The Laminar Flow of Blood

 The Laminar Flow of Blood

 The Laminar Flow of Blood

The blood flow in blood vessels is complex and fascinating. The circulatory system manages to maintain a laminar flow in vessels even when the heart pumps blood rhythmically. The pulsatile waves of blood from the heart makes the blood flow pulsatile but not turbulent.

Laminar flow is when fluid particles move in smooth, parallel layers, with no disruption or mixing between the layers (Imagine a deck of cards sliding smoothly over each other).

The major factors contributing to the laminar flow of the blood are the elastic nature of arteries, diastole of the heart, shear rate of blood and Reynolds number. When blood gushes out of the ventricles during systole, the pressure increases significantly in the arteries and they expand slightly (as they are elastic). Subsequently during the diastole when the pressure decreases, the elastic energy is restored and serves as a driving force for the blood flow. This ensures continuous flow of blood. The diastole also gives time for incoming blood to become laminar and cancel out the turbulence caused by systole. The shear rate explains blood flowing faster in the center and slower near the walls of the blood vessel. This difference in speed is directly related to the vessel diameter and explains the viscosity of the blood. This viscosity of blood resists minor turbulence in the blood flow (similar to static friction resisting movement of a body at rest).

Reynolds number is a dimensionless quantity that relates inertial forces (due to fluid motion) to viscous forces (due to fluid resistance). In simpler terms, it tells us whether the flow is dominated by momentum (inertia) or by friction (viscosity). A value less than 2300 ensures laminar flow and vice versa.

Though the blood flow cannot be comprehended accurately as blood is not a Newtonian fluid and the contents of blood keep changing from time to time, considering above-mentioned factors (especially Reynolds number which condenses every factor into one), we can conclude that the blood flow favours laminar flow in larger vessels and turbulent flow in smaller vessels.