In mammals and humans, the circulatory system is the most complex. This is a closed system consisting of two circles of blood circulation. Providing warm-bloodedness, it is more energetically beneficial and allows a person to occupy the niche of habitat in which he is now located.
The circulatory system is a group of hollow muscle organs responsible for the circulation of blood through the vessels of the body. It is represented by the heart and blood vessels of various calibers. These are the muscular organs that form the circles of blood circulation. Their outline is proposed in all textbooks on anatomy and is described in this publication.
The concept of blood circulation
The circulatory system consists of two circles - body (large) and pulmonary (small). The circulatory system is the system of vessels of the arterial, capillary, lymphatic and venous type, which delivers blood from the heart to the vessels and its movement in the opposite direction. The central organ of blood circulation is the heart, since two circles of blood circulation cross in it without mixing arterial and venous blood.
Large circle of blood circulation
A large circle of blood circulation is a system for providing peripheral tissues with arterial blood and its return to the heart. It starts from the left ventricle, from where blood flows into the aorta through the aortic orifice with a tricuspid valve. Blood flows from the aorta to the smaller body arteries and reaches the capillaries. This is a collection of organs that forms the leading link.
Here oxygen enters the tissue, and carbon dioxide is captured from them by red blood cells. Also, blood transports amino acids, lipoproteins, glucose to the tissue, the metabolic products of which are carried out from the capillaries into venules and further into larger veins. They flow into the vena cava, which return blood directly to the heart in the right atrium.
The right atrium ends with a large circle of blood circulation. The scheme looks like this (in the course of blood circulation): the left ventricle, aorta, elastic arteries, muscular-elastic arteries, muscle arteries, arterioles, capillaries, venules, veins and vena cava, returning blood to the heart in the right atrium. From a large circle of blood circulation, the brain, all skin, bones are fed. In general, all human tissues are fed from the blood vessels of a large circle of blood circulation, and the small one is only a place for blood oxygenation.
Pulmonary circulation
The pulmonary (small) circle of blood circulation, the scheme of which is presented below, originates from the right ventricle. Blood enters into it from the right atrium through the atrioventricular opening. From the cavity of the right ventricle, oxygen-depleted (venous) blood enters the pulmonary trunk through the exit (pulmonary) tract. This artery is thinner than the aorta. It is divided into two branches, which are directed to both lungs.
The lungs are the central organ that forms the pulmonary circulation. The human diagram described in the textbooks on anatomy explains that pulmonary blood flow is needed for blood oxygenation. Here she gives off carbon dioxide and absorbs oxygen. In sinusoidal capillaries of the lungs with a diameter of about 30 microns atypical for the body, gas exchange takes place.
Subsequently, oxygenated blood is sent through the system of intrapulmonary veins and collected in 4 pulmonary veins. All of them are attached to the left atrium and carry oxygen-rich blood there. This is where the circles of blood circulation end. The scheme of the small pulmonary circle looks like this (in the direction of blood flow): the right ventricle, pulmonary artery, intrapulmonary arteries, pulmonary arterioles, pulmonary sinusoids, venules, pulmonary veins, left atrium.
Features of the circulatory system
A key feature of the circulatory system, which consists of two circles, is the need for a heart with two or more chambers. The fish have one blood circulation, because they have no lungs, and all gas exchange takes place in the vessels of the gills. As a result, the fish heart is single chamber - it is a pump that pushes blood in only one direction.
Amphibians and reptiles have respiratory organs and, accordingly, circulatory circles. The scheme of their work is simple: from the ventricle, the blood goes to the vessels of a large circle, from the arteries to the capillaries and veins. Venous return to the heart is also realized, however, from the right atrium, the blood enters the common ventricle for two circles of blood circulation. Since the heart of these animals is three-chambered, the blood from both circles (venous and arterial) is mixed.
In humans (and mammals), the heart has a 4-chamber structure. In it, two ventricles and two atria are divided by partitions. The lack of mixing of two types of blood (arterial and venous) became a gigantic evolutionary invention, which provided warm-blooded mammals.
Blood supply to the lungs and heart
In the circulatory system, which consists of two circles, nutrition of the lung and heart is of particular importance. These are the most important organs that ensure the isolation of the bloodstream and the integrity of the respiratory and circulatory systems. So, the lungs have in their thickness two circles of blood circulation. But their tissue feeds on the vessels of a large circle: from the aorta and from the intrathoracic arteries, bronchial and pulmonary vessels branch, carrying blood to the lung parenchyma. And the organ cannot eat from the right departments, although part of the oxygen diffuses from there. This means that the large and small circles of blood circulation, the scheme of which is described above, perform different functions (one enriches the blood with oxygen, and the second sends it to the organs, taking deoxygenated blood from them).
The heart also feeds on the vessels of a large circle, but the blood located in its cavities is able to provide oxygen to the endocardium. In this case, part of the myocardial veins, mainly small ones, flows directly into the chambers of the heart. It is noteworthy that the pulse wave to the coronary arteries propagates into the cardiac diastole. Therefore, the organ is supplied with blood only when it "rests".
It is interesting
The circulatory system of a person, the scheme of which is presented above in the relevant sections, provides both warm-bloodedness and high endurance. Suppose a person is not the animal that often uses its power to survive, but this allowed other mammals to populate certain habitats. Previously, they were inaccessible to amphibians and reptiles, and even more so to fish.
In phylogenesis, a large circle appeared earlier and was characteristic of fish. And the small circle supplemented it only with those animals that completely or completely came to land and settled it. Since its inception, the respiratory and circulatory systems are considered together. They are connected functionally and structurally.
This is an important and already indestructible evolutionary mechanism of exit from the aquatic environment and the settlement of land. Therefore, the continuing complication of mammalian organisms will now go not along the path of complicating the respiratory and circulatory system, but in the direction of enhancing the oxygen-binding function of the blood and increasing the area of the lungs.