Immediately distal to vessel stenosis, blood velocity is increased such that the Reynolds number exceeds critical. Despite this, the Reynolds number retains its use as a guiding relationship for predicting likely blood flow patterns. These factors, combined with the pulsatile nature of the cardiac cycle, lead to the approximation of laminar flow only along short distances in small abdominal vessels 2. The critical Reynolds number is highly variable depending on pipe geometry, which in the human body varies considerably due to vessel curvature, branching and viscoelastic properties 2. When turbulence reigns, a greater driving pressure is required to generate an equivalent degree of flow in the same vessel. At low Reynolds numbers (2000), flow becomes chaotic, generating vortices and eddies characteristic of a turbulent pattern 1,2. Laminar flow is characterised by a parabolic flow profile comprised of concentric fluid laminae, with each layer increasing in velocity towards the vessel centre 2. Where: ρ = fluid density v = fluid velocity D = diameter of pipe μ = dynamic viscosity of fluid. In a straight pipe of constant diameter 2:
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