Blood clotting is a normal process that prevents loss of blood. However, sometimes disorders of the clotting system or injuries cause blood clots to form when they are not needed. In this case, the clots may cause significant complications. Blood clots can form in the veins (blood vessels that return blood to the heart after oxygen has been used by the tissues) and in the arteries (blood vessels that carry oxygenated blood from the heart to all parts of the body). Symptoms of a blood clot depend on the location of the clot. Symptoms of a blood clot in the venous system can include swelling of the affected area, warmth, redness, and pain. Venous blood clots occur most commonly in the arms and legs. Symptoms of an arterial blood clot result from a lack of or decrease in oxygen delivery to the tissues supplied by the involved artery. Pain in the involved area is often the first symptom. Other symptoms can occur when arterial clots form in a particular area. For example, clots in the coronary arteries can cause chest pain and the accompanying symptoms of a heart attack. If a clot forms in an artery of the brain, a stroke can occur, which can cause a number of different symptoms, depending on the precise area of the brain that is affected. Clots in the extremities can cause a pallor or whitening of the area, weakness, loss of sensation, or paralysis. Clots in the intestinal arteries can cause intense pain and bloody diarrhea.