A clot, or blood clot, is a clump of blood that forms over an injury to prevent excessive bleeding. The process by which clots develop is called blood clotting, or coagulation. When a blood vessel suffers damage, the formation of a clot stops the bleeding so that the blood vessel can heal. While blood clotting is an important process that repairs the body, it can also be dangerous because clots can sometimes form even if the body does not need them for healing. In addition, sometimes clots will not dissolve as they normally should; instead, they block blood flow by staying within the blood vessel, or traveling from one area of the body to another.
Blood is a liquid made up of red blood cells and white blood cells, as well as platelets and blood plasma. This liquid continuously circulates inside the blood vessels, with the arteries pumping it away from the heart and the veins returning it to the heart. When damage occurs to a blood vessel, bleeding takes place, and to repair this injury, the body sends platelets to the damaged area to serve as a plug. The platelets then release chemicals to start the clotting process using clotting factors that the body produces. Eventually, a protein known as fibrin forms and creates a mesh to finalize the clot.
Although clots form because the body needs to repair itself, they can develop due to other circumstances. For example, immobility is a cause of clots. When a person has been immobile for quite some time, his blood does not circulate as well as it normally would if he was mobile. This is because movement uses muscles, and it is the muscles that help pump blood through the veins back towards the heart. Thus, an immobile person, such as someone who is spending a long time sitting on an airplane ride, can experience a type of clot called a deep-vein thrombosis (DVT).
While a clot should dissolve by itself, sometimes it does not do so, leading to a block in blood flow, which is dangerous. When a blood vessel is unable to deliver blood and oxygen to parts of the body, it causes tissue damage or tissue death in those areas. In addition, a clot that does not dissolve can break off from where it originally formed and travel through the bloodstream to other areas of the body. For example, a DVT that breaks off can travel to the lungs and cause a pulmonary embolism (PE).