If viscosity of a fluid is constant, it is considered "Newtonian". What makes blood exhibit non-Newtonian behavior?

Blood exhibits non-Newtonian behavior primarily due to the complex interactions between its components, namely the cells, plasma, and other particles. The viscosity of blood varies depending on shear rate, which is influenced by the concentration of red blood cells, their interactions, and the overall composition of the blood.

The effective red cell volume impacts viscosity because as the shear rate changes, red blood cells can deform and align in the direction of flow, which reduces resistance and alters viscosity. This behavior indicates that blood does not maintain a constant viscosity under varying flow conditions, which is a hallmark of non-Newtonian fluids.

Additionally, the presence of solid particles, such as platelets and leukocytes, along with red blood cells, further complicates blood's rheological properties. These particles can influence the fluid's viscosity and flow characteristics by interacting with each other and with the plasma.

Red blood cells being deformable also contributes significantly to blood's non-Newtonian nature. Their ability to change shape enables them to pass through narrow capillaries more easily and adapt to varying shear forces, which alters blood viscosity dynamically.

In summary, blood is recognized as a non-Newtonian fluid due to its complex composition and the influence of red blood cells and other particles on its viscosity