The heart is located between the lungs, with its point or apex directed toward the left. The thick muscle layer of the heart wall is the myocardium. This is lined on the inside with a thin endocardium and is covered on the outside with a thin epicardium. The heart is contained within a fibrous sac, the pericardium.
Each of the upper receiving chambers of the heart is an atrium (plural, atria). Each of the lower pumping chambers is a ventricle (plural, ventricles). The chambers of the heart are divided by walls, each of which is called a septum. The interventricular septum separates the two ventricles; the interatrial septum divides the two atria. There is also a septum between the atrium and ventricle on each side.
The heart pumps blood through two circuits. The right side pumps blood to the lungs to be oxygenated through the pulmonary circuit. The left side pumps to the remainder of the body through the systemic circuit.
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