To prevent excessive bleeding after a blood vessel is injured, the blood clotting process is initiated. When blood reaches the site of injury, it includes red blood cells that carry oxygen throughout the body, platelets that come from fragments of special cells found in bone marrow, and clotting factors that help the blood in the area to coagulate. The platelets adhere to the injury and release chemical signals for other platelets to rush to the injury site. This causes a plug to form and stop the bleeding. On the inside of the injured blood vessel, clotting factors promote fibrin (long, thin proteins) to stick together and form a barrier for the inside of the wound. Once the blood vessel is healed, the blood clot that was formed by the platelets is dissolved.^[1]

Blood clotting can also be initiated on the inside of blood vessels in the absence of an injury, and this type of clot is called a thrombosis. A thrombosis can be caused by atherosclerosis, an etiology of cardiovascular disease, which involves the buildup and hardening of plaque on artery walls. This can restrict blood flow, which breaks open the plaques, and causes a blood clot to form. This type of blood clot can become dangerous if the clot breaks free from the blood vessel and travels throughout the body. The freed blood clot can block blood flow to major organs in the body, like the heart, which can cause a heart attack.