pericardium(redirected from heart sac)
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muscular organ that pumps blood to all parts of the body. The rhythmic beating of the heart is a ceaseless activity, lasting from before birth to the end of life. Anatomy and Function
The human heart is a pear-shaped structure about the size of a fist.
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the closed, saclike structure surrounding the heart in vertebrate animals and in man. The pericardium is an isolated part of the secondary body cavity, the coelom. It consists of two layers: an external, parietal layer—the pericardium proper—and an internal, visceral layer—the epicardium. The pericardium proper consists, in turn, of an internal, serous layer and an external, fibrous layer. The epicardium, being an external membrane of the heart, directly covers and is fused to the heart muscle (myocardium). In the region where large blood vessels enter and leave the heart, the epicardium is turned back and is continuous with the pericardium proper. Between the pericardium layers is a slitlike cavity containing a small quantity of transparent pale-yellow serous fluid. The pericardium contains blood vessels, lymphatics, and numerous nerve endings and is surrounded by areolar connective tissue. It has many receptors, whose stimulation causes hemodynamic and respiratory changes.