Thrombosis is the formation, development, or existence of a blood clot or thrombus within the vascular system. Clotting is a life-saving process when it occurs during hemorrhage, as when a person has cut their finger and is bleeding. But it is a life-threatening event when it occurs at any other time because the clot can occlude a blood vessel and interrupt the blood supply to an organ or body part. The thrombus, if detached, becomes an embolus and occludes a vessel at a distance from the original site; for example, a clot in the leg may break off and cause a pulmonary embolus. Medical nanorobots can patrol the bloodstream, searching for unwanted developing internal clots. If a blood vessel occlusion has already occurred, in vivo nanorobots can immediately clear an opening so that free blood flow may resume, avoiding tissue ischemia.