What structure directly surrounds and protects the heart?

The pericardium is the structure that directly surrounds and protects the heart. It is a double-walled sac composed of an outer fibrous layer and an inner serous layer. The fibrous pericardium provides a protective barrier, preventing over-expansion of the heart and anchoring it within the mediastinum. The serous layer of the pericardium produces a small amount of fluid that allows for smooth movement of the heart as it beats. This protective function is critical, as it not only safeguards the heart from physical trauma but also maintains its position within the thoracic cavity.

The other structures listed do serve important functions within the thoracic cavity, but they do not provide the specific protective and supportive role that the pericardium does for the heart. The pleura encase the lungs, the diaphragm plays a vital role in respiration, and the lung apices are the upper parts of the lungs. While these structures contribute to the overall anatomy of the thoracic region, none surrounds or protects the heart in the same manner as the pericardium.