Micro- and Macrocirculation

Abstract

In physiology, capillary blood flow is identified with microcirculation. Flow in small blood vessels supplying and draining the capillaries, the arterioles and venules, respectively, are also included in microcirculation, but the question of how many orders are to be included in microcirculation is sometimes debated, because different organs seem to demand different answers. From fluid mechanical point of view, the distinction between micro and macro circulation can be based on the Reynolds number, VD/v, and Womersley number, (D/2)ω/ν−−−√ (Sec. 5.16), where V represents the mean velocity of flow in the vessel, D is the vessel diameter, v is the kinematic viscosity of the blood, co is the circular frequency of oscillation of the blood velocity fluctuations. If the Reynolds number and Womersley numbers are both much smaller than 1, then the inertial force can be ignored, and the flow is said to be microcirculation. If both numbers are much greater than 1, then the fluid viscosity can be ignored, and the flow is said to be macrocirculation. In between these limits the fluid mechanical equations are harder to solve, and it is immaterial whether you classify them as micro or macro circulation.