The heart is a muscular organ, which constitutes the motor center of the circulatory system and propeller of blood and lymph in several animal organisms, including humans, in which it is formed by a particular tissue, the myocardium, and is covered by a membrane, the pericardium.
Most primitive worms do not possess a circulatory system: it is instead present in many phyla of higher invertebrates: molluscs, annelids, arthropods, echinoderms. The circulatory system of echinoderms is rather rudimentary. Annelids have a closed circulatory system, while arthropods, although related to annelids, have developed an open circulatory system. The heart is located dorsal to the digestive tract, within an expanded portion known as the pericardial sinus. It has a simple structure: it is divided into two parts (atrium and ventricle), and an anterior aorta and a posterior aorta branch off from it. In molluscs there is always only one ventricle while the atria are as many as the gills from which the arterial blood flows.
In all vertebrates the circulatory system is closed. The heart is always located ventral to the digestive tract and presents in different classes morphological and functional aspects sometimes very different, especially in relation to different types of respiration (gill or lung) and the corresponding types of circulation (double or simple, complete or incomplete). In cyclostomes the heart is formed by a single atrium and a single ventricle communicating with each other through an opening equipped with valves.
In the cartilaginous fishes, during the embryonic development, the vascular segment from which the heart originates is folded to “S” so that the ventricle is in a ventral position with respect to the atrium; in these animals is also characteristic the presence of an arterial cone interposed between the ventricle and the root of the aorta equipped with 2-5 series of valves and that can be considered a portion of the heart as it has a striated musculature typically cardiac. In the exclusively gill-breathing bony fishes, the anatomical conformation of the heart is more or less the same but, with the exception of few species, the arterial cone becomes very short while the aortic bulb develops.
In the dipnoi, the heart is partially divided into a right and a left half by a longitudinal septum extended both to the atrium and to the ventricle, even if during the systole, when the lumen of the cardiac cavities narrows, the separation can be considered almost perfect; there is an arterial cone also divided into two halves by an incomplete septum resulting from the fusion of the valves. In adult amphibians, the heart is formed by two atria located in a cranial position with respect to the ventricle, which is unique; the atria are separated by a septum (continuous in frogs, perforated in other amphibians) and communicate with the underlying ventricle through an elongated foramen, unique for both; the ventricle is not septate but has walls that prevent quite effectively the mixing of blood from the two atria. Also in these animals there is an arterial cone incompletely divided by a free-margin septum (spiral valve) that accomplishes an effective separation of the cavity into two halves when the cone in systole narrows. In dipnoi and amphibians, the major circulatory problem is related to the fact that lungs develop in more or less complete replacement of the gills as respiratory organs.
The heart therefore receives two kinds of blood, venous from the various parts of the body, and oxygenated from the lungs, which must be kept separate. The perfect solution of this problem has been achieved only in birds and mammals. In reptiles there are two separate atria, communicating with the ventricles by means of two distinct foramina; the ventricles are separated by an incomplete septum in chelonians and squamates, almost complete in crocodiles; the conus arteriosus is missing. In reptiles, therefore, some mixing of the two types of blood is possible.
In birds and mammals, animals with complete double circulation, there is a complete ventricular septum so that the two circulatory streams are separated along the length of the heart chambers. The heart of birds and mammals is divided into a left arterial half and a right venous half with anatomical features almost identical to those of the human heart. Venous blood from the body enters the right atrium, into which the primitive venous sinus has been incorporated. Arterial blood coming from the lungs enters the left atrium. From the atria, the blood passes into the ventricles and from there to the pulmonary artery, which, starting in the right ventricle, leads to the lungs, and to the arch of the aorta, coming from the left ventricle, which supplies the rest of the body.
The heart is situated 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 height of the IV and V intercostal spaces. It is a muscular organ approximately the size of a man’s fist, cone-shaped, slightly flattened vertically, with the base turned upward and the apex downward; its axis is rotated obliquely, directed forward and downward, so that the right ventricle is located a little more anterior than the left. The measurements of the heart vary according to the build of the individual and, in the adult man, the weight is about 300 g.
Externally, the heart is covered by the pericardium, a serous membrane that wraps around it like a soft sac, formed by two sheets, usually matching each other and appearing as a thin connective lamina, covered by simple pavimentous epithelium. Internally it is divided into four cavities, two upper, the right and left atria (or auricles) and two lower, the ventricles, right and left.
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 continue 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, appreciable on the external surface, at the base and diaphragmatic face of the atria themselves.
While the atria communicate, through valvular orifices, with the underlying ipsilateral ventricles, both the two atria and the two ventricles do not 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 circulates the venous blood, and a left one (left heart) in which circulates the arterial one.
During intrauterine life the two parts are communicating with each other, through an orifice 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, extending into a sort of diverticulum, similar to a triangular pyramid, the auricle. In the right atrium three veins flow: the superior cava, the inferior cava and the great coronary vein, which, just before ending in the heart, has a dilation (coronary sinus); in the left atrium come, instead, the pulmonary veins, which carry oxygenated blood and therefore red.
The ventricles are cone-shaped flattened on the septal face, also with capacity around 150-200 cm3 each. From the right ventricle begins the pulmonary artery (in which venous blood flows), from the left one starts the aorta. The orifices of the two arteries are formed by valves consisting of three membranous flaps (cusps) with a semi-lunar shape, which, like swallow’s nests, appear similar to pockets open upwards: the blood introduced into the arteries can no longer flow back into the heart, because these pockets, as they fill up, dilate and fit closely together, thus occluding the valve orifice.
Between the atria and ventricles, the passage of blood is regulated by two other valves (atrioventricular), similar to a funnel protruding into the ventricles; on the right there is the tricuspid, on the left the bicuspid or mitral. The flaps of the valves (three and two, respectively), consisting of strong fibrous tissue and the endocardium, allow the flow of blood from the auricles to the underlying atria and not vice versa, because they are held by cords of tendon that, starting from the papillary muscles, engage the free edges of the flaps.
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 meat, although they are covered by the endocardium; in the adult these columns are intertwined and anastomosed between them, forming a network of trabeculae. These muscular prominences are of three types: some are adherent to the heart with only one of the extremities, while the other is free (first-order fleshy columns); others are fixed at both ends, but free in their course (second-order columns); still others all remain adherent to the walls of the myocardium (third-order columns).
There are some differences between the two ventricles: the muscular walls of the left ventricle are stronger (even three times thicker) than those of the right one, whose cavity (a little more like a pyramid than a cone) is wider than the contralateral one. The inner structure of the heart can be considered composed of three tunics or layers: the outermost is the pericardium, which has already been mentioned, the innermost is the endocardium, thin membrane with a shiny appearance (formed by endothelium supported by connective tissue) that covers entirely, adapting to all irregularities, the walls of the cardiac cavities. Between these two tunics there is a muscular layer, the myocardium proper, or cardiac muscle, which is the thickest part, the one on which depends precisely the volume of the walls of the heart and is formed by striated muscle fibers, each distinct from the other, but anastomosed to each other.
The myocardium, externally, has a reddish-brown color, similar to that of muscles, with some yellowish streaks due to the presence of adipose tissue; in older individuals are also visible yellowish-white stripes and plaques, a consequence of fibrous degeneration. The consistency of the myocardium is not uniform: it appears greater in the pumping portion, corresponding to the ventricles, where the muscle tissue is thicker and more robust, while it is thinner in the atrial part. The myocardium is fed by two arteries, the right and left coronary arteries, whose branches, anastomosed to each other, penetrate into the interstitial tissue of the heart muscle; the blood is then collected by veins that lead to the coronary sinus or that flow, sometimes, directly into the atria. The heart is innervated by nerves arising from the cardiac plexus, which is formed by sympathetic nerve fibers, branches of the vagus and the pulmonary plexus.
The heart can be compared to a suction and pressure pump that, in its internal cavities, receives the waste blood from the veins and pushes it into the arteries. During its physiological activity, the heart changes continuously in shape and volume, alternating between diastole (relaxation of the myocardium) and systole (contraction of the heart muscle). If in the first phase the appearance of the heart is rather similar to that already described, in systole it presents modifications especially in the ventricles, on whose left anterior wall appears a kind of hump (which comes to impact against the chest wall, allowing the perception of the so-called heartbeat), while the cavities are reduced to a jagged and branched fissure.
The myocardium contracts, as a rule, 70-75 times per minute (equivalent to about one hundred thousand times in a day). During the cycle or cardiac revolution can be distinguished three essential phases: a presystole, characterized by contraction of the atria only and ventricular relaxation (or diastole) (during which the blood passes from the atria to the ventricles); a systole proper (with ventricular contraction, during which the blood is pushed into the arterial circulation) and a perisystole, characterized by complete rest of the heart. These functions occur rhythmically and their complex allows the circulation of blood in the body. The contraction of the myocardium begins at a point (sinus node) of the terminal groove of the right atrium, spreads to the two atria and finally reaches the ventricles, investing them from the apex towards the base.
The propagation of contraction is due to specialized muscle tissue: in the atria, presystole is under the control of the sinus node (consisting of the Keith and Flack node on the right and the Pace and Bruni node on the left); the Tawara node and the His bundle, on the other hand, are responsible for the spread of contraction from the atria to the ventricles. These nodes and bundles are respectively composed of entanglements and cords of contractile fibers, stimulated by excitatory nerve fibers from the cardiac plexus and by inhibitory fibers arising from the vagus. Specific characteristics of the myocardium are therefore the automatism (the ability to produce stimuli), the excitability and, finally, the contractility (the ability to contract, performing a job).
Diseases of the heart (heart disease) occupy today statistically the first place among the causes of death. They are grouped into acquired, congenital, traumatic and tumor-related heart diseases. Acquired heart disease can arise from infectious diseases, degenerative diseases (sclerosis), metabolic diseases (gout, diabetes), endocrine glands (Basedow’s disease), toxic factors (tobacco, alcohol), hypoxia, work and eating disorders. According to the localization of the morbid process we distinguish: endocarditis, when the endocardium and in particular the heart valves are affected with deforming scarring outcomes (valvular defects); myocarditis, when the disease attacks the myocardium (stenocardial syndrome or angina pectoris, myocardial infarction, abnormalities of cardiac rhythm, heart failure); pericarditis, when the disease affects the pericardium.
Congenital heart diseases, almost always associated with other anomalies, are transmitted by direct inheritance. The most frequent are: dextrocardias, interatrial communication (persistence of Botallo’s hole) and interventricular communication (or Roger’s disease), valvular stenosis and atresias, persistence of Botallo’s ductus arteriosus, tetrad and triad of Fallot, Eisenmenger’s complex. The possibility of a corrective surgical therapy is conditioned by the precocity of the diagnosis, often difficult even using the most advanced techniques (angiocardiography, cardiac catheterization, ether test, phonocardiography, tomography and fluorocardiography).
Traumatic heart disease is caused by trauma to the chest (impact, compression, crushing), such as to cause an immediate increase in blood pressure in the cardiac cavities; if the myocardium has, but not necessarily, an area of lower resistance (scar tissue in the aftermath of an infarction), this gives way, it becomes exhausted and there is an aneurysm or even rupture. Atherosclerotic heart disease is caused by atherosclerosis. Hypertensive heart disease is caused by elevated blood pressure. In addition, the rhythm of the heart may vary from the normal heartbeat and therefore sinus arrhythmia, premature heartbeat, heart block, d atrial fibrillation, atrial flutter, alternating pulse or paroxysmal tachycardia may result.
Tumors of the heart are rare; they are divided into primary, benign (myxoma, rhabdomyoma, fibroma, lipoma, angioma, leiomyoma) and malignant (sarcoma) and secondary metastatic, arising from tumors of organs in the chest and any other site.