Thrombosis and embolism are two mainstays of general pathology and their discussion begins with the definition of each.
A thrombus is a blood clot that is formed within a blood vessel by the contents of the flowing blood.
An embolus is a mass that is carried by the blood and impacts in a blood vessel distant to the site at which the embolus was initially formed. There are several different types of embolus.
Thromboembolism
Thromboemboli are the most common form of embolus. They are composed of intravascular blood clot that has broken off from its place of generation and been carried by the circulation to a distant site where it becomes lodged. The thrombus can be created in either arteries or veins. Arterial thrombi, often in the carotid arteries, are a cause of cerebrovascular accidents. Venous thrombi in the deep veins of the lower limbs or the pelvis are the origin of pulmonary emboli.
Fat
Trauma can force globules of fat into the blood stream. This is most likely to occur as a complication of a fracture, in which situation the embolus can also include haematopoietic bone marrow. Common sites of impact are the pulmonary arteries or the small arteries within the brain (this can cause widespread cerebral ischaemia and infarction).
Cholesterol
Manipulation of blood vessels that contain atherosclerotic plaques can occasionally dislodge part of the plaque. Atherosclerotic plaques are rich in cholesterol, which as a lipid is not soluble in the blood.
Air
One of the main claims to fame for air embolism is its occasional occurrence in thrillers and crime novels. The actual volume of air that has to be injected into the circulation to have a serious effect may need to be at least 50ml, rather than the tiny 2ml size syringes that are sometimes shown on television. The rapid entry of 300ml can readily be fatal of air. However, various factors that relate to the solubulity of the gas and the environmental pressure can modify these values.
Rather than being a popular mode of homicide, the main circumstance in which air embolism can take place is surgery, especially that on blood vessels of the head and neck region. Air embolism is also a rare complication of the canulation of central veins.
Air emboli can become lodged and obstruct a blood vessel. If a large volume of air enters the heart via the venous route it can become trapped in the right atrium and ventricle and prevent blood flow (the heart simply repeatedly compresses the air rather than expelling it).
Decompression sickness (the bends) is also a form of air embolism. The increase in ambient pressure with depth during a dive means that more nitrogen can be forced into solution in the plasma. When a diver ascends the reduction in the pressure causes the nitrogen to come out of solution and return to the gaseous state. If the ascent is too rapid, the nitrogen will form bubbles before the lungs can expel it. Decompression chambers reduce the pressure to which the divers are exposed more slowly and therefore allow the lungs to clear the nitrogen.
Amniotic Fluid
The trauma and high intrauterine pressure that happens during labour can rarely force a significant volume of amniotic fluid into the circulation. As well as causing problems through embolism, the amniotic fluid can induce disseminated intravascular coagulation.
Exogenous Material
It is theoretically possible for substances external to the body to be injected into the circulation.
Complicity by the late Iain Banks, involves a particularly unpleasant example.