Blood vessel clot. Coloured scanning electron micrograph (SEM) of a thrombus (abnormal clot, red) blocking a pulmonary blood vessel in a human lung. The blood vessel wall (brown) surrounds the clot. Surrounding the wall is lung fibrous tissue and alveoli (pink). This clot fills the entire lumen of the blood vessel and blocks the flow of blood. This prevents blood oxygenation and can be swiftly fatal (pulmonary embolism). Another danger is that part of the clot may break away, and travel in the bloodstream to lodge in the brain (stroke) or in the heart (heart attack). Magnification: x30 when printed 10cm wide