Chapter 2 Fluid dynamic and hemorheologic considerations
Abstract The previous edition of this work, published in 1985 (1), included a section on fluid dynamic and hemorheologic considerations in which the basic concepts of fluid flow, protein and cell transport, and blood rheology were presented. That material will not be reiterated in depth in this chapter but rather referred to as appropriate. Instead, the present chapter will attempt to consider the current status of research in this field and, to the extent possible, indicate those areas where our understanding is incomplete and in need of additional research efforts. Since the publication of the previous edition, our appreciation of the ability of fluid dynamic forces to influence blood-surface interactions has increased considerably. Progress has been made, not necessarily with respect to a detailed understanding of the manner in which fluid forces affect the behavior of materials in contact with blood in vivo, but rather in the growing recognition that the various blood and vascular cells exist in a dynamic environment in which flow contributes to an essential modulation of homeostatic events. In recognition of the complexity of the various biological processes that can occur in the presence of a graft implant or an organ replacement, a mechanistic rather than phenomenologic approach has slowly evolved that emphasizes the manner by which changes in fluid flow can affect the behavior of individual cells or proteins and their possible interaction with foreign materials. The approach here will be to consider the simplest case of blood flow and mass transport in a tube, as was done in the previous edition, and to delineate not only how basic characteristics of the system influence blood-material interactions, but also to distinguish where such simplifications can be misleading with respect to intravascular events or extracorporeal circuits. A description will be made of how the principles of blood flow and mass transport are being applied to such complicated events as cell proliferation and thromboembolism. In addition, the ability of shear stresses generated as a consequence of blood flow to modulate the biological activity of various cells will be discussed. Finally, several devices currently used to study the role of fluid flow in thrombotic and hemostatic processes will be described.