This is a common question I get from patients, in fact it is pretty rare for a day to go by without hearing it.
Our teeth have nerves and blood vessels inside. It is a unique place in our bodies because it is entirely enclosed in hard tooth structure. When a tooth is traumatized enough (usually by something like a cavity or a crack), our bodies respond to that insult the same as other bodily injuries – an inflammatory response. Our body’s immune system is our natural way of combating an infection and results in swelling. The problem with that enclosed tooth is it increases the pressure inside a tooth that frequently results in pain. This response to a damaged tooth was a great idea the body had, but the result is not only pain, but also a strangled blood supply to the tooth. Consequently the tissue inside the tooth dies which now causes the body to continue the response to not only the original crack or cavity, but now also to dead tissue inside the tooth. Left alone, the body will dissolve all the bone around the root of the tooth and eventually eject it (like a James Bond passenger) from the body!
Now enter the Root Canal. The root canal removes all the material (blood vessels and nerves) from the inside of the tooth, and thoroughly cleans (ultrasonic instruments and sometimes lasers are used for this) and fills that space with a biocompatible plastic material called gutta percha). This leaves the body to heal the surrounding area and allows you to keep the tooth assuming we have been able to repair the original crack or cavity.
Root canals are great ways to save teeth and are largely successful. In the event a tooth cannot be saved with a root canal (often due to very large cracks or cavities) a patient will likely end up losing a tooth.
Here is a good example of before and after a root canal on my friend Carla! You can see the dark area near the tip of the premolar root in the before xray which is the body destroying bone in that area. In the after xray, the white line going down the root is the gutta percha fill.