Rigid Pericardium


Another mechanism sharks and other elasmobranchs have to overcome the limitations imposed upon them by the nature of their closed circulatory system is the possession of a rigid pericardium. In this closed circulatory system, the pressure of the blood exiting the gills still has to make it to the rest of the body tissues and back to the heart. In instances of larger sharks, the pressure of blood exiting the gills is not enough to return the volume of blood to the heart. In order to compensate for this, the sharks have evolved a rigid pericardium surrounding their heart.

The series of contractions exhibited by an elasmobranch heart,
and arrows denoting pressures resulting from such contractions.

This functions to generate a negative pressure for the blood preceding the heart, and acts to draw the blood into the heart. The rigid pericardium acts upon the heart by introducing a fixed volume to the surrounding space. In order to maintain a constant volume when the ventricle contracts in the fluid media inside the pericardium, the atrium is drawn open, generating a negative pressure in the sinus venosus. This allows the shark to overcome the pressure deficiency of blood exiting the gills. This interaction of chambers within a fixed volume acts similarly to Boyle's gas laws, which correlate pressure, temperature, and volume. This leads to a rough equation showing the relative pressures and volumes of the atrium and ventricle:

P_atrium V_atrium + P_ventricle V_ventricle = Constant