This chapter is concerned with the mechanical properties of the blood and its constituents. We shall examine in Chapters 12 to 15 the flow of blood in blood vessels and its contact with their walls. The mechanics of fluids, discussed in Chapters 4 and 5, provide a background to the material that follows.

Blood is a suspension of the formed elements (the various blood cells) and some liquid particles (the chylomicrons) in the plasma. Plasma itself is an aqueous solution containing numerous low molecular weight organic and inorganic materials in low concentration, and about 7% by weight of protein (Table 10.1). The mechanical property of blood which is of principal interest to us is its viscosity. In order to understand what determines the viscosity of whole blood we must first consider what governs the viscosity of simple fluids and suspensions, then the mechanical properties of the plasma (p. 155) and the suspended elements (p. 157), and finally whole blood (p. 169).

Viscosity of fluids and suspensions

It was noted in Chapter 1 that the physical features of liquids, gases and solids are directly related to their molecular structure and that both liquids and gases are classed as fluids, because they flow when a shear stress is applied. The property which relates the rate of shearing to the shear stress is the viscosity (p. 37) and we must now consider the factors that determine the viscosity of a fluid.

It helps in understanding the physics of a liquid if at the same time we consider a gas. Gases are much less dense than liquids; therefore, the molecules of a gas are farther apart than those of a liquid.