Unlike water, blood is non-Newtonian because its viscosity increases at low flow velocities (e.g., during circulatory shock). Low flow states permit increased molecular interactions to occur between red cells and between plasma proteins and red cells. Therefore, flow is reduced at a given driving pressure when viscosity is elevated. Whole blood has a much higher viscosity than water and therefore the slope of the flow-pressure relationship is less steep (see figure). There is an inverse relationship between flow and viscosity therefore, the greater the viscosity, the smaller the slope of the flow-pressure relationship. 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).
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