Résumé
This chapter presents how questions about the microcirculation of blood and blood cells are addressed by physicists. It introduces the fundamental concepts and quantities at the basis of the physical approach. The chapter aims to develop a more formal approach that will give technical tools to the most expert and to those who want to get to the heart of the physics and mechanics. It describes the components of blood, the geometry of flow in the microvascular system and defines the notion of suspension and its specificity. White blood cells (WBCs) make up less than 1% of the cells in blood. The two types of WBCs found in blood flow are neutrophils and monocytes. The volume fraction in whole blood of Red blood cells (RBCs) ranges between 40% and 50%. RBCs are filled with a solution of hemoglobin, the key protein used in oxygen transport. The chapter also presents an overview of the key concepts discussed in this book.