Deep vein thrombosis. Light micrograph showing a blood clot (thrombus, dark pink) completely blocking the lumen (inner space) of a vein in the calf muscle. This is known as deep vein thrombosis (DVT). DVT can be provoked by situations that encourage sluggish blood flow, which increases the tendency of the blood to coagulate, such as prolonged bed rest, surgery, and long-haul flights. Treatment is through monitoring and blood-thinning drugs to prevent further clotting.