(124i) Blood Flow in Cortical Microvessels

Koh, Hookam, and Leal (JFM, 1994) adapted laser-Doppler anemometry to study velocity and concentration profiles in concentrated suspensions flowing through rectangular channels.   To allow measurements inside the concentrated suspensions, indices of refraction of the particles and fluid were closely matched.

We report the results of measurements for a similar problem in physiology – the flow of blood, a highly concentrated suspension of red blood cells, in cortical vessels of live, anesthetized rodents.  We are especially interested in measuring flow when the hematocrit – the volume fraction occupied by the red cells – exceeds normal physiological values, which are typically around 42%.  Our interest stems from observations of flow deficits associated with various myeloproliferative diseases, which are characterized by an overproduction of blood cells. 

Using a two-photon excited fluorescence (2PEF) microscope as a velocimeter, we have measured red cell velocity profiles inside arterioles, capillaries, and venules, both in healthy animals and in animals with elevated hematocrit (up to 60%) due to myeloproliferative disease.  Results demonstrate that the spatial resolution of the method is sufficient to construct detailed velocity profiles, despite the high concentration of red cells that limits other measurement methods.  Furthermore, the temporal resolution of 2PEF microscopy is sufficient to determine how local velocity profiles depend on heartbeat and respiration in individual blood vessels and at vessel bifurcations.

The data show that velocity profiles are blunted at all times during a cardiac cycle.  The time-averaged blood flow speed decreases with vessel diameter in arterioles and in capillaries, and increases in venules. Furthermore, animals with elevated hematocrit show an abnormally large number of “stalled” capillaries that have little or no blood flow.  A simple empirical model of capillary networks suggests these capillary stalls start as a result of hemodynamics and rheology.  The experiments show that they can persist for long times, which can be a consequence of cell adhesion to the vessel wall.  Diminished cerebral blood flow may be a cause of cognitive decline associated with myeoloproliferative disease.