• Shear stress is shear force divided by the area of contact.
  • Shear rate is difference in velocity between two layers of liquid divided by the distance between the two layers.
  • Viscosity is shear stress divided by shear rate.
  • Fluids with a straight line relation between shear stress and shear rate are called Newtonian fluids.
  • Blood is a non-Newtonian fluid. Its viscosity increases when velocity increases.
  • Increased hematocrit increases oxygen delivery. But, when hematocrit increases, viscosity of blood increases leading to decreased rate of oxygen delivery. The optimum balance is at hematocrit of 45, which is the normal level.
  • When blood vessel size is less than 1 mm, blood viscosity decreases. This is called Fahraeus-Lindqvist effect.
  • When temperature decreases by 1degree C, viscosity increases by 2%.