· Definitions
· Relationships between viscosity/density and flow rate
· Clinical applications
Viscosity 
· Internal friction of a fluid · Ratio shear stress (dyn/s) to shear rate (cm/s/cm) hence units dyn.s.cm^{2} · Shear stress: force per unit area · Shear rate: ratio of rate of motion of imaginary plate on surface of liquid, relative to the depth of the liquid 
Density 
· Ratio mass:volume · Units kgm^{3} or g/cm^{3 } 
Laminar flow 
· Smooth, organized flow of fluid in concentric rings · Likely in long, narrow tubes · Parabolic flow profile: fast moving in the middle, in theory stationary at the walls · Resistance = (8 x length x viscosity) / (π x radius^{4}) (Poiseuille equation) · Pressure gradient ∝ velocity (i.e. linear) · Wall properties matter less due to peripheral stasis. 
Turbulent flow 
· Chaotic flow · Likely at branching points, bends etc. · Squareish flow profile (necessary to expunging the periphery) · Pressure gradient ∝ (density x length)/radius^{5} · Pressure gradient ∝ velocity^{2} (i.e. exponential) · Wall properties (e.g. smoothness) matter more due to rapid peripheral flow. 
Transitional flow 
Laminar + turbulent 
Reynolds number 
· Dimensionless index · Ratio of inertial forces to viscous forces · Re = (2 x radius x density x velocity) / viscosity · < 2000 likely laminar, >2000 likely turbulent · ↑Reynolds number > ↑likelihood of turbulence if: o Wider tube o ↑ flow rate o ↑density o ↓viscosity 
Entrance length 
Length of tube before flow can become laminar =0.03 x Re x diameter 

Laminar flow 
Turbulent flow 
↑Viscosity 
↑Resistance 
↑Likelihood laminar 
↑Density 
↑Likelihood turbulent 
↑Resistance 

Laminar 
Turbulent 
Airflow: 
In small airways (from 11^{th} generation) · ↓Velocity · ↓Diameter · Infrequent branching
>If inspiratory flow rate and RR are low, parabolic flow profile augments alveolar ventilation 
In large airways · ↑Velocity · ↑Diameter · Frequent branching
>Heliox (low density gas) is used in upper airway obstruction 
Blood flow 
Small veins: · ↓Velocity · ↓Diameter · Entrance length reached
>Risk of thrombosis 
Large arteries: · ↑Velocity · ↑Diameter · Branching before entrance length
>Important for coronary artery perfusion 
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