Vascular system.

The vascular system is a system of tubes through which fluids (blood and lymph) circulate through them, on the one hand, supplying the nutrients necessary for them to the cells and tissues of the body, on the other hand, removal of the waste products of cellular elements and transferring of these products to the excretory organs (kidneys). In the intestinal cavity, the digestive cavity gives numerous outgrowths from itself, which facilitates the delivery of nutrients to certain parts of the body. But already in nemertin (a subtype of worms) there appear three separate blood vessels. The lancelet has a closed circulatory system, which is, however, still devoid of heart; the movement of the colorless blood of the lancelet is caused by the pulsation of the vessels themselves. In the circulatory system of vertebrates, the heart appears as a pulsating organ, gradually becoming more complex in its structure during phylogenesis.

The heart of fish consists of two chambers: the blood receptor – the atria, in front of which is the venous sinus, sinus venosus, and the expulsion – the ventricle, followed by the arterial cone, conus arteriosus. Through the whole heart venous blood flows, which flows further through the gill arteries to the gills, where it is enriched with oxygen (gill breathing). In amphibians, due to the beginning of the emergence from water and the emergence along with the gill and pulmonary type of respiration, the formation of the pulmonary circulation begins: the pulmonary artery develops from the last gill artery, which carries blood from the heart to the lungs, where gas exchange takes place. In this regard, the perceiving part of the heart – the atrium – is divided by a septum into two separate atria (right and left), as a result of which the heart becomes three-chambered. At the same time, venous blood flows in the right atrium, arterial blood flows in the left, and mixed blood in the common ventricle. In the larval state, the gill blood circulation functions, in the adult – pulmonary circulation, which reflects the beginning of the transition from the aquatic environment to the air.