Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Cardiovascular System
Heart
Chambers of the Heart

In adults, the right and left halves of The Heart are separated by a solid longitudinal septum (Fig. 2.1). The part of the septum dividing the atria is called the interatrial septum, and the one dividing the ventricles is the interventricular septum. On each side, the atrium communicates with the ventricle through the atrioventricular opening, through which Blood is pumped into the ventricle during atrial contraction.

The right atrium (atrium dextrum) receives the SUPERIOR VENA CAVA from above and the wider INFERIOR VENA CAVA from below and behind (Atl. Figs. 52, 53). Just below the latter, near the septum, lies the opening of the coronary sinus—which drains blood from the heart walls—along with several inconstant openings of the small cardiac Veins. The atrium communicates with the cavity of the right ventricle via the atrioventricular opening. On the interatrial septum, opposite the orifice of the inferior vena cava, the oval fossa is clearly visible as a remnant of the fetal foramen ovale. In the fetus, blood flows through this opening directly from the inferior vena cava into the left atrium. The septum is thinner at this site and may occasionally feature a slit-like opening as an anatomical variation.

The right auricle is an accessory reservoir of the atrium; it is relatively voluminous and shaped like a triangular pouch. Due to the uneven distribution of cardiac Muscle bundles across the internal surface of the atrium and its auricle, nearly parallel ridges known as pectinate Muscles stand out.

The right ventricle (ventriculus dexter) is located slightly anterior to the left one (Atl. Figs. 52, 53) and is demarcated on the heart's surface by the anterior and posterior interventricular sulci. It is separated from the atrium by the coronary sulcus. The outer margin of the ventricle is acute, the anterior wall is convex, and the posterior wall is flattened. The interventricular septum is concave toward the left ventricle. In cross-section, the cavity of the right ventricle is slit-like at the margins and more pronounced near the outer edge (Fig. 2.2). The internal surface of the ventricle is uneven, featuring a complex network of trabeculae carneae that merge into cone-shaped papillary muscles, which vary in number (typically three) and size.

In the upper part of the ventricle, There are two openings: the posterior atrioventricular opening and the anterior arterial opening, which leads into the pulmonary trunk. Toward the latter, the ventricle tapers conically upward, forward, and to the left.

The atrioventricular opening is guarded by the tricuspid valve (Fig. 2.3), which consists of thin, smooth, triangular cusps. These are folds (duplications) of the inner lining of the heart—the endocardium—containing a small amount of Connective Tissue and muscle fibers. Delicate tendinous cords (chordae tendineae), originating from the papillary muscles, attach to the free margins of the cusps, with each muscle sending fibers to multiple cusps. When blood flows from the atrium into the ventricle, the valve cusps are pressed against the ventricular walls. During ventricular contraction, the blood pressure forces the cusps to close, while the taut chordae tendineae prevent them from everting into the atrium, thereby ensuring complete Separation of the chambers.

The entrance from the ventricle into the pulmonary trunk is closed by three semilunar Valves (Fig. 2.3), formed, like the tricuspid valve, by duplications of the endocardium. These valves form pocket-like recesses facing the pulmonary trunk. Their outer margins feature thickened nodules that ensure a tight seal. During ventricular diastole, the valves fill with blood, which prevents backflow from the pulmonary trunk into the right ventricle.

The left atrium (atrium sinistrum) lies posterior to the right atrium (see Atl. pp. 58, 59). Its walls are thinner than those of the right atrium. Its right wall is formed by the interatrial septum, and its floor by the wall of the left ventricle. The left auricle extends from the anterosuperior wall, overlapping the Base of the pulmonary trunk. Posteriorly and superiorly, there are four openings through which the Pulmonary veins enter—two from each lung. These veins convey oxygenated blood from the Lungs to the heart. Like the openings of the venae cavae, these orifices lack valves. Most of the internal surface of the atrium is smooth, except for the interatrial septum, where the oval fossa—the remnant of the foramen ovale—is visible. The internal surface of the left auricle is lined with numerous pectinate muscles.

The left ventricle (ventriculus sinister) occupies the posteroinferior part of The Heart and appears oval or circular in cross-section (Fig. 2.2). Its internal surface is covered by a multitude of muscular trabeculae that form an intricate network, which is particularly prominent near the apex of the heart. These trabeculae continue into the papillary muscles.

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Fig. 2.2. Heart — cross-section through the ventricles:

1 — anterior interventricular sulcus; 2 — right ventricle; 3 — interventricular septum; 4 — posterior interventricular sulcus; 5 — epicardium (visceral layer); 6 — myocardium; 7 — chordae tendineae; 8 — left ventricle; 9 — papillary muscles

The left atrium communicates with the left ventricle through the atrioventricular opening. The cusps of the bicuspid (mitral) valve attach to its margins. The Structure of these cusps is similar to that of the corresponding valve in the right half of the heart. Chordae tendineae, originating from the papillary Muscles of the ventricular wall, attach to the serrated edges of the cusps.

The anterior part of the left ventricle forms the aortic vestibule, communicates with the aortic orifice, and is bounded by three semilunar valves (Fig. 2.3). It is located anterior and to the right of the atrioventricular opening. The structure of these valve cusps resembles that of the semilunar valves of the pulmonary trunk. The functional mechanism of all valves in the left ventricle is identical to that in the right.

Fig. 2.3. Valve apparatus of the heart:

1 — aortic orifice (semilunar valve); 2 — right atrioventricular orifice (tricuspid valve); 3 — right ventricle; 4 — left atrioventricular orifice (bicuspid/mitral valve); 5 — left ventricle; 6 — orifice of the pulmonary trunk (semilunar valve)