A severe knock or heavy fall may cause bruising without a fracture. The bruise is a result of blood leaking from damaged blood vessels within soft tissue, underneath the skin. The eventual green and yellow discoloration that results is caused by the red haemoglobin pigment in the blood being broken down into its constituent pigments.
Someone with extensive bruising should be given a hospital examination, because he or she may have suffered a fracture. Minor bruising can be treated at home. The principles are the same as for treatment of a sprain: try to reduce the swelling by cooling and elevating the injured part.