Primary hemostasis and blood clotting is known to be influenced by the red blood cell volume fraction (hematocrit) in blood. Depressed or elevated levels of red blood cells can lead to vascular perfusion problems ranging from bleeding to thrombus formation. The early stage of hemostasis and thus blood clotting in all vessel sections from the arterial to the venous system involves the adhesion of platelets to von Willebrand factor. Here we present experimental and theoretical results showing that the adhesion probability of platelets to von Willebrand factor is strongly and nonlinearly dependent on hematocrit and flow rate. Interestingly, the actual binding forces are not markedly different, which suggest that the origin of such behavior is in the distribution of platelets. Using hydrodynamic simulations of a simple model, we explicitly show that the higher the hematocrit and the flow rate, the larger the amount of platelets residing close to the wall. Our simulation results, which are in excellent agreement with the experimental observations, explain why such phenomena occur. We believe that the nonhomogeneous red blood cell distribution as well as the shear dependent hydrodynamic interaction is key for the accumulation of platelets on the vessel wall. The work we present here is an important step forward from our earlier work on single molecules and extends into the collective cellular behavior of whole blood. It sheds new light on the correlation between hematocrit and the initial steps in hemostasis and thrombosis, and outlines advances for the treatment of vascular diseases associated with high levels of red blood cells. These results are not only highly relevant for the field of hemostasis and the physics of blood clotting but are also of powerful impact in applied science most obviously in drug delivery and colloidal science.

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2104 Thrombus growth rate has been reported to be a linear function of very high shear rates beyond 40,000 s-1 (Ku DN, Bioreology. 2007;44(4):273-84), which are thought to occur in severe arterial stenosis. We therefore applied shear rates continuously increasing from 2,000 s-1 to 40,000 s-1 and examined platelet and von Willebrand factor (VWF) interaction in a parallel plate flow chamber under direct visualization. Above a critical threshold of 10,000 to 15,000 s-1 aggregates of inactivated platelets formed when VWF was present in solution and immobilized on the perfused surface (Ruggeri, … Reininger; Blood, 108: 1903–1910, 2006). A new finding was the shear dependent simultaneous formation of VWF strands in and around those platelet aggregates, growing to strands up to several microns thick and up to several tens of microns long, thus forming large VWF networks. Platelets appeared to be enmeshed in the VWF networks but were nevertheless a prerequsite for their formation, mutually fostering each other. Platelet-VWF conglomerates formed at the perfused VWF surface, were constantly rolling in the flow direction, grew in size with increasing shear rate, and were completely reversible below the critical shear rate threshold of 10,000 s-1. VWF networks assembled in whole blood as well as in washed blood cell suspensions reconstituted with VWF. Recombinant full length VWF generated larger VWF networks than a commercial VWF preparation from pooled human plasma. Perfusion of collagen coated surfaces caused assembly of VWF networks anchored to the collagen fibrils and capture of activation-blocked platelets within them at shear rates of 2,000 s-1, leading to residence times of platelet-VWF conglomerates of more than a minute. Newly formed platelet conglomerates could be visualized rolling on the surfaces of already attached conglomerates. When we used a gain of function mutation of VWF, rolling platelet aggregates could be detected at a shear rate as low as 1,500 s-1. We conclude that shear rate activation of VWF leads to reversible platelet-VWF network generation, which may be a crucial mechanism of fast platelet accrual and prolonged arrest times at sites of thrombosis as well as physiological hemostasis. Disclosures: No relevant conflicts of interest to declare.