The anatomical space between the two pleural cavities containing the heart and major vessels is the mediastinum.
The mediastinum is the central compartment of the thoracic cavity. It lies between the right and left pleural cavities, which surround the lungs, and extends from the sternum in front to the vertebral column behind. Although often described as a “space,” it is a region filled with organs, connective tissue, blood vessels, nerves, and lymphatic structures.
Its contents include the heart within the pericardium, the aorta, pulmonary trunk, superior vena cava, trachea, esophagus, thymus, thoracic duct, and major nerves. Anatomists divide it into superior and inferior portions; the inferior mediastinum is further divided into anterior, middle, and posterior regions.
The key distinction is that the mediastinum is not the same as the pericardium. The pericardium is the sac surrounding the heart, while the mediastinum is the larger central thoracic compartment that contains it. The pleural cavities, by contrast, are associated with the lungs.