Effect of microrheology of blood on the apparent flow instability in a rotational viscometer

Flow instability (formation of vortices and a concurrent increase in the apparent viscosity) was studied in the rotational rhombospheroid viscometer of 3 degrees, 5 degrees and 10 degrees gaps over a range of speeds from 10 to 300 r.p.m.. Comparisons between different blood systems were carried out mainly at 250 r.p.m. Experiments were carried out on blood samples obtained directly from human subjects, or from the Blood Bank, or from horses. Reconstituted suspensions of red cells in albumin or dextran were also used. Apparent flow instability was found to be not solely a function of blood viscosity, but a multiple function of many viscosity factors or blood subphases, including instability-decreasing factors such as haematocrit and aggregation of red cells; and instability-increasing factors such as rigidity of red cells; and thus specific to and characteristic of individual blood samples. Apparent instability can be described by multiple regressions as a function, Z, of red cell rigidity, Tk, blood viscosity, napp, and aggregation of red cells, AG; for example: Z = -28.29 + 26.24 Tk + 0.109 napp (r = 0.816; P less than 0.001), or Z = 5.90 - 0.0165 AG - 0.752 napp (r = 0.573; P less than 0.05). The apparent instability can be seen only in one-third of blood samples obtained from horses, and in more than half of blood samples obtained from human donors; majority of human donors shows apparent instability below 3 per cent.