What makes peritonitis, pleurisy, and pericarditis cause a great deal of pain?

Inflammation: All of the conditions are caused by inflammation. That means there is irritation of nerve endings leading to pain. Pleurisy is also called pleuritis. In medical terminology, the ending -itis means inflammation.

Inflammation: Peritonitis is an inflammation of the lining of the abdomen, pleurisy an inflammation of the lining around the lungs and pericarditis an inflammation of the lining around the heart. In each case it is likely that the inflammation irritates some of the nerve endings in the region and causes pain. In some cases, like pericarditis, the stretching of the pericardium from fluid accumulation contributes.