Human Anatomy, Part 2 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2008

Special Part
Cardiovascular system, systema cardiovasculare - Special Part
Heart - Heart Structure - Structure of the Heart Walls

The wall of The Heart is composed of three layers: the epicardium, epicardium; the myocardium, myocardium; and the endocardium, endocardium.

The epicardium is a serous membrane consisting of a Connective Tissue plate covered with mesothelium (a single-layered squamous epithelium) derived from the ventral mesoderm. Nerve and vascular plexuses are located within the epicardium.

The myocardium, or the muscular tunic of the heart, makes up the bulk of its wall (Fig. 14). It consists of specialized striated Muscle fibers, interspersed with Blood Vessels and connective tissue. The heart musculature comprises two anatomical divisions: the atrial muscle layers and the ventricular muscle layers. The atrial muscle layers are separated from the ventricular muscle layers by fibrous rings, anuli fibrosi, which allows the atria to contract independently of the ventricles.

The atrial wall consists of superficial and deep muscle layers. The superficial layer is formed by circular or transversely oriented fibers, while the deep layer consists of longitudinal fibers originating from the fibrous rings and looping around the atria. The fibers of the superficial layer enclose both atria, whereas the deep fibers surround each atrium separately.

The muscular tunic of the ventricles consists of three layers: outer longitudinal, middle circular, and inner longitudinal (Fig. 14). The outer and inner layers are common to both ventricles and merge into one another at the apex of the heart. The inner layer forms muscular trabeculae and papillary Muscles to which the chordae tendineae attach. The middle layer does not pass from one ventricle to the other, remaining independent for each of them.

The interventricular septum, septum interventriculare, is formed mainly by muscular tissue, pars muscularis; only in its upper region is there a connective tissue plate, pars membranacea. Both the muscular and membranous PARTS OF THE septum are covered with endocardium. Anomalies of the septum can occur in this area, manifesting as a septal defect that requires surgical intervention.

The Conduction system of the Heart, systerna conducens cordis, is located within the myocardium (Fig. 15). It is represented by the sinuatrial node, nodus sinuatrialis, the atrioventricular node, nodus atrioventriularis, the bundle of His*, and Purkinje fibers**. The conduction system has The ability to generate the cardiac contraction impulse and conduct it to all parts of the myocardium, thereby ensuring their coordinated contraction.

The sinuatrial node, nodus sinuatrialis (Keith-Flack node), is 1-2 mm in diameter and located in the wall of the right atrium between the right auricle and the opening of the SUPERIOR VENA CAVA. Bundles of fibers extend from the node to the atrial myocardium, ensuring its rhythmic contraction, as well as toward the orifices of the venae cavae and the atrioventricular node.

The atrioventricular node, nodus atrioventricualaris (Aschoff-Tawara node), is located in the wall of the right atrium near the septal cusp of the tricuspid valve. It is up to 5 mm long and up to 4 mm wide. The atrioventricular bundle, fasciculus atrioventricualaris (bundle of His), extends from the node into the interventricular septum. The bundle divides into right and left branches, crus dextrum et sinistrum, which branch beneath the endocardium into subendocardial branches, rami subendocardiales. The atrioventricular bundle ensures the rhythmic action of the atria and ventricles by transmitting the contraction wave from the atria to the ventricles (hence it is called the pacemaker). Excitation originates in the sinuatrial node, travels via specialized internodal pathways (Bachmann's bundle) to the atrial musculature and the atrioventricular node, and then via the atrioventricular bundle, bundle branches, and subendocardial branches reaches the ventricular muscle fibers, ensuring the necessary sequential contraction of the atria and ventricles.

The endocardium, endocardium, lines the inner surface of the atria and ventricles, including the papillary muscles, Valves, trabeculae, and chordae tendineae. The endocardium is formed by connective tissue rich in elastic fibers and smooth muscle Cells, covered by endothelium. The valve cusps are folds of the endocardium supported by a connective tissue core.

Blood supply to the heart is provided by the right and left coronary Arteries, a. coronaria dextra and a. coronaria sinistra (Figs. 16, 17). The coronary arteries originate from the ascending aorta within the aortic bulb, just below the upper margins of the semilunar valves. Blood enters the coronary arteries during diastole when the ostia of the coronary arteries are not obstructed by the semilunar valves.

The right coronary artery, a. coronaria dextra (Fig. 16), arises from the aorta to the right and posteriorly, lies in the coronary sulcus between the aorta and the right auricle, curves around the right border of the heart to reach the posterior surface, where it continues as the posterior interventricular branch, ramus interventricularis posterior, descending along the sulcus of the same name to the apex of the heart. The main trunk of the artery gives off the following branches within the coronary sulcus:

1. Atrioventricular branches, rami atrioventriculares;

2. Conus arteriosus branch, ramus coni arteriosi;

3. Sinuatrial node branch, ramus nodi sinuatrialis;

4. Atrial branches, rami atriales;

5. Right marginal branch, ramus marginalis dexter;

6. Intermediate atrial branch, ramus atrialis;

7. Posterior interventricular branch, ramus interventricularis posterior;

8. Interventricular septal branches, rami interventriculares septales;

9. Atrioventricular node branch, ramus nodi atrioventricularis.

* Wilhelm His Jr. (1863–1934), German clinician and professor of internal medicine in Göttingen and Berlin. Between 1880 and 1894, he described the atrioventricular bundle of the heart's conduction system, which was named after him (bundle of His).

** Jan Evangelista Purkyně (Purkinje), Iohannes Evangelista Purkinje (1787–1869), Czech scientist, professor of physiology at the University of Wrocław, founder of the Prague school of Histology, who described the specialized conduction fibers of the heart named after him (Purkinje fibers).

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Fig. 13. Heart from the left. Left atrium and left ventricle (opened), after R. D. Sinelnikov

Fig. 14. Heart from the left. Arrangement of the myocardial layers (after R. D. Sinelnikov)

Fig. 15. Conduction System of the heart, systema conducente cordis

Fig. 16. Arteries and Veins OF THE Heart (anterior view) (after R. D. Sinelnikov)

The right coronary artery, a. coronaria dextra, supplies blood to the right atrium, part of the anterior wall and the entire posterior wall of the right ventricle, a small area of the posterior wall of the left ventricle, the interatrial septum, and the posterior third of the interventricular septum.

The left coronary artery, a. coronaria sinistra (Fig. 17), arises from the aorta and lies in the coronary sulcus between the pulmonary trunk and the left auricle. In 70% of cases, it divides into two branches: the anterior interventricular branch, ramus interventricularis anterior, and the circumflex branch, ramus circumflexus. In approximately 30% of cases, the left coronary artery branches into three branches, and rarely into 4–5 branches.

Fig. 17. Arteries and veins of the heart (posterior view) (after R. D. Sinelnikov)

The anterior interventricular branch, ramus interventricularis anterior, in most cases has a diameter of 3 mm, runs together with the great cardiac vein in the sulcus of the same name to the apex of the heart, where it anastomoses with the posterior interventricular branch of the right coronary artery. The anterior interventricular branch gives rise to:

1. Conus arterial branch, ramus coni arteriosi;

2. Lateral branch, ramus lateralis;

3. Interventricular septal branches, rami interventriculares septales.

The circumflex branch, ramus circumflexus, runs in the coronary sulcus and gives off the following branches:

1. Anastomotic atrial branch, ramus atrialis anastomaticus;

2. Atrioventricular branches, rami atrioventriculares;

3. Left marginal branch, ramus marginalis sinister;

4. Intermediate atrial branch, ramus atrialis intermedius;

5. Posterior branch of the left ventricle, ramus posterior ventriculi sinistri;

6. Inconstant sinuatrial nodal branch, ramus nodi sinuatrialis;

7. Atrioventricular nodal branch, ramus nodi atrioventricularis;

8. Atrial branches, rami atriales.

The left coronary artery supplies the left atrium, the entire anterior wall and the greater part of the posterior wall of the left ventricle, part of the anterior wall of the right ventricle, and the anterior 2/3 of the interventricular septum.

The CORONARY ARTERIES AND their areas of blood supply are variable. There are Three types of cardiac blood supply: right-coronary type (60–84%) — with a predominance of blood supply to the heart areas by the right coronary artery (Fig. 18); left-coronary type (7–14%) — with a predominance of blood supply to the heart areas by the left coronary artery (Fig. 19); and balanced, or symmetric type (10–28%) — in which the branching of both arteries is approximately equal.

The cardiac arteries branch into smaller arteries and further into arterioles within all layers of the heart wall. These arterioles divide into the intramural capillary bed (Figs. 18, 19), the capillaries of which run parallel to the muscle bundles and periodically form anastomotic loops. Anastomoses exist between coronary arteries of various branching levels, providing collateral Coronary Circulation when necessary.

The primary collateral pathways in the heart include:

✵ connections between the circumflex branch of the left coronary artery and the right coronary artery;

✵ anastomoses between the septal interventricular Branches of the right and left coronary arteries;

✵ connections between the anterior and posterior interventricular branches;

✵ anastomoses between the Arteries of the epicardial arterial network, including anastomoses between epicardial and pericardial arteries;

✵ anastomoses between the arteries of the subendocardial arterial network;

✵ anastomoses with the arteries of the adventitia and vascular walls.

Fig. 18. Right-dominant type of cardiac blood supply. Coronary Vessels of the heart injected with a lead contrast medium (according to P. A. Sokolov)

Fig. 19. Left-dominant type of cardiac blood supply. Coronary vessels of the heart injected with a lead contrast medium (according to P. A. Sokolov)