Intervention during circulatory insufficiency invariably produces certain basic changes in the blood itself and, as a consequence, in the functional aspects of blood-tissue exchange. Several categories of disturbances can be recognized: 1) Changes in the fluidity of the blood as a consequence of activation of blood coagulation mechanisms; 2) alterations in the physical properties of the red blood cells leading to abnormally high shear forces at the blood stream-vessel wall interface and to hemolysis; 3) inability of artificial systems to maintain integrity of vascular endothelium, and the blood capillary barrier in particular; 4) red blood cell aggregation and changes in the rheological properties of the blood.