A blood clot is often a deposit usually of fat (cholesterol) combined with blood clotting factors that forms on, or has been dislodged from a blood vessel wall. Blood flow is either blocked or markedly reduced. Clots stop bleeding and while clot formation is useful if the skin has been cut, it is dangerous when clots form in a blood vessel. A stroke or heart attack often follows when blood has a tendency to clot too much, serious health problems can occur.