Capillaries are actually very tiny little arteries and veins.
Sickle cell disease causes the sickled red blood cells to clog the small capillaries and prevent blood from passing through. This causes a decrease in the amount of oxygen getting to cells which leads to cell death and pain. Sickle cell crises occur spontaneously or with infections. Crises are times when the symptoms of the disease are most evident. The pain is severe in the bones, joints, back and abdomen.