The most common bruises on the body are the rupture of capillaries under the skin caused by external forces, but also allergic purpura, senile purpura and thrombocytopenic purpura. 1, inattentive bump or external force hit: causing the rupture of capillaries under the skin bleeding, blood pooling under the skin will form bruises, time can gradually fade. 2, allergic purpura: is a capillary allergic disease, capillary permeability, blood extravasation caused by petechiae.