Cell Distribution and Segregation Phenomena During Blood Flow

Blood is the archetype of a biological complex fluid. It is complex in the microstructural and mechanical sense, as a multiphase non-Newtonian viscoelastic fluid, and also in the biological sense, as a tissue that has a wide range of functions from delivery of oxygen and nutrients to response to injury and inflammation. These forms of complexity are interconnected, as the physical nature of blood as a multiphase fluid is intimately related to its biological functions. In the present chapter, we summarize basic features of the structure and biology of blood as well as observations of its dynamics during flow in the body. Emphasis will be put on flow at small scales, where the particulate nature of blood as a suspension of many different types of cells becomes important both physically and functionally. The first part of the chapter describes the nature and biological functions of the various components of blood, as well as the distribution of these components in blood vessels. In particular, it has long been observed that the various cellular components of blood are distributed very nonuniformly, a phenomenon that is physiologically important as well as fascinating from the fluid-dynamical point of flow. The second part of the chapter focuses on computational and theoretical approaches for predicting and understanding the distribution and segregation of blood cells in flow. Various numerical methods are described, with a focus on one of the most widely used for multiphase small-scale flows, the boundary integral method. Computational results for model suspensions are presented that allow careful study of the basic mechanisms underlying segregation phenomena and a model framework is introduced that incorporates these mechanisms in an idealized way. This framework is a stepping stone toward a unified understanding of these segregation phenomena that will hopefully be useful in aiding the development of therapies to modify and exploit blood flow phenomena for disorders as varied as cancer, sickle cell disease, and hemorrhage.