The Fahraeus effect of aggregating red cell suspensions flowing through narrow tubes is theoretically considered using a mathematical model. This model was developed in our previous paper to analyze the flow properties of aggregating red cell suspensions in narrow vertical tubes. The ratio of tube hematocrit H_T to discharge hematocrit H_D is calculated as a function of pseudo-shear rate u, which is defined as a mean velocity divided by a tube diameter. At low shear rates, it is shown that the ratio H_T/H_D increases with increasing u and increases with increasing tube diameter. These theoretical predictions are in good qualitative agreeement with experimental results.

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To claritfy the detailed in-vivo hemodynamics of aorto-renal bifurcation, we measured the velocity profiles and bifurcation geometry at a surgically exposed left renal artery in 10 anesthetized dogs using a 20 MHz, 80 ch. pulsed Doppler velocimeter and 30 MHz, high resolution ultrasound tomography, respectively. The angle between the aorta and the left renal artery ranged from 60° to 90°. The diameter of the left renal artery ranged from 1.5 to 3.5mm. The velocity profile in the proximal renal artery in the plane parallel to the bifurcation showed a skewed pattern toward the caudal-side wall through a cardiac cycle. Reverse flow near cranial-side wall was observed even during systole, indicating flow separation. When the probe was placed on the cranial-side wall perpendicular to the vessel, the velocity component from the cranial to caudal-side wall was observed, indicating the existence of secondary flow in that direction. At 4 diameters distal to the renal ostia, velocity profiles were almost parabolic. In conclusion, the in-vivo flow pattern at the cranial-side wall of the renal ostia, where early development of atheromatous plaque is prone to localize, was characterized by (1) low shear rate, (2) oscillation of flow and (3) separation of flow.

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