Heart
In humans, like other mammals, the heart (fig. 189) is a four-chambered muscular organ, the function of which consists in) rhythmic absorption of blood (during the relaxation of the walls of the heart chambers - diastole) and pumping it into the blood network during systole - contraction of the heart chambers. Each person's heart is the size of a fist and shaped like a cone.
The apex of the heart is directed down, to the left and forward, and the base - up, to the right and back. Thus, the long axis of the heart goes from its apex from the bottom up, from left to right and from front to back (approximately 12-13 cm long). The largest transverse size of the heart is 9-10 cm, and the anteroposterior one is 6-7 cm. The average weight of a man's heart is 300 g, and nothing - 200 g.
There are four surfaces of the heart (sternocostal, diaphragmatic, two pulmonary) and the right edge. The sternocostal (front) surface is convex, touching the chest wall only with the top part. The diaphragmatic (lower) surface is completely adjacent to the diaphragm located on the dense liver, as a result of which this surface of the heart is flattened. Pulmonary surfaces are elevated to the lungs. The right edge belongs to the wall of the right ventricle.
On the surface of the heart are located: the coronal furrow, at the border between the atria and ventricles, the anterior and posterior interventricular furrows. The last two furrows, which run longitudinally on the border between the ventricles. At the top of the heart, they merge into one another, and from above (in front and behind) connect with the coronal groove of the heart (see fig. 190).
The heart has the right and left atria, which occupy the smaller part of the heart at its base, and the right and left ventricles, which make up the larger part.
The atria are separated by the atrial septum, and the ventricles by the interventricular septum.
The shape of the right atrium resembles an irregular parallelepiped with six walls. The upper wall is the confluence, or sinus, of the vena cava. Here, on the side of the atrial cavity, there are two large openings of the upper and lower vena cava, between which the intervenous tubercle is located.
The back, right and front walls of the right atrium are formed by crestal muscles. The left wall is the interatrial septum, on the right surface of which an oval fossa with a clearly demarcated edge – the border of the oval fossa – is clearly visible. At the place of the fossa, the fetus has an open oval hole, which, as a rule, grows over soon after birth. At the junction of the front and right walls, the atrial cavity continues into the hollow conical right auricle. A valve extends from the lower edge of the fossa ovalis to the lower part of the opening of the inferior vena cava, thanks to which in the fetus almost all blood from the inferior vena cava is directed through the oval opening into the left atrium. Below the valve, on the border of the back and left walls of the right atrium, the coronary sinus opens, which collects venous blood from most of the veins of the heart. On the right side, the hole of the coronary sinus is covered by the valve of the coronary sinus.
Thus, the upper and lower vena cava and the coronary sinus carry venous blood to the right atrium. In addition, part of the smallest veins of the heart flow into the right atrium (some of them flow into other chambers of the heart. The lower wall of the right atrium is occupied by a wide right atrioventricular opening leading to the right ventricle.
The right ventricle is very similar to an irregular pyramid, the base of which is directed upwards, and the apex - downwards. The walls of the right ventricle are relatively thin (5-8 mm). The anterior (sternocostal) and posterior (diaphragmatic) surfaces are separated by the right edge. The left wall is formed by the interventricular septum, which is very protruding into the cavity of the right ventricle. The front (narrowed) part of the right ventricle forms an arterial cone, which passes directly into the pulmonary trunk at the top.
The right ventricle has two openings: the right atrioventricular opening, located above, to the right and behind, and the pulmonary trunk opening.
The inner surface of the ventricle is covered with numerous multidirectional interstices, among which the interstice-marginal one stands out the most, connecting the septum with the base of the anterior papillary muscle. Between the interstices are located (sometimes 2-4) cone-shaped *anterior, posterior and septal) papillary muscles, which freely protrude into the cavity of the ventricle.
The right atrioventricular opening has the same name, or tricuspid, valve, which consists of three leaflets: anterior, posterior and septal. Thin tendon strings are fixed to the free edge of each leaflet, which depart from the papillary muscles. These strings keep the valve tightly closed during ventricular systole.
The pulmonary trunk opening also has a valve of the same name, which consists of three semilunar (like flaps) valves: anterior, right and left. In the middle of the free edge of each semilunar valve there is a nodule the size of a poppy seed. These nodules during ventricular diastole contribute to tighter closure of the valves.
The left atrium differs little from the right one externally. From the inside, the walls of the left atrium, unlike the walls of the right one, are smooth, with the exception of a small area near the left auricle. In the dorsal part of the upper wall of the left atrium, the openings of the right and left pulmonary veins open in pairs (at some distance from each other). On the lower wall of the left atrium there is a left atrioventricular opening, leading to the left ventricle.
The left ventricle resembles a pyramid with the base turned upwards. The anterior and left (corresponding to the pulmonary surface) walls of the left ventricle are the most powerful walls of the heart (the thickness reaches 2 cm). The right wall of the left ventricle is formed by the interventricular septum, the upper third of which is membranous, and the lower 2/3 - muscular tissue.
In the cavity of the left ventricle there are numerous fleshy folds, and as a rule, in (anterior and posterior) large papillary muscles. At the base, that is, on the upper wall of the left ventricle, there are two large openings: the left atrioventricular opening and to the right of it - the aortic opening.
The left atrioventricular opening during systole closes the valve of the same name, or mitral, consisting of anterior and posterior leaflets. Tendon chords from the anterior and posterior papillary muscles are fixed to the free edge of each valve.
The heart wall consists of three membranes: the outer one - epicardium, the middle one - myocardium and the inner one - endocardium. It has a different thickness, depending on the development of the muscular membrane.
The atrial myocardium is separated from the ventricular myocardium by the fibrous skeleton of the heart. It consists of fibrous rings lying around the right and left atrioventricular openings, and the right and left fibrous triangles lying in the same plane between the left atrioventricular and aortic openings.
The ventricular myocardium consists of three layers of muscle bundles: the outer and inner oblique longitudinal (common to both ventricles) and the middle circular, separate for each ventricle.
The work of the heart is regulated by the central nervous system and the stylish complex of the heart, or the conduction system of the heart, which consists of nodes of nerve cells and neuromuscular fibers. The nerve impulse arises in the cells of the sinoatrial node, located under the epicardium between the superior and inferior vena cava. From here, the fibers go to the muscles of the atrium and the atrioventricular node, which is located under the endocardium in the atrial wall near the septal leaflet of the atrioventricular valve. From this node, the atrioventricular bundle leaves, which at the beginning of the interventricular septum splits into the right and left legs, which branch in the walls of the corresponding ventricles.
Blood supply. Unlike other organs, the heart has its own circulatory system, almost not connected with the general bloodstream. The heart has right and left coronary arteries, which begin at the bulb of the aorta immediately after it exits the left ventricle, below the free edge of the right and left halves of the local aortic valves.