ABSTRACT
Before considering the mechanics of biological žuids in the circulation, it is necessary to Ÿrst consider some key deŸnitions and speciŸc properties. Once established, we will use these “pieces” to construct important laws and principles which are the foundation of žuid mechanics. To begin, we will deŸne what we mean by a Œuid. In general, a material can be characterized as a Œuid if it deforms continuously under the action of a shear stress produced by a force that acts parallel to the line of motion. In other words, a žuid is a material that cannot resist the action of a shear stress. Conversely, a žuid at rest cannot sustain a shearing stress. For our applications, we will treat a žuid as a continuum (i.e., it is a homogeneous material) even though both liquids and gases are made up of individual molecules. On a macroscopic scale, however, žuid properties such as density, viscosity, and so on are reasonably considered to be continuous.
