the growingthrombi.

That a regime in which platelet-surface reactivity influences ad-

TABLE II

Axial Dependence of Platelet Interaction

Wall Shear Rate(sec:")

Blood(Human)

Exponent of Axial Dependence-Platelet Adhesion Thrombus Formation

(% coverage) (% coverage)

130013003300

citratenativenative

0.16(0.10)a0.03(0.10)0.34(0.25)

0.30(0.11)0.20(0.16)0.45(0.27)

• Numbers in parentheses are standard error of the mean.



hesion is indeed present at high shear rates is indicated by experiments in which the platelet-surface reactivity was reduced comparedto that in normal citrated blood. By using blood either (1) depletedof von Willebrand's factor 24•25 (a protein in blood important fornormal platelet adhesion), (2) anticoagulated with increased levelsof sodium citrate." or (3) treated with prostacyclin, a potent plateletinhibitor.s" adhesion rates were reduced compared to normals onlyin the high shear rate regime; at low shear rates adhesion values werecomparable to normals. The relationship between platelet adhesionand thrombus formation is complex; a study of the influence ofthrombus formation on adhesion is currently being prepared.P

It should be noted that the low, intermediate, and high range of wallshear rate correspond directly to flowin veins, larger arteries, and themicrocirculation, respectively.P In fact, wall shear rates appear tobe maximal in arterioles of 20--S0-p.m diameter-? and may reach valuesas high as 16,000 sec"! under normal circulatory flow.3o It is in themicrocirculation that the events of platelet thrombosis and hemostasisare most important and the present work suggests that these eventspotentially can be influenced by changes in platelet-surface reactivity.The work also suggests that measurement of changes produced byvarious inhibitors should be performed in the high shear rate regime;testing of platelet function in the diffusion-controlled region at lowshear rates may lead to erroneous conclusions concerning plateletreactivity.

Red Cell Concentration

The effects of hematocrit on platelet adhesion and thrombus formation in human blood are shown in Fig. 2. At a wall shear rate of800 sec", platelet adhesion increases as hematocrit is increased from10 to 70%. A parallel increase in surface covered by platelet thrombiis observed for two subjects; in the others, few thrombi were foundat all hematocrits. A similar increase in adhesion and thrombusformation occurred on a-chymotrypsin-digested subendotheliumexposed simultaneously with the subendothelial segments above. Onthe digested surfaces, thrombi are more readily formed than onsubendothelium under these flow conditions2o,22 and all four subjectsshowed significant thrombus formation (average values were 0.2,15.7,and 24%at hematocrits of 10, 40, and 70%, respectively).

As indicated from the shear rate results earlier (Fig. 1), conducted



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~z~4Owxc< 20I-W....W

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20 30 40 50 60

HEMATOCRIT ('l'oRED CELL VOLUME)

Fig. 2. Values of platelet adhesion and thrombus formation on subendotheliumexposed to blood of varying hematocrit and constant platelet concentration flowingat 160 ml/rnin for 10 min (wall shear rate of 800 sec'). Individual runs conductedwith blood from the same donor are connected.

under normal hematocrits (40%), adhesion may be influenced by bothdiffusional and kinetic transport mechanisms at 800 sec". Thus,the decrease in adhesion at 10%hematocrit and the increase at 70%hematocrit may be due to acornbination of physical and kinetic factors. We are in the process of submitting a more complete study ofthe influence of shear and hematocrit on platelet interaction withsubendothelium.P Briefly, at a wall shear rate of 200 sec" (diffusion-controlled region), red cells cause an increased adhesion up tohematocrits of 40%. Adhesion becomes independent of hematocritabove 40%. It is interesting to note that Goldsmith and Karino" haveobserved microscopically that the radial displacements of red celltracers are maximal in flowingsuspensions containing 30-45% red cellghosts (cellsmade transparent by removal of hemoglobin). At higherwall shear rates (2600 sec-I) under which adhesion is independentof shear at normal hematocrit (Fig. 1), adhesion continues to increaseas hematocrit varies from 10 to 70%. The increase in adhesion athematocrits greater than 40%is due to the direct effect ofred cells onthe adhesion mechanism that is operating under these flowconditions.This mechanism appears to be associated with kinetic factors (seeprevious discussion), thus suggesting that red cells have a chemical


744 TURITrO, WEISS, AND BAUMGARTNER

influence on platelet adhesion. While early investigations31,32 of theinfluence of red cells on platelet reactivity concluded that the red cellrole was kinetic, due perhaps to release of adenosine diphosphate (apotent platelet-aggregating agent secreted by platelets) from red cells,recent work33- 35 has indicated that much of the effect ascribed tokinetic mechanisms was due instead to increased platelet transportphysically induced by red cell motions. Current studies suggest thatunder high shear conditions, the kinetic influence of red cells maybecome dominant for hematocrits greater than 40%. At lower hematocrits the decrease in adhesion is probably due to both reducedtransport and kinetic mechanisms.

These results may be applicable in certain disease states characterized by abnormal red cell concentrations. For example thebleeding observed in certain anemias is corrected by red cell infusion36; also the number of thrombotic episodes which occur in polycythemia have been directly related to red cell concentration.s?

Platelet Concentration

Asshown in Fig. 3, there is a strong dependence of platelet adhesionon platelet concentration in whole blood. Under these flowconditions

...."'"a::....>oo....o~a::::>III

~20 iii

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is 40III

'"x!i 20....'"...J'" 0~...JlL

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200 300

PLATELET NUMBER (nL'1l

Fig. 3. Values of platelet adhesion and thrombus formation on subendotheliumexposed to blood of varying platelet concentration and constant hematocrit flowingat 160 ml/min for 10 min (wall shear rate of 800 sec I ) . Individual runs conductedwith blood from the same donor are connected.



(wall shear rate of 800 sec-I), the dependence decreases as plateletconcentration exceeds 200/nl, but this effect is due primarily to saturation of the surface with adherent platelets. Below a platelet countof 150/nl (surface coverages of less than 70%),the data can be fit bya regression line passing through the origin with a slope, b = 0.52 ±0.13 (S.E.). For either diffusion-controlled or reaction-controlledadhesion, this slope would be directly related to the rate of diffusivetransport [Eq. (5)] or the reaction rate coefficient [Eq. (4)]; howeverfor these shear conditions h'w =800 sec-I), adhesion appears to beintermediately controlled (Fig. 1) and the general equation [Eq. (3)]is applicable in which the slope is determined by both platelet-vesselreactivity and diffusive transport.

Adhesion results with rabbit blood exposed under identical flowconditions showed a similar behavior-s; with rabbit blood, thrombidid not form below platelet concentration of 200/nl and increasedexponentially with concentration. In human blood at these shearconditions the formation of thrombi is more variable (previously notedin Fig. 2) and five of six subjects failed to show appreciable amountsof thrombi on subendothelium at all platelet concentrations. We arecurrently investigating the dependence of thrombus formation onplatelet concentration at higher shear rates where formation is moreextensive (Table n.

The normal range for platelet count in human blood is 150-4oo/n1.At these flow conditions a relatively saturated subendothelial surfaceis attained within 10 min. Clinically, platelet counts as low as 100/nlare not considered consequential and bleeding due to lack of plateletsdoes not normally occur until the count is below 50/n1. At suchconcentrations the perfusion results indicate that virtually no thrombiform and adhesion is 20-30% of that normally observed within 10min.

Axial Dependence

We have previously investigated the effect of axial position onplatelet adhesion in rabbit blood." At a wall shear rate of 800 sec-t,adhesion to subendothelium decreased as distance from the leadingedge increased. Nonlinear regression of the form, adhesion = (l/x )a,gave a value of a = 0.17 ± 0.05 (S.E.). This value of a lies betweenthat predicted by theory for a diffusion-controlled rate [0.33,Eq. (5)]and reaction-controlled rate [0.0,Eq. (4)] and is consistent with the



intermediate rate-controlling mechanism indicated at these shearconditions by the flow studies (Fig. 1).

In recent experiments, subendothelium was exposed at higher wallshear rates (1300and 3300sec:") to both native (nonanticoagulated)and citrated human blood--; platelet adhesion and thrombus formation were evaluated at five axial positions, as indicated in Fig. 4.In citra ted human blood, adhesion decreased in a manner similar tothat observed with rabbit blood (a = 0.16 ± 0.10). The exponentsindicating the dependence of adhesion and thrombus formation onaxial position in citrated and native blood are shown in Table II. Atall conditions, adhesion and thrombus formation decrease with increasing axial position.

In native blood the dependence of adhesion on position is very weakat 1300 sec-\ but at 3300 sec-1 increases to the maximum valuepredicted by theory. Theoretical considerations predict a decreasein the exponent, a, as shear rate is increased and as platelet-surfacereactivity becomes more dominant. The apparent increase in a with

70

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>-'j.... 10~oJ0.

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AXI AL POSITION (Mil FROM LEADING EDGE)

Fig. 4, Values of platelet adhesion and thrombus formation on subendothelialsegments (20 mm in length) exposed to human citrated blood flowing at a wall shearrate of 1300SeC-I. Evaluations were performed at distances approximately 4,7,10,13,and 16 mm from the leading edgeof the vesselsegment. Individual runs conductedwith blood from the same donor are connected.



shear rate is not understood. Shear forces are known to affect theability of platelets to release chemical moieties which influence subsequent platelet reactivity.39-41 The formation of an axially dependent concentration boundary layer of released materials at the highestshear rate studied might produce the enhanced axial dependence ofadhesion.

Axial position influences thrombi more than adhesion; values ofa for thrombi are increased over those for adhesion and occasionallyare greater in magnitude than predicted by theory. Values of a forthrombi also increase with increasing shear rate in native blood. Aphysical explanation for the stronger dependence of thrombi on position may be the accelerated growth of thrombi upstream due to increased interception of platelets. As thrombi form, newly arrivingplatelets encounter upstream thrombi first because of location; inaddition, these thrombi probably protrude further into the bloodstream since platelet flux is inherently greater upstream. Interception of platelets by upstream thrombi enhances their rate of growthand thus results in greater axial dependence of thrombi (and perhaps-adhesion to a lesser degree, since the growth of thrombi breadthwiseleads to increased platelet-surface attachments).

It is clear that the growth of platelet thrombi in flowing blood iscomplex. Adhesion is a prior necessity for this growth; however,thrombus mass may also influence subsequent adhesion by alteringflow streamlines and perhaps the reactivity of platelets which passnear or are released from thrombi.14,2o.26 We have attempted tomeasure platelet adhesion and thrombus formation on subendothelium as a function of various physical factors and to analyze theirbehavior using analytical expressions derived from classical masstransfer theory. Strictly, such equations are applicable only to theinitial stages of platelet attachment prior to the development ofthrombi, such as encountered presently. However, the analyticalexpressions have proved valuable in defining the variables which influence platelet adhesion even in the presence of extensive thrombusformation and have led to the following observations concerningplatelet-subendothelial interaction in flowing blood:

1. An increase in wall shear rate leads to increased platelet flux(adhesion and/or thrombus formation); further, as shear rate increases, three kinetic zones can be distinguished in which diffusion-,intermediate, and reaction-controlled rates of attachment predominate.



2. Red cells increase the diffusive transport of platelets severalorders of magnitude and may in addition affect platelet-surface reactivity. The physical versus kinetic role for red cells is dependenton the shear conditions.

3. Platelet concentration strongly influences platelet interaction;it is not known if shear conditions influence this dependence.

4. Axial position affects platelet thrombus formation more thanplatelet adhesion, but the effect is weak over the limited changes inaxial position attainable in a perfusion system.

This work was done in part during the tenure of an Established Investigatorship ofthe American Heart Association and was supported in part by Grant HL 15495 fromthe U.S. Public Health Service and a Grant-in-Aid from the American Heart Association with funds contributed in part by the New York Heart Association. The authorsgratefully acknowledge the skillful technical assistance of Mr. Thomas Hoffmann andMs. Sachi Senrui.

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Wiley, New York, 1960.16. H. R. Baumgartner, Microvasc. Res., 5,167 (1973).17. V, T. Turitto and H. R. Baumgartner, Thromb. Diath. Haem., Suppl. 60, 17-24

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Received April 24, 1979Accepted as revised July 6, 1979


rheological factors influencing platelet interaction with vessel surfaces

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