As in all infections, the body answers with increased circulation (hyperaemia): the supplying blood vessels expand. Hyperaemia can transform into an acute or chronic pulpitis. A tooth with acute pulpitis is extremely sensitive to temperature. Cool air is sufficient to trigger the pain. In pulpitis acuta serosa, extended capillary vessels cause the excretion of granulocytes and serum. Activated enzymes, causing the breakdown of proteins, create pus, leading to severe pain in pulpitis acuta purulenta.

Chronic pulpitis on the other hand is often completely without symptoms. It is usually caused by caries. White blood cells (leucocytes) accumulate in the pulpa to combat inflammation. The bacteria cause the blood vessels of the pulpa to become permeable to serum (pulpitis serosa) and the number of infection combating cells increases (e.g., lymphocytes). Bacteria aggravate the infection. Pus-forming granulocytes supervene and the pain intensifies.

In reversible, acute pulpitis, the tooth reacts to sweetness, cold and heat. The pain lasts for a short while only. The sensitivity test is positive. If caries can be removed without opening the pulpa, the tooth loses its symptoms. The pulpa can be kept vital. This acute form of pulpitis is therefore reversible.

In irreversible pulpitis, which may be acute or chronic, the tooth is permanently painful. The dentin is frequently destroyed up to the pulpal cavity and cariously altered. Pain continues even after caries removal and medicamentous filling. The tooth's sensitivity to touch and biting is joined by decreasing or lacking reaction to a sensitivity test. The damaged pulpa cannot be reversed to its original healthy state, it is irreversible.

Vitality loss of the tooth marrow leads to pulpal necrosis which initially shows no symptoms. This may occur through a bacterial infection such as gangrene or after trauma without the participation of bacteria. If infection occurs in the jaw bone via the foramen apicale, it results in acute or chronic apical parodontitis. Diagnosis is confirmed with a negative sensitivity test. Therapy consists of trepanation with subsequent root canal preparation and filling.

If the disease is left untreated, anaerobic putrefactive bacteria decompose the dead tissue and result in gangrene. Putrefactive bacteria create a gas. This gas creates pressure in the tooth which in turn causes severe pain. Dental trepanation often alleviates the pain, as these putrefactive gases, pus and secretion can escape, whereby the root canals are massively infested with bacteria, which can be recognised in a foul odour and taste. In pulpal gangrene, the result of a sensitivity test is negative. If the tooth is knock-sensitive, bacteria have entered the jaw bone through the foramen apicale. The body is trying to defend itself by forming defensive tissue, an apical granuloma. This can be recognised as a dark apical spot on the X-ray. Cysts and abscesses may form later under certain conditions.

A chronic apical parodontitis often displays no pain. The treatment principle in irreversible pulpitis and pulpal gangrene consists of the removal (extirpation) of the painful and infected or devital pulpa and the antibacterial filling of root canals with tissue compatible pastes and gutta-percha pins.