The circulatory system of animals has come a long way in the formation during the evolutionary development of the world. It was formed on the site of the rudimentary parts of the primary cavity of the body, which in higher animals was displaced by the celloma, or secondary cavity of the body. In the process of ontogenesis, the circulatory system of animals began to play a universal role, namely the transfer of nutrients, oxygen, metabolic products, hormones and other biologically significant elements. Thus, it is the most important integrating system of a living organism, which ensures its integrity.
Scientists-zoologists distinguish a full-fledged circulatory system from such primitive fauna representatives as nemerthins, annelids, echinoderms, mollusks and insects, that is, those classes that are of the invertebrate type. The structure of the circulatory system was very primitive among their common ancestors - a small system of gaps, which are cavities without epithelial cells in which the direction of blood flow was not established. In modern invertebrates, the principle of the structure of the circulatory system is also very primitive: the two main longitudinal vessels, the abdominal and dorsal, lie under and above the intestine, respectively. And they are interconnected with the help of transverse blood vessels that enclose the intestines in the ring and have numerous branches to the internal organs and integuments of the body. At the same time, a feature of the functioning of the system is that the blood moves backward along the abdominal vessel, and forward along the dorsal vessel. The function of blood movement is provided by the rhythmic contraction of the dorsal vessel.
In the process of evolution, the circulatory system of animals improved in different ways in all classes. Its development does not originate from a highly organized circulatory system of invertebrates. This explains its most important functional feature: the presence of a muscular pulsating organ called the heart, which appears in primitively organized representatives of the chordates - in cyclostomes. Also, the movement of blood occurs along the abdominal vessel forward, and along the dorsal - back.
The circulatory system of vertebrates has a single structural plan, the complication and improvement of which occurred gradually with the course of the evolutionary process. But not all evolutionary rearrangements were aromorphoses, that is, progressive morpho-physiological changes. The increasing complexity of the organization or structure cannot be considered a progressive feature of the changes that will make the animal that acquired it more highly developed. In the evolutionary transformations of the circulatory system of chordates, an undoubted aromorphosis was the development of a real heart in the most ancient representatives, as well as the intensification of its activity and the formation of a closed circulatory system in the class of maxillary. Also, the most important aromorphoses are considered to be the transformation of the main vessels and the heart during the separation of venous and arterial blood flows in reptiles, birds and animals.
The circulatory system of animals is formed by the heart and blood vessels. The heart is a hollow muscular organ that provides blood movement through the vessels, pumping it. From it to all organs, blood moves through the arteries, and from organs to the heart - through the veins. The heart in different classes of vertebrates can be either single-chamber (in cyclostomes), or two-chamber (fish), three-chamber (amphibians, reptiles) and four-chamber (birds, mammals). Thus, the anatomy of the heart can determine the degree of basic metabolism of the body and allows you to divide the animals into cold-blooded and warm-blooded. In primitive chordates, constantly living in water, the simplest principle is the organization of the circulatory system - it has one circle of blood circulation. The release of animals on land involves a more intense metabolism, characterized by the appearance of pulmonary respiration, and, as a result, the emergence of a second circle of blood circulation, which is responsible for the gas exchange of blood in the lungs. The circulatory system of animals is complexly organized, together with the complete separation of the circles of blood circulation, it is characterized by a four-chambered heart, as well as the presence of additional circles, such as the placental, cardiac, and willis blood circulation.