Blood clots form when there is damage to the lining of a blood vessel, either an artery or a vein. The damage may be obvious, such as a laceration, or may occur on the microscopic level. As well, blood will begin to clot if it stops moving and becomes stagnant.
Venous thrombosis or blood clots in a vein occur when a person becomes immobilized and muscles are not contracting to push blood back to the heart. This stagnant blood begins to form small clots along the walls of the vein. This initial clot can gradually grow to partially or completely occlude or block the vein and prevent blood from returning to the heart. An analogy to this process is a slow moving river. Over time, weeds and algae start to accumulate along the banks of the river where the water flows more slowly. Gradually, as the weeds start to grow, they begin to invade the center of the river because they can withstand the pressure of the oncoming water flow.
Arterial thrombi (blood clots in an artery) occur by a different mechanism. For those with atherosclerotic disease, plaque deposits form along the lining of the artery and grow to cause narrowing of the vessel. This is the disease process that may cause heart attack, stroke, or peripheral artery disease. If a plaque ruptures, a blood clot can form at the site of that rupture and can completely or partially occlude the blood flow at that point.
Blood clots in the heart. In atrial fibrillation, the atrium or upper chamber of the heart does not beat in an organized manner. Instead, it jiggles, and blood tends to become stagnant along the walls of the atrium. Over time, this may cause small blood clots to form. Clots can also form in the ventricle after a heart attack when part of the heart muscle is injured and unable to contract normally. Since the damaged area doesn't contract with the rest of the heart, blood can start to pool or stagnate, leading to clot formation.
Blood leaking out of a blood vessel. Blood clots can form when blood leaks out of a blood vessel. This is very beneficial when a person gets a cut or scrape wound, because the clot helps stop further bleeding at the wound site. The clotting mechanism works well following trauma as well. Broken bones, sprains and strains, and nosebleeds all result in bleeding that is controlled by the body's clotting mechanism.
Blood clots causing other medical problems. Sometimes, normal blood clotting can cause medical problems because of its location. For example, if bleeding occurs in the urine from any of a variety of reasons (such as infection, trauma, or tumor) clots may form and prevent the bladder from emptying, causing urinary retention. Clot formation in the uterus may cause pain when the clots are passed through the cervix and can lead to vaginal bleeding, either as part of menstruation or as abnormal vaginal bleeding (menorrhagia, dysmenorrhea).
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