Cardiac and vascular disorders: Blood clot

A thrombus is a blood clot that forms inside a blood vessel, often causing a blockage of the flow of blood through the vascular system.

The process of thrombus formation, called “thrombosis”, results from the activation of the normal blood clotting system. In pathological thrombosis, the process of coagulation far exceeds the body’s capacity to dissolve the clot. As additional layers of platelets and fibrin are deposited, the thrombus continues to grow larger until a mass is formed. This thrombus can eventually block the space within a blood vessel called the “lumen”, resulting in complete blockage.

An embolus is the sudden blockage of an artery by a mass that is carried through the bloodstream, eventually lodging in a blood vessel that is too small to allow its passage. The mass can be composed of fat, cancer cells, air, or even a blood clot. For example, a thromboembolism is a thrombus that detached and traveled through the circulatory system eventually blocking a distant artery.

When a long bone is fractured, or during orthopedic surgery, damaged fat cells in the bone marrow may release fragments of fat into the bloodstream – that can form a fat embolism. Similarly, large fragments of bone marrow including hematopoietic tissue may be released into the bloodstream, which can cause a bone marrow embolism. Clumps of cancer cells may break free into the circulation to form tumor emboli.

An air embolus can form as a result of the inadvertent introduction of air bubbles during a catheterization procedure. There are two main types of embolus depending on whether it lodges in the pulmonary or systemic circulation. A pulmonary embolus originates in peripheral veins and is carried to the heart where it enters the pulmonary circulation, eventually becoming lodged in the pulmonary arterial system. A systemic embolus originates in the heart and travels via the aorta to the systemic arterial circulation. They commonly affect the arteries leading to the brain, kidneys, spleen, gut, and lower limbs.

Review Date:5/27/2024
Reviewed By:Michael A. Chen, MD, PhD, Associate Professor of Medicine, Division of Cardiology, Harborview Medical Center, University of Washington Medical School, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997-A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

The Agency for Health Care Administration (Agency) and this website do not claim the information on, or referred to by, this site is error free. This site may include links to websites of other government agencies or private groups. Our Agency and this website do not control such sites and are not responsible for their content. Reference to or links to any other group, product, service, or information does not mean our Agency or this website approves of that group, product, service, or information.

Additionally, while health information provided through this website may be a valuable resource for the public, it is not designed to offer medical advice. Talk with your doctor about medical care questions you may have.