(RBCs) generally deform to adopt a parachute-like, torpedo-like, or other configuration to align and flow through a capillary that is narrower than their major axis. As described herein, even in a narrow tube (25 m) with diameter much larger than that of a capillary, flowing RBCs at 1 mm/s align axially and deform to a paraboloid shape in a viscous Newtonian fluid (505 kDa dextran medium) with viscosity of 23.4 -57.1 mPa ⅐ s. A high-speed digital camera image showed that the silhouette of the tip of RBCs fits a parabola, unlike the shape of RBCs in capillaries, because of the longer distance of the RBC-free layer between the tube wall and the RBC surface (ϳ8.8 m). However, when RBCs are suspended in a "non-Newtonian" viscous fluid (liposome-40 kDa dextran medium) with a shear-thinning profile, they migrate toward the tube wall to avoid the axial lining, as "near-wall-excess," which is usually observed for platelets. This migration results from the presence of flocculated liposomes at the tube center. In contrast, such near-wall excess was not observed when RBCs were suspended in a nearly Newtonian liposome-albumin medium. Such unusual flow patterns of RBCs would be explainable by the principle; a larger particle tends to flow near the centerline, and a small one tends to go to the wall to flow with least resistance. However, we visualized for the first time the complete axial aligning and near-wall excess of RBCs in the noncapillary size tube in some extreme conditions. hemorheology; erythrocytes; viscometry; artificial red cells; microcirculation RED BLOOD CELLS (RBCs) or erythrocytes in mammals lost their nuclei during their evolution and specialization for their role of oxygen transport. Microcirculatory observation of capillaries (Ͼ4 m diameter) that are narrower than the RBC diameter (8 m) revealed that flowing RBCs alter their own morphology from a biconcave disk to various configurations resembling a parachute, umbrella, jellyfish, or torpedo, as though they were alive (10,11,29). This phenomenon was first reported in the 1960s.During blood flow in a capillary, a pressure gradient exists: a pressure drop in the direction of flow. The higher pressure in the rear tends to compress the rear portion of the RBCs. In response to such stress, the elastic cellular structure of RBCs (biconcave disc) is known to be effective to deform and stir the intracellular viscous hemoglobin (Hb) solution (Ͼ35 g/dl), thereby facilitating gas exchange (1,6,14). From conventional microscopic observation of peripheral tissues, it is difficult to discern the capillary shape, especially at a reduced hematocrit resulting from the Fahraeus effect and plasma skimming, but we can infer the presence of functional capillary walls near the RBCs by the presence of deformed and aligned RBC flow (25,29). In glass tubes (inner diameter, Ͻ12 m), similar deformation of RBCs is apparent (8, 35). On the other hand, the glycocalyx layer of the capillary endothelium interacts with the plasma layer fluid and retards its flow. Its hydraulic resista...