Reynolds Number

Reynolds Number

Reynolds number Re = (V x d x rho) / eta
V = velocity of blood stream; d = diameter of the vessel; rho = density of blood; eta = viscosity of blood
When the number is above 2000, flow becomes turbulent. When viscosity (eta) increases, the number decreases. It may be noted that it is a dimensionless number. It is a ratio of local and convective forces to viscous forces. Viscous forces are secondary to viscosity of the blood and shear forces developed along the vessel wall [1].

What is the significance of this number in the cardiovascular system? As mentioned above, when the number is above 2000, flow becomes turbulent. Otherwise the chance of having a murmur or bruit increases. Now we know the parameters which will determine a murmur. Velocity of blood stream, diameter of the vessel and density of the blood increases the number while viscosity of blood has an inverse relationship. That is why anemia amplifies most murmurs while polycythemia decreases the murmurs.

It has been shown that production of turbulent flow necessary for the production cardiac murmur is related to the number, ejection flow rates and cardiovascular geometry [1].

Reference

  1. J P Murgo. Systolic ejection murmurs in the era of modern cardiology: What do we really know? J Am Coll Cardiol. 1998 Nov 15;32(6):1596-602.