Blood clot. Light micrograph of a section through human muscle fibres, with a blood clot (thrombus, centre) in a capillary. Damage to a blood vessel, or other vascular disease, can cause clot formation. Blockage (occlusion) such as this may reduce blood flow, or stop it altogether. A danger is that part of the clot may break away, and travel in the bloodstream to lodge elsewhere in the body. Magnification unknown.