Blood Clots

What are blood clots?
Blood is a liquid that flows within blood vessels. It is constantly in motion as the heart pumps blood through arteries to the different organs and cells of the body. The blood is propelled back to the heart in the veins. When muscles contract, they squeeze the veins and allow the blood to be pushed back to the heart.
Blood clotting is an important mechanism to help the body repair injured blood vessels. Blood consists of:
red blood cells containing hemoglobin that carry oxygen to cells and remove carbon dioxide,
white blood cells that fight infection, and
platelets that are part of the clotting process of the body, and
blood plasma, which contains fluid, chemicals and proteins that are key to bodily functions.
Complex mechanisms exist in the bloodstream to form clots where they are needed. If the lining of the blood vessels becomes damaged, platelets are recruited to the injured area to form an initial plug. These activated platelets release chemicals that start the clotting cascade, activating a series of clotting factors. Ultimately, fibrin is formed, the protein that crosslinks with itself to form a mesh that makes up the final blood clot.
The medical term for a blood clot is a thrombus (plural= thrombi). When a thrombus is formed as part of a normal repair process of the body, there is little consequence. Unfortunately, there are times when a thrombus (blood clot) will form when it is not needed, and this can have potentially significant consequences.

What causes blood clots?
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 lining walls of the vein that gradually grow to partially or completely occlude the vein. 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 water.
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, and peripheral artery disease. If a plaque ruptures, a blood clot can form at the site of the ruptured plaque that can completely or partially occlude the blood flow at that point.
Blood clots can also form 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.