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While it is convenient to describe the flow of blood through the right side of the heart and then through the left side, it is important to realize that both atria and ventricles contract at the same time.

When the ventricles relax, semilunar valves close to prevent blood from flowing back into the ventricles. When the ventricles contract, atrioventricular valves close to prevent blood from flowing back into the atria. The valve between the left ventricle and the aorta is the aortic semilunar valve. The valve between the right ventricle and pulmonary trunk is the pulmonary semilunar valve. The left atrioventricular valve is the bicuspid, or mitral, valve. The right atrioventricular valve is the tricuspid valve. The valves between the atria and ventricles are called atrioventricular valves (also called cuspid valves), while those at the bases of the large vessels leaving the ventricles are called semilunar valves. The heart has two types of valves that keep the blood flowing in the correct direction. Pumps need a set of valves to keep the fluid flowing in one direction and the heart is no exception. The right atrium receives deoxygenated blood from systemic veins the left atrium receives oxygenated blood from the pulmonary veins. Differences in thickness of the heart chamber walls are due to variations in the amount of myocardium present, which reflects the amount of force each chamber is required to generate. The two ventricles are thick-walled chambers that forcefully pump blood out of the heart. The two atria are thin-walled chambers that receive blood from the veins. The internal cavity of the heart is divided into four chambers: The outer layer of the heart wall is the epicardium, the middle layer is the myocardium, and the inner layer is the endocardium. Three layers of tissue form the heart wall. The visceral layer of the serous membrane forms the epicardium. The heart is enclosed in a pericardial sac that is lined with the parietal layers of a serous membrane. 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.