The heart is a muscular pump that maintains unidirectional flow of blood.
The heart contains four chambers—the right and left atria and right and left ventricles—through which blood is pumped (see Fig. 13.1). Valves guard the exits of the chambers, preventing backflow of blood. An interatrial septum and an interventricular septum separate the right and left sides of the heart. The right side of the heart pumps blood through pulmonary circulation. The right atrium receives blood returning from the body via the inferior and superior venae cavae, the two largest veins of the body (Fig. 13.3). The right ventricle receives blood from the right atrium and pumps it to the lungs for oxygenation via the pulmonary arteries. The left side of the heart pumps blood through systemic circulation. The left atrium receives the oxygenated blood returning from the lungs via the four pulmonary veins. The left ventricle receives blood from the left atrium and pumps it into the aorta for distribution into the remainder of the body.
FIGURE 13.3 • Diagram of the blood circulation. This diagram shows the right and left side of the heart artificially separated. The right side of the heart pumps blood through the low-pressure pulmonary circulation. The right atrium receives deoxygenated blood returning from the body via the inferior and superior venae cavae. The right ventricle receives blood from the right atrium and pumps it to the lungs for oxygenation via the pulmonary arteries. The left side of the heart pumps blood through the high-pressure systemic circulation. The left atrium receives the oxygenated blood returning from the lungs via the four pulmonary veins. The left ventricle receives blood from the left atrium and pumps it into the aorta for systemic distribution.
The heart contains the following:  A musculature of cardiac muscle for contraction to propel the blood.
 A fibrous skeleton that consists of four fibrous rings surrounding the valve orifices, two fibrous trigones connecting the rings, and the membranous part of the inter-ventricular and interatrial septa. The fibrous rings are composed of dense irregular connective tissue. They encircle the base of the two arteries, leaving the heart (aorta and pulmonary trunk) and the openings between the atria and the ventricles (right and left atrioventricular [AV] orifices) (Fig. 13.4). These rings provide the attachment site for the leaflets of all four valves of the heart that allow blood flow in only one direction through the openings. The membranous part of the interventricular septum is devoid of cardiac muscle; it consists of dense connective tissue that contains a short length of the atrioventricular bundle of the conducting system of the heart. The fibrous skeleton provides independent attachments for the atrial and ventricular myocardium. It also acts as an electrical insulator by preventing the free flow of electrical impulses between atria and ventricles.
 A conducting system for initiation and propagation of rhythmic depolarizations, which results in rhythmic cardiac muscle contractions (Fig. 13.5). This system is formed by modified cardiac muscle cells (Purkinje fibers), which generate and conduct electrical impulses rapidly through the heart. In the sudden cessation of normal heart rhythm leading to abrupt cessation of blood circulation called cardiac arrest, the conducting system of the heart fails to produce or conduct electrical fibrous ring of pulmonary conus trunk ligament left fibrous fibrous ring trigone of tricuspid fibrous ring of mitral valve
FIGURE 13.4 • Fibrous skeleton of the heart as seen with the two atria removed. This fibrous network (indicated in blue) serves for the attachment of cardiac muscle; it also serves for the attachment of the cuspid valves between the atria and ventricles and for the semilunar valves of the aorta and the pulmonary artery. The atrioventricular bundle passes from the right atrium to the ventricular septum via the membranous septum of the fibrous skeleton.