The Heart To move blood within the circulatory system a pump is necessary. This is accomplished by a four chambered pump composed of specialized muscle tissue, connective tissue and an electrical conduction system. The interior of the heart is lined by the endocardium that is homologous with the intima of the blood vessels. The heart is surrounded by a fibrous double-walled pericardial sac that contains a proteinaceous lubricating fluid between the layers. The upper thin walled chambers of the heart, the atria, are separated from the lower, more muscular chambers, the ventricles, by valves that open and close in a coordinated manner to efficiently pump blood to the lungs for gas exchange and to the rest of the body.
Blood enters the heart via the vena cava into the right atrium.
From the right atrium the blood flows through the tricuspid valve into the right ventricle that serves to pump blood to the lungs for the exchange of gases. The blood is ejected through valves in the pulmonary artery. Upon return from the pulmonary vascular system the blood enters the left atrium via the pulmonary veins. Blood flows through the mitral valve to the thicker walled left ventricle that serves to forcefully pump blood through the aortic valve into the aorta and to the systemic vascular system. The heart beats in an orderly sequence due to the characteristics of the heart muscle and a specialized electrical conduction system.