Function of the pericardium and pericardioperitoneal canal in elasmobranch fishes

Previous studies of cardiac function in elasmobranch fishes have not included the influence of the pericardioperitoneal canal on pericardial pressure and volume and thus on cardiac function. Accordingly, we studied the function of the pericardium and pericardioperitoneal canal in sharks and rays. We found negative pericardial pressure that rose to a plateau of approximately 0 mmHg when fluid was infused into the pericardium with the canal undisturbed. However, this pericardial pressure elevation caused severe cardiac tamponade. After the canal was occluded, the pressure plateau was substituted with an exponential rise. We injected radioisotopes into the pericardial cavity and obtained scintigrams several hours later. The scans and counts of body fluids and tissues indicated absorption, disputing the suggestion that the primary function of the canal may be inadequate absorption of pericardial fluid. We conclude that the pericardioperitoneal canal maintains negative pericardial pressure, which is a prerequisite in elasmobranch fishes and may serve to regulate pericardial pressure level to optimize cardiac function in relation to changes in cardiac size.