Light micrograph of human lung tissue showing a branch of the pulmonary artery occluded (blocked) by an embolus (red) originating from a thrombus (clot) in the deep veins of the leg. Fragments of thrombus may become detached from their site of formation & travel in the circulation as thrombo- emboli. On reaching vessels small enough to prevent further passage, the thrombo-emboli become lodged and cause obstruction to blood flow. Depending on the degree of obstruction & extent of alternative blood supply, the result is either a reduced blood supply to tissue (ischaemia) or total cessation leading to infarction (tissue death). UFIG. 8.3 BASIC HISTOPATHOLOGY