Thrombosis is the formation of a blood clot inside a blood vessel, obstructing the flow of blood through the circulatory system. When a blood vessel is injured, the body uses platelets (thrombocytes) and fibrin to form a blood clot to prevent blood loss. Even when a blood vessel is not injured, blood clots may form in the body under certain conditions. A clot that breaks free and begins to travel around the body is known as an embolus.

When a thrombus is significantly large enough to reduce the blood flow to a tissue, hypoxia (oxygen deprivation) can occur and metabolic products such as lactic acid can accumulate. A larger thrombus causing a much greater obstruction to the blood flow may result in anoxia, the complete deprivation of oxygen and infarction, tissue death. There are also a number of other conditions that can arise according to the location of the thrombus and the organs affected.