This introductory chapter for Section 4 will overview the anatomical, histological and functional characteristics of the vascular system and will briefly review the physical principles governing blood flow in various sections of the circulatory system.

The first part of this chapter will review the anatomical and histological characteristics of blood vessels, including arteries, arterioles, capillaries, postcapillary venules and veins. Particular emphasis will be on exploring the strikingly close correlation between their wall structure and physiological function of transporting and distributing oxygen and other essential substances to the tissues and also removing by-products of metabolism.

The second part of the chapter will review the progressive changes of luminal diameter and vascular cross-sectional area along the circulatory system. The consequent changes in intravascular pressure, vascular resistance, wall shear stress and blood flow dynamics will be discussed, also highlighting the critical importance of the macro- and microrheological properties of blood. In addition, the chapter will introduce some basic physical principles governing nutritional exchange between tissues and the vascular channels (i.e., Starling law).

The concluding part of the chapter, as a transition to chapter 4.2, will highlight the critical importance of vascular control mechanisms responsible for altering blood flow distribution necessary to meet the ever changing tissue requirements in response to physiologic and pathologic conditions.