Clot forming in lung tissue, coloured scanning electron micrograph (SEM). Tendrils of fibrinogen (a glycoprotein) enmeshing red blood cells (red) during the process of blood clotting. This clot (thrombus) is forming in lung tissue. An abnormal blood clot both here in the lungs and if it detaches and lodges elsewhere, can be the cause of conditions such as a stroke, a heart attack or a pulmonary embolism.