The role of stagnation point flow in the formation of platelet thrombi on glass surfaces in tubes with various geometry.

Glass models of small rectangularly branched and 360° curved arteries have been perfused with diluted bovine platelet rich plasma activated with ADP. Whereas straight tube flow did not lead to platelet deposits, branched tube flow caused thrombi at two sites. First site was the region of the distal rim of the orifice of the side branch, second site was the wall of the main tube opposite and downstream the side branch origin. Curved tube flow produced deposits on the wall with the smaller radius and on the wall with the greater radius of curvature. The formation of deposits at the specific sites required definite flow conditions. Thrombi at sites corresponding to the predilection sites of atherosclerosis in small arteries, i.e. the wall opposite the side branch and the wall with the smaller radius of curvature, occured where flow separated from the wall, reattaches forming a stagnation point flow. The influence of stagnation point flows on thrombus formation on glass walls was studied quantitatively for a rotationally symmetric stagnation point flow. The experiments revealed that, activated bovine platelets adhere only where they are transported by flows to the wall exhibiting velocity components directed perpendicularly towards the wall; polycellular stripe like thrombi form only when the velocity components of platelets parallel to the wall overcomes a critical value; the mass of deposited polycellular thrombi increases with the velocity component parallel to the wall. For all deposits a critical parallel velocity component exists which dislodges thrombi

Hemodynamically induced blood platelet deposits in branched, curved, and constricted glass tubes.

Branched, curved, and constricted glass tubes were perfused with bovine platelets/platelet rich plasma (50000 platelets/μl activated by 2.5 x 10-6 M ADP; at 37°C) for 3 to 12 min. Straight tube flow does not lead to adhesions on the tube wall. As a rule, deposits start with individual adherent platelets and are observed at following sites: on both sides of and distally from the orifice of the side branch (main tube ID (inside diameter) 3 mm, side branch ID 1.5 mm; rectangular); on the outer wall of a 180° curved tube (ID 3 mm, curve radius 6.5 mm); and on the beginning of the constriction (ID 4 mm constricted to 2.1 mm over 4 mm). The deposits form streaks following the streamlines. A minimum flow rate and ADP concentration is necessary to produce deposits; these values are higher in curved and constricted tubes than in a branched tube. High flow rates prevent mural adhesion. Addition of washed red blood cells up to a concentration of 10 fold the platelet concentration does not alter the platelet deposits. It was shown earlier that red blood cells deposit in contrary in the stagnation water opposite the orifice of the side branch if a rectangular branched tube is perfused with a suspension of sticky red blood cells. At this site platelet aggregations rarely and not yet reproducible deposit on the wall