Rheological features of the whole blood and blood components are of potential importance for microcirculatory blood flow in circulatory shock. The relative contribution of vascular geometric and rheological factors to the resistance to blood flow in the various shock states in man, however, are hard to elucidate [73]. Blood viscosity is mainly determined by haematocrit. In low flow states RBC aggregation in the venules might sustain the arrest of flow and hamper reflow. Lowering haematocrit may be beneficial in improving microcirculatory flow in hypovolaemic conditions as haemorrhagic shock. A decrease in RBC deformability is seen in critically ill, septic patients and this may reduce capillary flow. The WBC is likely to contribute to the maldistribution of microcirculatory flow in both hypovolaemic and septic shock and might even block capillary flow. The exact contribution of changes in haematocrit, RBC aggregation, RBC deformability, WBC deformability and plasma viscosity on blood viscosity and thus (micro)circulatory flow in human shock states, especially in septic shock, remains to be elucidated.