Blood clots in the leg usually appear red and swollen, but they can take on different appearances depending on the location. In contrast, blood clots in the heart and lungs are not immediately visible.
Blood clotting is sometimes essential to the body's processes. It prevents excessive bleeding when blood vessels are injured. Certain blood cells, called platelets, work together with the blood's plasma to stop bleeding by acting as a knot-shaped barrier over the injury.
Blood clots can also occur in veins and arteries, which must remain free-flowing as an essential attribute to the body's circulatory system. These types of clots may require medical attention.