Decreased hydrodynamic resistance in the two-phase flow of blood through small vertical tubes at low flow rates.

Abstract

The aggregation of red blood cells in blood flowing through small tubes at very low shear rates leads to the two-phase flow of an inner core of rouleaux surrounded by a cell-depleted peripheral layer. The formation of this layer is known to be accompanied by a decrease in hydrodynamic resistance to flow. To quantitate this effect, we measured the pressure gradient, flow rate, and the radius of the red blood cell core in suspensions flowing through tubes of 172-microns radius at mean linear flow rates (U) from 50 to 0.15 tube diameters.sec-1. Washed red blood cells were suspended in 1.5% buffered dextran 110 at hematocrits of 34-52%. Using syringe pumps, blood flowed from a stirred reservoir through a vertical 12-cm length of tube in either the upward or downward direction. The pressure drop was measured with transducers. Mean values in distributions in the core radius were obtained by analyzing cine films of flow taken through a microscope with flow in the upward direction, measuring the core radius at five equally spaced axial positions of the tube in each of 100 frames. At 34% and 46% hematocrit, the hydrodynamic resistance increased as U decreased from 50 sec-1, reaching a maximum at U-2 sec-1. It then decreased to a minimum at U less than 0.5 sec-1 as the red blood cell core formed in the tube, and the mean core radius/tube radius ratio decreased from 0.98 to 0.74 with marked axial fluctuations at the lower U. At higher hematocrits, both the increase and decrease in hydrodynamic resistance were greater. In a red blood cell albumin-saline suspension, where there is no aggregation of red blood cells and no two-phase flow, hydrodynamic resistance increases linearly with decreasing U. The experimental results were compared with the predictions of a two-phase steady-flow model, assuming axisymmetric flow of a core surrounded by cell-free suspending medium. Two models were considered, one in which the core is solid, the other in which the rheological properties of the suspension in the core are given by the Quemada equation. The effects of sedimentation of the core resulting in a zero net flow pressure gradient were taken into account. Provided that an experimentally extrapolated value for the zero pressure gradient was used, the Quemada-fluid model gave good agreement with the experimentally observed core radius as a function of U and hematocrit.

Modulation of endothelial nitric oxide synthase expression by red blood cell aggregation

“…It has been previously demonstrated that blood flow and wall shear stress in the vascular system are important determinants of NO synthesis by endothelial cells (15,39,46,53,59), with decreased blood flow shown to suppress eNOS expression and FMD responses in small arteries (55). Enhanced RBC aggregation would also tend to promote axial accumulation of RBC in blood vessels, resulting in a less-viscous, plasma-rich region near vessel walls (16). Diminished wall shear stress resulting from this nonuniform radial composition of blood should be expected to influence the NO-related mechanisms in blood vessels (16,20,33).…”

“…It is well known that the composition of blood across the diameter of a blood vessel is not uniform (20). Rather, red blood cells (RBCs) tend to migrate toward the axis, resulting in a plasmarich, cell-poor marginal layer with relatively lower viscosity (16,20). The extent of RBC axial migration is strongly affected by their tendency for aggregation, such that an enhanced tendency for RBC aggregation promotes axial migration (16) and, under the appropriate conditions, decreases flow resis-…”

“…Rather, red blood cells (RBCs) tend to migrate toward the axis, resulting in a plasmarich, cell-poor marginal layer with relatively lower viscosity (16,20). The extent of RBC axial migration is strongly affected by their tendency for aggregation, such that an enhanced tendency for RBC aggregation promotes axial migration (16) and, under the appropriate conditions, decreases flow resis-tance (6,16,47). Soutani et al (50) clearly demonstrated the existence of this cell-poor layer and the influence of RBC aggregation on its thickness in an in vivo study using rabbit mesentery.…”

Cerebral Blood Flow Modeling in Primate Cortex

“…Under normal physiological conditions, the blood cells (the majority being red blood cells) have an essential function in the flow of blood in small vessels. This subtle rheological behavior can be lumped into an apparent viscosity m a , which is a function that depends on the tube diameter as well as on the input hematocrit (Cokelet and Goldsmith, 1991). Using this apparent viscosity leads to correct prediction of the observed relationship between the imposed flux and the observed pressure drop.…”

“…The influence of red blood cells' behavior inside vessels, leading to an effective viscosity of confined blood flows, as well as their behavior at vessel branching points (the so-called phase separation effect), need to be taken into consideration (Cokelet and Goldsmith, 1991). This is an important improvement over other recent works (Reichold et al, 2009) where rectilinear, single diameter tubes with homogeneous plasma flows were used to model CBF in rat cortex.…”