The pericardial sac is best described as which of the following?

• A. Outer membrane of the lungs
• B. Fibrous sac that holds the heart, is double-walled, and anchors it to the diaphragm
• C. Inner lining of the heart chambers
• D. Layer of muscle around the heart

Answer: (B)

The pericardial sac is a fibrous protective envelope around the heart that is double-walled because of the serous inner layer. The outer fibrous pericardium tightly encases the heart and helps anchor it in the middle of the chest, attaching to the diaphragm via ligaments so the heart stays in place and doesn’t over-move. Inside, the serous pericardium forms a parietal layer that lines the fibrous sac and a visceral layer that covers the heart surface, with a small pericardial cavity between them containing a little lubricant. This arrangement reduces friction as the heart beats and helps limit overdistension. This structure is not the outer membrane of the lungs, the inner lining of the heart chambers, or the muscle layer around the heart.