Blood clots are gel-like clumps of blood. When they form in response to a cut or other injury, they stop the bleeding by plugging the injured blood vessel. These blood clots help the body heal.

But some blood clots form inside the veins without a good reason. They don't dissolve naturally. These clots may require medical attention, especially if they are in the legs, lungs or brain. A number of conditions can cause this type of blood clot.