Some of the most challenging medical problems we face have to do with the formation of blood clots. For instance, clots can create heart attacks and can cause lack of blood flow to the extremities, resulting in loss of fingers, toes, arms or legs. Clots — when lodged in large blood vessels in the lungs — produce chest pain, difficulty breathing, or even death from what we call a pulmonary embolism. Despite their relatively small size, clots in the brain can produce a type of stroke. People who live with atrial fibrillation — an irregular heart rhythm — can form clots in the atrium of the heart, which can then move out into the circulation and cause some of the complications above.