Medicine
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Anatomy of human heart
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Anatomy of human heart
The heart is located in the mediastinum, between the two lungs, above the tendinous center of the diaphragm; anteriorly it is in direct contact with the internal thoracic wall, immediately to the left of the sternum, approximately at the level of the IV and V intercostal spaces. It is a muscular organ about the size of a man’s fist, cone-shaped, slightly flattened vertically, with the base turned upward and the apex downward; its axis is obliquely rotated, directed forward and downward, so that the right ventricle is slightly more anterior than the left. The dimensions of the heart vary according to the physique of the individual, and the weight of the adult male is about 300 grams.
Externally, the heart is covered by the pericardium, a serous membrane that surrounds it like a soft sac, consisting of two layers that usually fit together and appear as a thin connective lamina covered by a simple pavement epithelium. Internally it is divided into four cavities, two upper, the right and left atria (or auricles) and two lower, the right and left ventricles.
The boundary between the atria and the ventricles appears to be marked externally by the coronary groove, which surrounds the entire heart in a ring; the boundary between the right and left ventricles is marked by the anterior and posterior longitudinal grooves, which originate from the coronary groove and merge into each other a little to the right of the apex of the heart. Between the two atria there is a boundary, the interatrial sulcus, noticeable on the external surface, at the base and the diaphragmatic surface of the atria themselves.
While the atria communicate with the underlying ipsilateral ventricles through valvular orifices, neither the two atria nor the two ventricles communicate with each other; for this reason, we can consider the heart as clearly divided into two lateral halves (separated by a septum, interatrial in the upper part, interventricular in the lower part), a right one (right heart) in which the venous blood circulates, and a left one (left heart) in which the arterial blood circulates.
During intrauterine life, the two parts communicate with each other through an opening that connects the two atria, the Botallo’s hole, which is obliterated after birth. The atria are irregularly cubic, smooth-walled, with a capacity of about 150-200 cm3 each, and end in a kind of diverticulum, similar to a triangular pyramid, the auricle. There are three veins in the right atrium: the inferior vena cava, the superior vena cava and the great coronary vein, which has a dilatation just before it enters the heart (coronary sinus); in the left atrium there are the pulmonary veins, which carry oxygenated blood and are therefore red.
The ventricles are cone-shaped, flattened on the septal surface, also with a capacity of about 150-200 cm3 each. From the right ventricle starts the pulmonary artery (in which venous blood flows), from the left ventricle starts the aorta. The orifices of the two arteries are formed by valves consisting of three membranous flaps (cusps) with a semilunar shape that resemble swallow’s nests, like pockets that open upwards: the blood introduced into the arteries can no longer flow back into the heart because these pockets, as they fill up, expand and close together, closing the valve orifice.
Between the atria and the ventricles, the passage of blood is regulated by two other valves (atrioventricular), similar to a funnel, which protrude into the ventricles; on the right, the tricuspid, and on the left, the bicuspid or mitral. The leaflets of the valves (three and two, respectively), made up of strong fibrous tissue and endocardium, allow the blood to flow from the auricles to the underlying atria and not vice versa, because they are held in place by tendons that, starting from the papillary muscles, engage the free edges of the leaflets.
The walls of the ventricles, thick and strong, are very irregular on the inner surface due to the presence of membranous formations, tendinous threads and muscular prominences called fleshy columns, due to their color similar to that of flesh, although they are covered by the endocardium; in the adult, these columns are intertwined and anastomosed among themselves, forming a network of trabeculae. These muscular prominences are of three types: some are attached to the heart by only one of their ends, while the other is free (fleshy columns of the first order); others are attached at both ends but free in their course (columns of the second order); still others remain all attached to the walls of the myocardium (columns of the third order).
There are some differences between the two ventricles: the muscular walls of the left ventricle are thicker (even three times thicker) than those of the right ventricle, whose cavity (more like a pyramid than a cone) is wider than the contralateral one. The internal structure of the heart can be considered as composed of three tunics or layers: the outermost is the pericardium, already mentioned, the innermost is the endocardium, a thin membrane with a shiny appearance (formed by an endothelium supported by connective tissue) that completely covers the walls of the cardiac cavities, adapting to all irregularities. Between these two tunics there is a muscular layer, the actual myocardium or myocardium, which is the thickest part, on which exactly depends the volume of the walls of the heart and which is formed by striated muscle fibers, each distinct from the other, but anastomosed to each other.
The myocardium has a reddish-brown color, similar to that of the muscles, with some yellowish streaks due to the presence of fatty tissue; in older individuals, yellowish-white streaks and plaques are also visible as a result of fibrous degeneration. The consistency of the myocardium is not uniform: it appears greater in the pumping part, corresponding to the ventricles, where the muscle tissue is thicker and more robust, while it is thinner in the atrial part. The myocardium is supplied by two arteries, the right and left coronary arteries, whose branches, anastomosed to each other, penetrate the interstitial tissue of the myocardium; the blood is then collected by veins that lead to the coronary sinus or, sometimes, flow directly into the atria. The heart is innervated by nerves that come from the cardiac plexus, which is made up of sympathetic nerve fibers, branches of the vagus nerve, and the pulmonary plexus.
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