The bruising on the arm after blood donation is due to the lack of pressure on the location of the punctured blood vessel after blood donation, resulting in localized bleeding in the vessel wall and penetration into the subcutaneous tissue. In the early stage, there is a swelling and lump in the area where the blood was drawn. Because of the blood inside, a large bruise is formed under the skin after the blood is slowly absorbed by the body tissues. Therefore, after the blood is drawn, the area where the blood was drawn should be pressed very tightly and not rubbed back and forth while pressing. When bruising occurs at a later stage, warm towels or hot compresses should be applied to the surface to speed up the circulation of blood around the area, so that the bruising will fade faster.