Dynamic or absolute viscosity is a measure of the resistance of a fluid or gas to flow. It is measured using a viscometer or rheometer.

The SI unit of dynamic (or absolute) viscosity is the pascal-second (Pa·s), equivalent to N·s/ m2.

Viscosity is independent of flow rate

Viscosity is independent of pressure

For a gas, viscosity decreases as its temperature increases

The relationship between viscosity and temperature and pressure only holds true for those fluids and gases that exhibit ‘Newtonian’ properties, i.e. substances that will always flow irrespective of forces acting upon them (e.g. crystalloid solutions and water).

Blood is a non-Newtonian fluid and its viscosity depends largely on haematocrit (and varies with flow rate).

Blood viscosity is increased by volatile anaesthetic agents, smoking and aging

A thixotropic substance is one that becomes less viscous over time when agitated. If it becomes more viscous with agitation, this is known as a rheopectic material.

Kinematic velocity is defined as the dynamic velocity of a substance divided by its density.