Vaso – occlusive crisis caused by stasis of blood with clumping of cells in the micro- circulation, ischemia and infraction. Splenic sequestration is caused by pooling and clumping of blood in the spleen. Hyperhemolytic crisis is an accelerated rate of red blood cell destruction. Aplastic crisis is caused by diminished production and increased destruction of red blood cells, triggered by viral infection or depletion of folic acid. Saunders: 371.