Blood clot in the lung. Coloured scanning electron micrograph of a section through a blood clot (thrombus) in a blood vessel of the lung. Red blood cells (erythrocytes, small, round) are trapped within a fibrin protein mesh (fibres), which is formed in response to chemicals secreted by platelets, fragments of white blood cells (not shown). Clots are formed in response to cardiovascular disease or injuries to blood vessels.