Steady flow through a double converging-diverging tube model for mild coronary stenoses.

Velocity profiles and the pressure drop across two mild (62 percent) coronary stenoses in series have been investigated numerically and experimentally in a perspex-tube model. The mean flow rate was varied to correspond to a Reynolds number range of 50-400. The pressure drop across two identical (62 percent) stenoses show that for low Reynolds numbers the total effect of two stenoses equals that of two single stenoses. A reduction of 10 percent is found for the higher Reynolds numbers investigated. Numerical and experimental results obtained for the velocity profiles agree very well. The effect of varying the converging angle of a single mild (62 percent) coronary stenosis on the fluid flow has been determined numerically using a finite element method. Pressure-flow relation, especially with respect to relative short stenoses, is discussed.

Blood platelets are concentrated near the wall and red blood cells, in the center in flowing blood.

Hematocrit and vessel wall shear rate are important factors in the transport and subsequent adherence of platelets to vessel wall subendothelium. When mass transport theory is applied to platelets in flowing blood, the blood is usually considered to be a fluid with platelet and red cell wall concentrations similar to the average tube concentration. With the laser-Doppler technique, we found how red blood cell ghosts and platelets were distributed radially for various hematocrits and wall shear rates. Red cell ghosts are crowded near the axis of the tube, with a local hematocrit higher than the average tube hematocrit, and they decrease steadily toward the wall. In the absence of ghosts, platelets exhibit the 'tubular pinch' effect (rigid particles crowding at 0.6 x tube radius). In the presence of ghosts, the platelets are expelled toward the wall region. This high concentration at the wall increases with higher average tube hematocrit and wall shear rates. Increasing the average tube platelet concentration 10 times causes the wall concentration to increase only three times. The increase in platelet adherence observed with increasing hematocrit and increasing wall shear rate can be partially ascribed to increased platelet concentration near the wall. The observation that the increased platelet concentration does not fully explain the platelet adherence data suggests that platelet transport may also be enhanced by a shear rate-dependent rotary motion.

Velocity profiles in annular perfusion chamber measured by laser-Doppler velocimetry.

p6e fluid flow in the annular perfusion chamber of Baumgartner developed to study platelet vessel wall interaction was examined with laser-Doppler velocimetry. A laminar and stable flow with a Reynolds number of less than or equal to 50 was measured at flow rates up to 3 ml s-1. No turbulence was found. The wall shear rate directly determined from measured velocity profiles agreed well with theory. The experiments underlined the necessity to work with vessels of uniform thickness and a smooth surface.