What is the purpose of the pericardial sac?

The main purpose of pericardial sac is the fixation of heart at mediastinum, protection and lubrication of heart.

Explanation:

Pericardial sac is also known as pericardium and contains heart and large vessels. Two main layers of pericardium are fibrous pericardium and serous pericardium.

Pericardial sac secretes the fluid that provide lubrication and reduces the friction of heart. The sac fixes the position of the heart and limits the heart movement. The pericardial sac also reduces the excessive dilation of heart during acute volume overload.