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Structure of the Heart
The heart is located within a protective sac called the pericardium. The pericardium consists of an outer fibrous layer, which functions to attach the heart to the rest of the body and to protect the heart form physical damage, and an inner serous layer. The inner serous layer can be further subdivided into a parietal layer and a visceral layer. In between the parietal and visceral layer is the paricardial cavity, which contains the percardial fluid. This fluid is used to lubricate the heart and decrease the amount of friction it experiences every time it contracts. The heart itself consists of three different layers - the epicardium, the myocardium and the endocardium. The epicardium is the outermost layer of the heart that is fused with the visceral layer of the inner pericardium mentioned above. The myocardium is the middle layer (usually the thickest) and consists of cardiac muscle cells that are responsible for contracting the heart. The inner layer of the heart is called the endocardium and this layer consists simple squamous endothelial cells. These three layers create the four chambers of the heart. The right atrium receives deoxygenated blood from the systemic circulation while the left atrium receives oxygenated blood from the lungs (pulmonary circulation). The right ventricle sends deoxygenated blood to the lungs (pulmonary circulation) while the left ventricle sends oxygenated blood to the rest of the body (systemic circulation). In order to ensure that the flow inside the heart is unidirectional, the heart uses a system of four valves to prevent back flow of blood. The right atrioventricular valve, also known as the tricuspid valve, separates the right atrium from the right ventricle. The left atrioventricular valve, also known as the mitral or bicuspid valve, separates the left atrium from the left ventricle. The pulmonary semilunar valve separates the right ventricle from the pulmonary arteries. The aortic semilunar valve separates the left ventricle from the aorta.
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