Heart

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.

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