The pericardium encloses most of the heart, the ascending aorta, the pulmonary trunk, and the terminal segment of the venae cavae. Serving primarily to anchor the heart in the central thorax and reduce its contact with surrounding structures, the pericardium is composed of two layers, a serous visceral layer adherent to the heart and a fibrous parietal layer. The space between the two layers is lubricated by a small amount of clear fluid, an ultrafiltrate of plasma. The pericardium receives its blood supply from the internal mammary arteries and is innervated by the phrenic nerves.
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