The root canal is actually a channel that runs from the root of the tooth, which connects to the bone, up to the top surface of the tooth. The canal contains blood vessels, nerves, and the complex cells that make up the living tissue inside the tooth. This lifeline inside the tooth is called the pulp.

When a tooth is decayed or cracked, bacteria can get to the pulp. The acid from the bacteria irritates the pulp and it becomes inflamed; it's the same process you watch when other parts of your body become infected. When the pulp tissue becomes inflamed, it's harder for blood to flow to the tissue, and the resulting pressure creates pain inside your tooth.