Mayo Clinic Health Manager
Get free personalized health guidance for you and your family.
Get StartedDefinition
By Mayo Clinic staffBlood clots normally form to stop the bleeding when an artery or vein is damaged, such as when you experience a cut. However, sometimes blood clots form when you don't need them, and they can block the flow of blood, usually through a vein. A number of conditions can cause you to develop abnormal blood clots, and they require medical attention.
Blood clots forming inside superficial veins may cause you to experience redness, pain and swelling in the affected area, usually a leg. Or, blood clots that form in deep veins can break off and possibly cause a potentially life-threatening condition by traveling from your vein, through your heart. If a clot travels to your heart vessels, it can cause a heart attack. If the clot goes to your lungs, it can cause a pulmonary embolism. In your brain, it can cause a stroke.