What is the pericardium?

The pericardium is a double-walled sac that surrounds the heart and the roots of the great vessels. It serves important functions, such as providing lubrication to reduce friction between the heart and surrounding structures during heartbeats, as well as helping to anchor the heart in place within the thoracic cavity.

The option that defines the pericardium accurately corresponds to its role related to the heart, indicating it's involved with the protective lining around the organ. Understanding this context clarifies that none of the other choices pertain to the heart or the protective structures associated with it. The membrane covering the diaphragm, lung tissue types, and outer lung structure all belong to different anatomical regions and functions, thus highlighting the uniqueness and significance of the pericardium in relation to heart health and function.