Unwanted blood clots can be very dangerous if they float through the bloodstream. A freely floating clot, also called an "embolus," can get stuck in a blood vessel, blocking it off so that blood can't get to the cells in that area. If the cells go without oxygen and nutrients for too long, they will die.
An example of this is when a blood clot travels to the brain, and blocks off a blood vessel there. If blood flow is blocked off for too long, it causes a stroke. The brain tissue that no longer gets blood from this vessel quickly dies. As you may know, a stroke can cause serious damage to the brain, and even death.
Clots that form in the veins of the legs can later break off and block blood vessels in the lungs. This is called a "pulmonary embolism," and it can lead to shortness of breath, a sudden decrease in blood pressure, or even death.
Other places that clots may become lodged include the vessels that lead to the kidneys, digestive organs, liver, or spleen. If a clot blocks off the blood supply to these organs, or any other organ of your body, it can be very serious and possibly fatal.
To decrease the risk of these types of problems, it's important to prevent unwanted blood clots from forming.