A tooth with an exposed nerve is highly susceptible to the symptoms of pulpitis. The pulp cavity, except for the apical foramen, is surrounded by a hard wall of dentin. The pulp tissue is located in it and is connected to the rest of the tooth only through the narrow apical foramen. It is not easy to establish proper drainage during inflammation, resulting in the accumulation of inflammatory exudates. The lack of elasticity of the dentin wall restricts its expansion, resulting in increased pressure in the pulp cavity. On the one hand, the infection can easily spread to the entire pulp, and on the other hand, the compression of the nerve produces severe pain. However, once the pulp cavity is penetrated, the pressure drops and the pain decreases abruptly, which is the anatomical reason why the pulp should be opened and depressurized first in the treatment of acute pulpitis. The pulp tissue lacks collateral circulation compared to other connective tissues in the body because only a few blood vessels enter the pulp through the apical foramen and the lateral root canals, and because of the hard dentin wall surrounding the pulp, collateral circulation can hardly be formed when it is inflamed, thus severely limiting its ability to remove necrotic tissues and inflammatory products as well as its healing ability. Once the pulp is inflamed, it is very easy to go to necrosis, so the tooth with exposed nerve can not be healed by itself.