According to the National Heart, Lung, and Blood Institute, bruising is a symptom of aplastic anemia. This disease occurs when there are too few red blood cells, white blood cells and platelets in the blood.
The University of Rochester Medical Center explains that bruising is the result of broken blood vessels under the skin. The blood is trapped when a blow, fall or other trauma breaks vessels. The function of platelets is to stop the bleeding from blood vessels, the NHLBI states. Platelets join together to seal off cuts or tears. If blood has a reduced number of platelets due to anemia, it is harder for the body to stop the bleeding, and a bruise appears.