In fact, increasing the hematocrit from 40 to 60% (a 50% increase) increases the relative viscosity from 4 to 8 (a 100% increase).В Increased viscosity increases the resistance to blood flow and thereby increases the work of the heart and impairs organ perfusion.

Patients with an abnormal elevation in red cell hematocrit (polycythemia)В have much higher blood viscosities.

At a normal hematocrit of 40%, the relative viscosity of blood is about 4. Therefore, blood viscosity strongly depends on hematocrit.

Note that the increase is non-linear В increased hematocrit causes a disproportionate increase inВ relative viscosity. IncreasingВ red cell hematocritВ increasesВ relative viscosity. In the figure,В the relative viscosity at 0% hematocrit (plasma without cells) is about 1.8 as shown by the y-intercept. Of these formed elements, red cells have the greatest effect on viscosity. The addition of formed elements to plasma (red cells, white cells, and platelets) further increases the viscosity. In fact, plasma at 37вЊ is about 1.8-times more viscous than water at the same temperature therefore, the relative viscosityВ (О· r) of plasma compared to waterВ is about 1.8. Because of molecular interactions between these different components of plasma, it is not surprising that plasma has a higher viscosity than water. This is referred to as the yield stress required to initiateВ flow.ДЄlthough plasma is mostly water, it also contains other molecules such as electrolytes, proteins (especially albumin and fibrinogen), and other macromolecules. Because of the high degree of interaction between the elements of blood when it is not flowing, a driving pressure significantly greater than zero is required for stationary blood to start flowing again. This can cause red cells to stick together and form chains of several cells ( rouleau formation) within the microcirculation, which increases the blood viscosity. Unlike water, blood is non-Newtonian because its viscosity increases at low flow velocities (e.g., during circulatory shock).В Low flow statesВ permit increased molecular interactions to occur between red cells and between plasma proteins and red cells. There is an inverse relationship between flow and viscosity therefore, the greater the viscosity the smaller the slope of the flow-pressure relationship, meaning that at a given driving pressure flow will be reduced at higher viscosities.В Whole blood has a much higher viscosity than water and therefore the slope of the flow-pressure relationship is less steep (see figure). This is shown in the figure as a linear dashed line for the flow-pressure relationship of water. Water behaves as a Newtonian fluid and therefore under non-turbulent conditions, its viscosity is independent of flow velocity (i.e., viscosity does not change with changes in velocity). For example, water is a homogeneous fluid and its viscosity is determined by molecular interactions between water molecules. The interactions between fluid layers depend on the chemical nature of the fluid, and whether it is homogeneous or heterogeneous in composition. This internal friction contributes to the resistance to flow as described by Poiseuille's equation. Viscosity is an intrinsic property of fluid related to the internal friction of adjacent fluid layers sliding past one another (see laminar flow).

Viscosity of water