What surrounds the heart in the thoracic cavity?

The pericardium is the double-walled sac that encases the heart within the thoracic cavity. It consists of two layers: the fibrous pericardium, which provides protection and limits the heart's movement, and the serous pericardium, which produces a fluid that lubricates the heart, reducing friction during heartbeats. This specialized structure allows the heart to function efficiently without damage from movement or external forces.

In contrast, the peritoneum refers to the membrane that lines the abdominal cavity and covers abdominal organs, not the thoracic cavity. The pleurae are membranes that envelop the lungs and are involved in respiratory mechanics, while the diaphragm serves as the primary muscle of respiration and separates the thoracic cavity from the abdominal cavity. These other structures are important in their own contexts but do not surround the heart specifically like the pericardium does.