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Differences in ventricular muscle thickness

In this figure the left panel shows the muscles of the heart in the relaxed position, and the right panel shows the muscles of the heart in contracted position.
The myocardium in the left ventricle is significantly thicker than that of the right ventricle. Both ventricles pump the same amount of blood, but the left ventricle must generate a much greater pressure to overcome greater resistance in the systemic circuit. The ventricles are shown in both relaxed and contracting states. Note the differences in the relative size of the lumens, the region inside each ventricle where the blood is contained.

Internal structure of the heart

Recall that the heart’s contraction cycle follows a dual pattern of circulation—the pulmonary and systemic circuits—because of the pairs of chambers that pump blood into the circulation. In order to develop a more precise understanding of cardiac function, it is first necessary to explore the internal anatomical structures in more detail.

Septa of the heart

There are four openings that allow blood to move from the atria into the ventricles and from the ventricles into the pulmonary trunk and aorta. Located in each of these openings between the atria and ventricles is a valve    , a specialized structure that ensures one-way flow of blood. The valves between the atria and ventricles are known generically as atrioventricular (AV) valves . The valves at the openings that lead to the pulmonary trunk and aorta are known generically as semilunar valves    .

Internal structures of the heart

In this figure the top panel shows the image of the heart with the major parts labeled. The bottom left panel shows a photo of the heart with the surface layer peeled off. The images on the bottom right show detailed musculature inside the heart.
This anterior view of the heart shows the four chambers, the major vessels and their early branches, as well as the valves.

Right atrium

The right atrium serves as the receiving chamber for blood returning to the heart from the systemic circulation. The two major systemic veins, the superior and inferior venae cavae, and the large coronary vein called the coronary sinus that drains the heart myocardium empty into the right atrium. The superior vena cava drains blood from regions abover the diaphragm: the head, neck, upper limbs, and the thoracic region. It empties into the superior and posterior portions of the right atrium. The inferior vena cava drains blood from areas below the diaphragm: the lower limbs and abdominal and pelvic region of the body. It, too, empties into the posterior portion of the atria, but is below the opening of the superior vena cava. The majority of the internal heart structures discussed in this and subsequent sections are illustrated in the provided figure.

The atria receive venous blood on a nearly continuous basis, preventing venous flow from stopping while the ventricles are contracting. While most ventricular filling occurs while the atria are relaxed, they do demonstrate a contractile phase and actively pump blood into the ventricles just prior to ventricular contraction. The opening between the atrium and ventricle is guarded by the tricuspid valve.

Right ventricle

The right ventricle receives blood from the right atrium through the tricuspid valve. Each flap of the valve is attached to strong strands of connective tissue, the chordae tendineae    , literally “tendinous cords,” or sometimes more poetically referred to as “heart strings.” There are several chordae tendineae associated with each of the flaps. They are composed of approximately 80 percent collagenous fibers with the remainder consisting of elastic fibers and endothelium. They connect each of the flaps to a papillary muscle    .

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