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Unformatted text preview: Structure of the Heart The human heart is a four- chambered muscular organ, shaped and sized roughly like a man's closed fist with two- thirds of the mass to the left of midline. The heart is enclosed in a pericardial sac that is lined with the parietal layers of a serous membrane. The visceral layer of the serous membrane forms the epicardium . Layers of the Heart Wall Three layers of tissue form the heart wall. The outer layer of the heart wall is the epicardium, the middle layer is the myocardium, and the inner layer is the endocardium. Chambers of the Heart The internal cavity of the heart is divided into four chambers: Right atrium Right ventricle Left atrium Left ventricle The two atria are thin-walled chambers that receive blood from the veins. The two ventricles are thick- walled chambers that forcefully pump blood out of the heart. Differences in thickness of the heart chamber walls are due to variations in the amount of myocardium present, which reflects the amount of...
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