Blood clots are rather simple in their structure, consisting of blood platelets, fibrin and red blood cells. If you take away one of these elements, then the entire blood clot would effectively fall apart. Onions and garlic both have the unique ability to reduce the fibrin content of the body, which can thus prevent the clumping of blood platelets and RBC to form blood clots. Garlic and onions are also important sources of antioxidants that can reduce oxidative stress in the cardiovascular system and prevent chronic health concerns.