1.3.7 Blood Viscosity Definition

Blood Viscosity Definition is the precise characterization of blood viscosity as the measure of blood's internal resistance to flow, arising from friction between its fluid components and determined primarily by the concentration of red blood cells, plasma protein content, and the rate of flow itself. This definition establishes blood viscosity as a physical property that directly influences the resistance encountered as blood moves through the vasculature, distinguishing it from pressure and flow, which are separate but related variables.

Elements of the Definition

Resistance to Deformation and Flow

Blood viscosity is defined as the internal resistance a fluid presents to deformation and flow, reflecting the frictional interactions between its cellular and plasma components as they move relative to one another.

Dependence on Cellular Concentration

Central to the definition is the strong dependence of blood viscosity on hematocrit, since a greater concentration of red blood cells increases internal friction and thus raises overall viscosity.

Non-Newtonian Behavior

The definition specifies that blood does not behave as a simple, uniform fluid but instead exhibits non-Newtonian properties, meaning its viscosity changes with the rate and conditions of flow rather than remaining constant.

Distinguishing Features

Contrast with Plasma Viscosity

Blood viscosity is distinguished from plasma viscosity, the latter referring only to the resistance of the fluid component alone, without accounting for the additional frictional contribution of suspended cellular elements.

Dependence on Vessel Diameter

A defining feature of blood viscosity is its variation with vessel diameter, decreasing in very small vessels due to the alignment of red blood cells along the central axis of flow, a phenomenon relevant to microcirculatory dynamics.

Sensitivity to Flow Velocity

The definition accounts for the fact that blood viscosity decreases at higher flow velocities, as increased shear forces cause red blood cells to deform and align, reducing internal resistance compared to conditions of slow or stagnant flow.

Purpose of the Definition

Establishing a Physical Property Relevant to Circulation

A precise definition of blood viscosity establishes it as a specific physical property distinct from pressure and flow, providing a variable that can be measured and related to overall circulatory resistance.

Foundation for Understanding Resistance to Flow

The definition provides the basis for understanding how blood's physical composition contributes to the resistance encountered throughout the vascular network, a factor in the broader relationship between pressure, flow, and resistance.

Clarifying Viscosity's Role Among Circulatory Variables

By specifying that viscosity is a property of the blood itself rather than of the vessels or the heart, the definition delineates its distinct contribution to circulatory resistance alongside vessel radius and length.