Cardiovascular system of fish. Cardiovascular system, circulatory circles Which animal has one circulatory circle

They have a closed circulatory system, represented by the heart and blood vessels. Unlike higher animals, fish have one circulation (with the exception of lungfishes and lobe-finned fish).

Pisces have a heart two-chamber: consists of the atrium, ventricle, sinus venosus and conus arteriosus, alternately contracting with their muscular walls. Contracting rhythmically, it moves the blood in a vicious circle.

Compared to land animals, the heart of fish is very small and weak. Its mass usually does not exceed 0.33–2.5%, on average 1% of body weight, while in mammals it reaches 4.6%, and in birds - 10–16%.
Blood pressure in fish is also weak.
Fish also have a low heart rate: 18–30 beats per minute, but at low temperatures it can decrease to 1–2; In fish that survive freezing into ice in winter, the heart pulsation stops altogether during this period.
In addition, fish have a small amount of blood compared to higher animals.

But all this is explained by the horizontal position of the fish in the environment (there is no need to push the blood upward), as well as by the life of the fish in water: in an environment in which the force of gravity affects much less than in the air.

Blood flows from the heart through arteries, and to the heart through veins.

From the atrium it is pushed into the ventricle, then into the conus arteriosus, and then into the large abdominal aorta and reaches, where gas exchange occurs: the blood in the gills is enriched with oxygen and freed from carbon dioxide. The red blood cells of fish - erythrocytes - contain hemoglobin, which binds oxygen in the gills, and carbon dioxide in organs and tissues.
The ability of hemoglobin in the blood of fish to extract oxygen varies among species. Fast-swimming fish that live in oxygen-rich running waters have hemoglobin cells that have a great ability to bind oxygen.

Oxygen-rich arterial blood has a bright scarlet color.

After the gills, the blood enters the head through the arteries and further into the dorsal aorta. Passing through the dorsal aorta, blood delivers oxygen to the organs and muscles of the trunk and tail. The dorsal aorta stretches to the end of the tail, from which large vessels extend to the internal organs along the way.

The venous blood of fish, depleted of oxygen and saturated with carbon dioxide, has a dark cherry color.

Having given oxygen to the organs and collected carbon dioxide, the blood flows through large veins to the heart and atrium.

The fish body also has its own characteristics in hematopoiesis:

Many organs can form blood: gill apparatus, intestines (mucosa), heart (epithelial layer and vascular endothelium), spleen, vascular blood, lymphoid organ (accumulations of hematopoietic tissue - reticular syncytium - under the roof of the skull).
The peripheral blood of fish may contain mature and young red blood cells.
Red blood cells, unlike mammalian blood, have a nucleus.

The blood of a fish has an internal osmotic pressure.

To date, 14 fish blood group systems have been established.

The cardiovascular system of fish consists of the following elements:

Circulatory system, lymphotic system and hematopoietic organs.

The circulatory system of fish differs from other vertebrates in one circle of blood circulation and a two-chambered heart filled with venous blood (with the exception of lungfishes and lobe-finned fish). The main elements are: Heart, blood vessels, blood (Fig. 1b

Figure 1. Circulatory system of fish.

Heart in fish it is located near the gills; and is enclosed in a small pericardial cavity, and in lampreys - in a cartilaginous capsule. The heart of fish is two-chambered and consists of a thin-walled atrium and a thick-walled muscular ventricle. In addition, fish are also characterized by accessory sections: the venous sinus, or venous sinus, and the conus arteriosus.

The venous sinus is a small thin-walled sac in which venous blood accumulates. From the venous sinus it enters the atrium and then into the ventricle. All openings between the parts of the heart are equipped with valves, which prevents the reverse flow of blood.

In many fish, with the exception of bony fish, the conus arteriosus, which is part of the heart, is adjacent to the ventricle. Its wall is also formed by the heart muscles, and on the inner surface there is a system of valves.

In teleost fish, instead of an arterial cone, there is an aortic bulb - a small white formation, which is an expanded part of the abdominal aorta. Unlike the conus arteriosus, the aortic bulb consists of smooth muscle and has no valves (Fig. 2).

Fig.2. Diagram of the circulatory system of a shark and the structure of the heart of a shark (I) and bony fish (II).

1 - atrium; 2 - ventricle; 3 - arterial cone; 4 - abdominal aorta;

5 - afferent branchial artery; 6 - efferent branchial artery; 7- carotid artery; 8 - dorsal aorta; 9 - renal artery; 10 - subclavian artery; I - caudal artery; 12 - venous sinus; 13 - duct of Cuvier; 14 - anterior cardinal vein; 15 - tail vein; 16 - renal portal system; 17 - posterior cardinal vein; 18 - lateral vein; 19 - subintestinal vein; 20 portal vein of the liver; 21 - hepatic vein; 22 - subclavian vein; 23 - aortic bulb.

In lungfish, due to the development of pulmonary respiration, the structure of the heart became more complex. The atrium is almost completely divided into two parts by a septum hanging from above, which, in the form of a fold, continues into the ventricle and conus arteriosus. The left side receives arterial blood from the lungs, the right side receives venous blood from the venous sinus, so more arterial blood flows in the left side of the heart, and more venous blood flows in the right.

Fish have a small heart. Its mass varies among different fish species and ranges from 0.1 (carp) to 2.5% (flying fish) of body weight.

The heart of cyclostomes and fish (with the exception of lungfishes) contains only venous blood. The heart rate is specific to each species, and also depends on the age, physiological state of the fish, water temperature and is approximately equal to the frequency of respiratory movements. In adult fish, the heart beats quite slowly - 20–35 times per minute, and in juvenile fish it beats much more often (for example, in sturgeon fry - up to 142 times per minute). When the temperature rises, the heart rate increases, and when it decreases, it decreases. In many fish during wintering (bream, carp), the heart beats only 1-2 times per minute.

The circulatory system of fish is closed. The vessels that carry blood out of the heart are called arteries, although venous blood flows in some of them (abdominal aorta, afferent branchial arteries), and the vessels bringing blood to the heart - veins. Fish (except lungfish) have only one circulation.

In bony fishes, venous blood from the heart flows through the aortic bulb into the abdominal aorta, and from it through the afferent gill arteries to the gills. Teleosts are characterized by four pairs of afferent and the same number of efferent gill arteries. Arterial blood through the efferent gill arteries enters the paired epibranchial vessels, or the roots of the dorsal aorta, passing along the bottom of the skull and closing in front, forming a head circle, from which vessels extend to different parts of the head. At the level of the last branchial arch, the roots of the dorsal aorta, merging together, form the dorsal aorta, which passes in the trunk region under the spine, and in the caudal region in the hemal canal of the spine and is called the caudal artery. Arteries that supply arterial blood to organs, muscles, and skin are separated from the dorsal aorta. All arteries break up into a network of capillaries, through the walls of which substances are exchanged between blood and tissues. From the capillaries, the blood collects into the veins (Fig. 3).

The main venous vessels are the anterior and posterior cardinal veins, which, merging at the level of the heart, form transverse vessels - the ducts of Cuvier, which flow into the venous sinus of the heart. The anterior cardinal veins carry blood from the top of the head. From the lower part of the head, mainly from the visceral apparatus, blood collects in the unpaired jugular (jugular) vein, which stretches under the abdominal aorta and near the heart divides into two vessels that independently flow into the Cuvier ducts.

From the caudal region, venous blood is collected in the tail vein, which passes in the hemal canal of the spine under the caudal artery. At the level of the posterior edge of the kidneys, the tail vein divides into two renal portal veins, which stretch for some distance along the dorsal side of the kidneys, and then branch in the kidneys into a network of capillaries, forming the renal portal system. The venous vessels leaving the kidneys are called the posterior cardinal veins, running along the underside of the kidneys to the heart.

On their way, they accept veins from the reproductive organs and body walls. At the level of the posterior end of the heart, the posterior cardinal veins merge with the anterior ones, forming paired ducts of Cuvier, carrying blood to the venous sinus.

From the digestive tract, digestive glands, spleen, swim bladder, blood collects in the portal vein of the liver, which, entering the liver, branches into a network of capillaries, forming the liver portal system. From here the blood flows through the paired hepatic veins into the venous sinus. Consequently, fish have two portal systems - kidneys and liver. However, the structure of the portal system of the kidneys and the posterior cardinal veins in bony fishes is not the same. Thus, in some cyprinids, pike, perch, and cod, the right portal system of the kidneys is underdeveloped and only a small part of the blood passes through the portal system.

Due to the wide variety of structure and living conditions of different groups of fish, they are characterized by significant deviations from the outlined scheme.

Cyclostomes have seven afferent and the same number of efferent gill arteries. The epibranchial vessel is unpaired, there are no aortic roots. The renal portal system and Cuvier's ducts are absent. There is only one hepatic vein. There is no inferior jugular vein.

In cartilaginous fish, there are five afferent gill arteries and ten efferent gill arteries. There are subclavian arteries and veins, which provide blood supply to the pectoral fins and shoulder girdle, as well as lateral veins starting from the ventral fins. They pass along the side walls of the abdominal cavity and in the region of the shoulder girdle merge with the subclavian veins.

The posterior cardinal veins at the level of the pectoral fins form extensions - cardinal sinuses.

In lungfish, more arterial blood, concentrated in the left half of the heart, enters the two anterior branchial arteries, from which it goes to the head and dorsal aorta. More venous blood from the right side of the heart passes into the two posterior branchial arteries and then into the lungs. During air breathing, the blood in the lungs is enriched with oxygen and flows through the pulmonary veins to the left side of the heart (Fig. 4).

In addition to the pulmonary veins, lungfish have abdominal and large cutaneous veins, and instead of the right cardinal vein, the posterior vena cava is formed.

Lymphatic system. The lymphatic system, which is of great importance in metabolism, is closely related to the circulatory system. Unlike the circulatory system, it is not closed. Lymph is similar in composition to blood plasma. As blood circulates through the blood capillaries, some of the plasma containing oxygen and nutrients leaves the capillaries, forming tissue fluid that bathes the cells. Part of the tissue fluid containing metabolic products again enters the blood capillaries, and the other part enters the lymphatic capillaries and is called lymph. It is colorless and contains only lymphocytes from the formed elements of blood.

The lymphatic system consists of lymphatic capillaries, which then turn into lymphatic vessels and larger trunks, through which lymph slowly moves in one direction - towards the heart. Consequently, the lymphatic system drains tissue fluid, complementing the function of the venous system.

The largest lymphatic trunks in fish are the paired subvertebral ones, which stretch along the sides of the dorsal aorta from the tail to the head, and the lateral ones, which run under the skin along the lateral line. Through these and the cephalic trunks, lymph flows into the posterior cardinal veins at the Cuvier ducts.

In addition, fish have several unpaired lymphatic vessels: dorsal, ventral, spinal. Fish do not have lymph nodes, but some species of fish under the last vertebra have pulsating paired lymphatic hearts in the form of small oval pink bodies that push lymph to the heart. The movement of lymph is also facilitated by the work of the trunk muscles and breathing movements. Cartilaginous fish do not have lymphatic hearts or lateral lymphatic trunks. In cyclostomes, the lymphatic system is separate from the circulatory system.

Blood. The functions of blood are diverse. It carries nutrients and oxygen throughout the body, frees it from metabolic products, communicates the endocrine glands with the corresponding organs, and also protects the body from harmful substances and microorganisms. The amount of blood in fish ranges from 1.5 (skate) to 7.3% (horse mackerel) of the total mass of the fish, while in mammals it is about 7.7%.

Rice. 5. Fish blood cells.

The blood of a fish consists of blood fluid, or plasma, formed elements - red - erythrocytes and white - leukocytes, as well as blood platelets - platelets (Fig. 5). Fish have a more complex morphological structure of blood compared to mammals, since in addition to specialized organs, the walls of blood vessels also participate in hematopoiesis. Therefore, the bloodstream contains formed elements at all phases of their development. Red blood cells are ellipsoidal in shape and contain a nucleus. Their number in different fish species ranges from 90 thousand/mm 3 (shark) to 4 million/mm 3 (bonito) and varies in the same species B: depending on the sex, age of the fish, as well as environmental conditions.

Most fish have red blood, which is due to the presence of hemoglobin in red blood cells, which carries oxygen from the respiratory organs to all cells of the body.

Rice. 6. Antarctic whitefish

However, in some Antarctic fish - white-blooded fish, which include ice fish, the blood contains almost no red blood cells, and therefore no hemoglobin or any other respiratory pigment. The blood and gills of these fish are colorless (Fig. 6). Under conditions of low water temperature and high oxygen content, respiration in this case is carried out by diffusion of oxygen into the blood plasma through the capillaries of the skin and gills. These fish are inactive, and the lack of hemoglobin in them is compensated by the increased work of the large heart and the entire circulatory system.

The main function of leukocytes is to protect the body from harmful substances and microorganisms. The number of leukocytes in fish is high, but varies

depends on the species, sex, physiological state of the fish, as well as the presence of a disease, etc.

The sculpin goby, for example, has about 30 thousand/mm 3 , the ruffe has from 75 to 325 thousand/mm 3 leukocytes, while in humans there are only 6-8 thousand/mm 3 . A large number of leukocytes in fish indicates a higher protective function of their blood.

Leukocytes are divided into granular (granulocytes) and non-granular (agranulocytes). In mammals, granular leukocytes are represented by neutrophils, eosinophils and basophils, and non-granular leukocytes are lymphocytes and monocytes. There is no generally accepted classification of leukocytes in fish. The blood of sturgeon and bony fish differs primarily in the composition of granular leukocytes. In sturgeons they are represented by neutrophils and eosinophils, and in teleosts - neutrophils, pseudoeosinophils and pseudobasophils.

Non-granular leukocytes of fish are represented by lymphocytes and monocytes.

One of the features of fish blood is that the leukocyte formula varies greatly depending on the physiological state of the fish, so all granulocytes characteristic of a given species are not always found in the blood.

Platelets in fish are numerous, and larger than in mammals, with a nucleus. They are important in blood clotting, which is also facilitated by skin mucus.

Thus, fish blood is characterized by signs of primitiveness: the presence of a nucleus in erythrocytes and platelets, a relatively small number of erythrocytes and a low hemoglobin content, causing low metabolism. At the same time, it is also characterized by highly specialized features: a huge number of leukocytes and platelets.

Hematopoietic organs. If in adult mammals hematopoiesis occurs in the red bone marrow, lymph nodes, spleen and thymus, then in fish that have neither bone marrow nor lymph nodes, various specialized organs and foci participate in hematopoiesis. Thus, in sturgeons, hematopoiesis mainly occurs in the so-called lymphoid organ, located in the head cartilages above the medulla oblongata and cerebellum. All types of formed elements are formed here. In bony fish, the main hematopoietic organ is located in the recesses of the outer part of the occipital part of the skull.

In addition, hematopoiesis in fish occurs in various foci - the head kidney, spleen, thymus, gill apparatus, intestinal mucosa, walls of blood vessels, as well as in the pericardium in teleosts and the endocardium in sturgeons.

Head kidney in fish it is not separated from the body and consists of lymphoid tissue, in which erythrocytes and lymphocytes are formed.

Spleen in fish it has a varied shape and location. Lampreys do not have a formed spleen, and its tissue lies in the shell of the spiral valve. In most fish, the spleen is a separate dark red organ located behind the stomach in the folds of the mesentery. In the spleen, red blood cells, white blood cells and platelets are formed, and dead red blood cells are also destroyed. In addition, the spleen performs a protective function (phagocytosis of leukocytes) and is a blood depot.

Thymus(thymus, or thymus gland) is located in the gill cavity. It distinguishes between the superficial layer, the cortex and the medulla. Lymphocytes are formed here. In addition, the thymus stimulates their formation in other organs. Thymic lymphocytes are capable of producing antibodies involved in the development of immunity. It reacts very sensitively to changes in the external and internal environment, responding by increasing or decreasing its volume. The thymus is a kind of guardian of the body, which in unfavorable conditions mobilizes its defenses. It reaches its greatest development in fish of younger age groups, and after they reach sexual maturity, its volume noticeably decreases.

In the circulatory system of fish, in comparison with lancelets, a real heart appears. It consists of two chambers, i.e. fish heart is two chambered. The first chamber is the atrium, the second chamber is the ventricle of the heart. Blood first enters the atrium, then is pushed into the ventricle by muscle contraction. Further, as a result of its contraction, it pours into a large blood vessel.

The heart of fish is located in the pericardial sac, located behind the last pair of gill arches in the body cavity.

Like all chordates, fish circulatory system is closed. This means that nowhere along its route does the blood leave the vessels and flow into the body cavities. To ensure the exchange of substances between blood and cells of the whole body, large arteries (vessels carrying oxygenated blood) gradually branch into smaller ones. The smallest vessels are capillaries. Having given up oxygen and taken in carbon dioxide, the capillaries again unite into larger vessels (but already venous).

In fish only one circle of blood circulation. With a two-chambered heart, it cannot be any other way. In more highly organized vertebrates (starting with amphibians), a second (pulmonary) circulation appears. But these animals also have a three-chambered or even four-chambered heart.

Venous blood flows through the heart, giving oxygen to the cells of the body. Next, the heart pushes this blood into the abdominal aorta, which goes to the gills and branches into the afferent branchial arteries (but despite the name “arteries” they contain venous blood). In the gills (specifically, in the gill filaments), carbon dioxide is released from the blood into the water, and oxygen leaks from the water into the blood. This happens as a result of the difference in their concentration (dissolved gases go where there are fewer of them). Enriched with oxygen, the blood becomes arterial. The efferent branchial arteries (already with arterial blood) flow into one large vessel - the dorsal aorta. It runs under the spine along the body of the fish and smaller vessels originate from it. The carotid arteries also branch from the dorsal aorta, leading to the head and supplying blood, including the brain.

Before entering the heart, venous blood passes through the liver, where it is cleared of harmful substances.

There are slight differences in the circulatory system of bony and cartilaginous fish. This mainly concerns the heart. In cartilaginous fishes (and some bony fishes) the expanded portion of the abdominal aorta contracts along with the heart, but in most bony fishes this does not.

The blood of fish is red, it contains red blood cells with hemoglobin, which binds oxygen. However, fish red blood cells are oval in shape, not disc-shaped (as, for example, in humans). The amount of blood flowing through the circulatory system is less in fish than in terrestrial vertebrates.

The heart of fish does not beat often (about 20-30 beats per minute), and the number of contractions depends on the ambient temperature (the warmer, the more often). Therefore, their blood does not flow as fast and therefore their metabolism is relatively slow. This, for example, affects the fact that fish are cold-blooded animals.

In fish, the hematopoietic organs are the spleen and the connective tissue of the kidneys.

Despite the fact that the described circulatory system of fish is characteristic of the vast majority of them, in lungfishes and lobe-finned fish it is somewhat different. In lungfishes, an incomplete septum appears in the heart and a semblance of a pulmonary (second) circulation appears. But this circle does not pass through the gills, but through the swim bladder, turned into a lung.

In the human body, the circulatory system is designed to fully meet its internal needs. An important role in the movement of blood is played by the presence of a closed system in which arterial and venous blood flows are separated. And this is done through the presence of blood circulation circles.

Historical reference

In the past, when scientists did not yet have informative instruments at hand that could study physiological processes in a living organism, the greatest scientists were forced to search for anatomical features in corpses. Naturally, the heart of a deceased person does not contract, so some nuances had to be figured out on their own, and sometimes simply fantasized. So, back in the second century AD Claudius Galen, self-learner Hippocrates, assumed that the arteries contained air instead of blood in their lumen. Over the next centuries, many attempts were made to combine and link together the existing anatomical data from the standpoint of physiology. All scientists knew and understood how the circulatory system works, but how does it work?

Scientists have made a tremendous contribution to the systematization of data on heart function. Miguel Servet and William Harvey in the 16th century. Harvey, scientist who first described the systemic and pulmonary circulation , in 1616 determined the presence of two circles, but he could not explain in his works how the arterial and venous beds were connected to each other. And only later, in the 17th century, Marcello Malpighi, one of the first to use a microscope in his practice, discovered and described the presence of tiny capillaries, invisible to the naked eye, which serve as a connecting link in the blood circulation.

Phylogeny, or the evolution of blood circulation

Due to the fact that, as animals of the vertebrate class evolved, they became more and more progressive in anatomical and physiological terms, they required a complex structure of the cardiovascular system. Thus, for faster movement of the liquid internal environment in the body of a vertebrate animal, the need for a closed blood circulation system arose. Compared to other classes of the animal kingdom (for example, arthropods or worms), the rudiments of a closed vascular system appear in chordates. And if the lancelet, for example, does not have a heart, but there is an abdominal and dorsal aorta, then in fish, amphibians (amphibians), reptiles (reptiles) a two- and three-chambered heart appears, respectively, and in birds and mammals a four-chambered heart appears, the peculiarity of which is is the focus in it of two circles of blood circulation that do not mix with each other.

Thus, the presence of two separated circulatory circles in birds, mammals and humans, in particular, is nothing more than the evolution of the circulatory system, necessary for better adaptation to environmental conditions.

Anatomical features of the blood circulation

The circulatory system is a set of blood vessels, which is a closed system for the supply of oxygen and nutrients to the internal organs through gas exchange and nutrient exchange, as well as for the removal of carbon dioxide and other metabolic products from cells. The human body is characterized by two circles - the systemic, or large circle, and the pulmonary, also called the small circle.

Video: blood circulation circles, mini-lecture and animation

Systemic circulation

The main function of the large circle is to ensure gas exchange in all internal organs except the lungs. It begins in the cavity of the left ventricle; represented by the aorta and its branches, the arterial bed of the liver, kidneys, brain, skeletal muscles and other organs. Further, this circle continues with the capillary network and venous bed of the listed organs; and through the entry of the vena cava into the cavity of the right atrium it ends in the latter.

So, as already said, the beginning of the great circle is the cavity of the left ventricle. Arterial blood flow, which contains more oxygen than carbon dioxide, is sent here. This flow enters the left ventricle directly from the circulatory system of the lungs, that is, from the small circle. The arterial flow from the left ventricle is pushed through the aortic valve into the largest great vessel - the aorta. The aorta can be figuratively compared to a kind of tree that has many branches, because arteries extend from it to the internal organs (to the liver, kidneys, gastrointestinal tract, to the brain - through the system of carotid arteries, to skeletal muscles, to the subcutaneous fat fiber, etc.) Organ arteries, which also have numerous branches and bear names corresponding to their anatomy, carry oxygen to each organ.

In the tissues of internal organs, arterial vessels are divided into vessels of smaller and smaller diameter, and as a result, a capillary network is formed. Capillaries are the smallest vessels, practically without a middle muscular layer, and are represented by an inner membrane - intima, lined with endothelial cells. The gaps between these cells at the microscopic level are so large compared to other vessels that they allow proteins, gases and even formed elements to easily penetrate into the intercellular fluid of the surrounding tissues. Thus, intense gas exchange and exchange of other substances occurs between the capillary with arterial blood and the liquid intercellular medium in a particular organ. Oxygen penetrates from the capillary, and carbon dioxide, as a product of cell metabolism, enters the capillary. The cellular stage of respiration occurs.

After more oxygen has passed into the tissues and all carbon dioxide has been removed from the tissues, the blood becomes venous. All gas exchange occurs with each new influx of blood, and during the period of time while it moves along the capillary towards the venule - a vessel that collects venous blood. That is, with each cardiac cycle, in one or another part of the body, oxygen enters the tissues and carbon dioxide is removed from them.

These venules unite into larger veins, and a venous bed is formed. Veins, similar to arteries, are named according to the organ in which they are located (renal, cerebral, etc.). From large venous trunks, tributaries of the superior and inferior vena cava are formed, and the latter then flow into the right atrium.

Features of blood flow in the organs of the systemic circle

Some of the internal organs have their own characteristics. So, for example, in the liver there is not only a hepatic vein, which “carries” the venous flow away from it, but also a portal vein, which, on the contrary, brings blood to the liver tissue, where blood purification is performed, and only then the blood collects in the tributaries of the hepatic vein to enter to a big circle. The portal vein brings blood from the stomach and intestines, so everything that a person eats or drinks must undergo a kind of “purification” in the liver.

In addition to the liver, certain nuances exist in other organs, for example, in the tissues of the pituitary gland and kidneys. Thus, in the pituitary gland the presence of a so-called “wonderful” capillary network is noted, because the arteries that bring blood to the pituitary gland from the hypothalamus are divided into capillaries, which then collect into venules. The venules, after the blood with the molecules of releasing hormones are collected, are again divided into capillaries, and then veins are formed that carry the blood from the pituitary gland. In the kidneys, the arterial network is divided twice into capillaries, which is associated with the processes of excretion and reabsorption in the kidney cells - in the nephrons.

Pulmonary circulation

Its function is to carry out gas exchange processes in the lung tissue in order to saturate the “waste” venous blood with oxygen molecules. It begins in the cavity of the right ventricle, where venous blood flow with an extremely small amount of oxygen and a large content of carbon dioxide enters from the right atrial chamber (from the “end point” of the great circle). This blood moves through the pulmonary valve into one of the large vessels called the pulmonary trunk. Next, the venous flow moves along the arterial bed in the lung tissue, which also breaks up into a network of capillaries. By analogy with capillaries in other tissues, gas exchange occurs in them, only oxygen molecules enter the lumen of the capillary, and carbon dioxide penetrates into the alveolocytes (cells of the alveoli). With each act of breathing, air enters the alveoli from the environment, from which oxygen penetrates through the cell membranes into the blood plasma. When exhaling, the carbon dioxide that enters the alveoli is expelled with the exhaled air.

After being saturated with O2 molecules, the blood acquires the properties of arterial blood, flows through the venules and ultimately reaches the pulmonary veins. The latter, consisting of four or five pieces, open into the cavity of the left atrium. As a result, venous blood flows through the right half of the heart, and arterial blood flows through the left half; and normally these flows should not mix.

Lung tissue has a double network of capillaries. With the help of the first, gas exchange processes are carried out in order to enrich the venous flow with oxygen molecules (relationship directly with the small circle), and in the second, the lung tissue itself is supplied with oxygen and nutrients (relationship with the large circle).

Additional circulation circles

These concepts are used to distinguish the blood supply of individual organs. For example, to the heart, which needs oxygen more than others, arterial inflow is carried out from the branches of the aorta at its very beginning, which are called the right and left coronary (coronary) arteries. Intense gas exchange occurs in the myocardial capillaries, and venous outflow occurs into the coronary veins. The latter collect in the coronary sinus, which opens directly into the right atrial chamber. In this way it is carried out cardiac or coronary circulation.

coronary (coronary) circle of blood circulation in the heart

Circle of Willis is a closed arterial network of cerebral arteries. The medulla provides additional blood supply to the brain when cerebral blood flow through other arteries is disrupted. This protects such an important organ from lack of oxygen, or hypoxia. The cerebral circulation is represented by the initial segment of the anterior cerebral artery, the initial segment of the posterior cerebral artery, anterior and posterior communicating arteries, and internal carotid arteries.

Circle of Willis in the brain (classical variant of the structure)

Placental circulation functions only during pregnancy by a woman and performs the function of “breathing” in a child. The placenta is formed starting from 3-6 weeks of pregnancy, and begins to function fully from the 12th week. Due to the fact that the fetus's lungs do not work, oxygen enters its blood through the flow of arterial blood into the baby's umbilical vein.

fetal circulation before birth

Thus, the entire human circulatory system can be divided into separate interconnected sections that perform their functions. The proper functioning of such areas, or blood circulation circles, is the key to the healthy functioning of the heart, blood vessels and the entire body as a whole.

Platelets in mammals they are irregularly shaped fragments of cells surrounded by a membrane and usually lacking a nucleus. They are formed from special cells in the bone marrow. Each platelet is approximately four times smaller than a red blood cell. Platelets are necessary to start the blood clotting process. 1 mm3 of blood contains approximately 250,000 platelets. The lifespan of platelets in humans is 5-9 days; they are then destroyed in the liver and spleen.

Circulation

Generalized human blood circulation diagram is presented in the figure and is characterized by the following features.

1. A person has two circles of blood circulation. This means that blood, passing throughout the body, enters the heart twice. The advantage of such a system is the ability to first enrich the blood with oxygen in the lungs (small, or pulmonary, circle), then return it to the heart and again push it out to the rest of the organs (large, or systemic, circle). The fact is that blood pressure in the pulmonary capillaries drops, and without an additional increase, the blood supply to most of the body would become ineffective. This pattern is not characteristic of all vertebrates. In fish, for example, blood from the heart is sent to the gills, enriched with oxygen there, then distributed throughout the body and only after that returns to the heart, i.e. in fish there is only one circle of blood circulation. Two circles of blood circulation appear in the evolutionary history of amphibians, but are completely separated only in birds and mammals. It is no coincidence that it was the last two groups of vertebrates that became warm-blooded. Warm-bloodedness requires intensive metabolism, which is only possible with a good supply of oxygen to the tissues, which is necessary for aerobic respiration (it is much more energetically beneficial than oxygen-free - anaerobic). And intensive metabolism allows you to maintain a high level of general activity of the body in a wide variety of environmental conditions. The presence of two completely separate circulations requires the division of the heart into two functional halves. One pumps deoxygenated blood to the lungs, and the other pumps oxygenated blood to the rest of the body. In fact, we have two hearts (right and left), which are fused together and contract simultaneously. In amphibians, the heart is not divided at all, but in reptiles it is incompletely divided (with the exception of crocodiles).

2. Blood supply organs is carried out not sequentially, but in parallel. Otherwise, the blood, passing from organ A to B, then to C, etc., would lose pressure, oxygen and nutrients at each stage, i.e., some parts of the body would sooner or later be deprived. In addition, damage to a blood vessel in any one location would cut off the blood supply to all downstream tissues.

3. Leads from the intestines to the liver portal vein. Portal veins are veins that connect two organs, neither of which is the heart (a similar system connects the hypothalamus with the pituitary gland). Thus, the intestines and liver are connected in series, and not in parallel, which entails the disadvantages mentioned above. However, they are offset by an important advantage. The fact is that the blood flowing from the intestines varies greatly in composition depending on what the individual ate or drank. And one of the functions of the liver is filtering blood in order to maintain its composition within physiologically acceptable limits. For example, here excess glucose is removed from the blood and stored as glycogen.