Define: i) Shear stress; ii) Reynolds number

• == poise

shear stress: force it takes for one lamina or layer of fluid to slip over another [force applied to a unit area - dynes/cm² - ie units of pressure]

shear rate: measure of how fast one lamina will slip over another

• Water viscosity at room temperature:0.01 poise [1 centipoise (cp)]

Blood viscosity 3-4 poise

• Blood is much more viscous than water because it contains formed elements & plasma proteins, hence it flows more slowly under the same conditions

• Shear stress: internal friction (eg: denatured proteins will increase internal friction of blood due to their new rough shape proteins, their aggregates & their altered electrochemical characteristics ‘thickens’ the blood)

• Diluted blood is thinner & less viscous, thereby reducing injurious effects of the high shear stresses

Reynolds number:

• Nondimensional number used to predict the transition from laminar to turbulent flow

• Turbulent flow generates spontaneous eddies from flow instabilities within the flow (such a flow would have a high Reynolds number)

• Defined as the ratio of inertial to viscous forces

• Turbulence occurs when disrupting forces (inertial) overcome the retaining forces (viscous); this relationship is expressed as Reynolds number: [= (density x velocity x diameter) / viscosity

• Empirically, turbulence may occur in blood when Reynolds > 1000, although its occurrence is also influenced by curvature, smoothness etc

Maximum flows to minimise turbulence
	Max flow (l/min)
Tubing size inch (ID)	To keep Reynolds No. < 1000	To keep Reynolds No. < 2000
1/4	1.1	2.2
3/8	1.8	3.3
1/2	2	4