Circulatory system. How the human heart and human circulatory system work How long are all the blood vessels of the human body

Circulatory system It consists of a central organ - the heart and closed tubes of various calibers connected to it, called blood vessels. The heart, with its rhythmic contractions, sets in motion the entire mass of blood contained in the vessels.

The circulatory system performs the following functions:

ü respiratory(participation in gas exchange) - the blood delivers oxygen to the tissues, and carbon dioxide enters the blood from the tissues;

ü trophic- blood carries nutrients received with food to organs and tissues;

ü protective- blood leukocytes are involved in the absorption of microbes entering the body (phagocytosis);

ü transport- on vascular system hormones, enzymes, etc. are carried;

ü thermoregulatory- helps to equalize body temperature;

ü excretory- the waste products of cellular elements are removed with the blood and transferred to the excretory organs (kidneys).

Blood is a liquid tissue consisting of plasma (intercellular substance) and shaped elements suspended in it, which develop not in vessels, but in hematopoietic organs. Formed elements make up 36-40%, and plasma - 60-64% of the blood volume (Fig. 32). A human body weighing 70 kg contains an average of 5.5-6 liters of blood. Blood circulates in the blood vessels and is separated from other tissues by the vascular wall, but the formed elements and plasma can pass into the connective tissue surrounding the vessels. This system ensures the constancy of the internal environment of the body.

blood plasma - This is a liquid intercellular substance consisting of water (up to 90%), a mixture of proteins, fats, salts, hormones, enzymes and dissolved gases, as well as end products of metabolism that are excreted from the body by the kidneys and partly by the skin.

To the formed elements of blood include erythrocytes or red blood cells, leukocytes or white blood cells, and platelets or platelets.

Fig.32. The composition of the blood.

red blood cells - These are highly differentiated cells that do not contain a nucleus and individual organelles and are not capable of dividing. The life span of an erythrocyte is 2-3 months. The number of red blood cells in the blood is variable, it is subject to individual, age, daily and climatic fluctuations. Normal at healthy person the number of red blood cells ranges from 4.5 to 5.5 million in one cubic millimeter. Erythrocytes contain a complex protein - hemoglobin. It has the ability to easily attach and split off oxygen and carbon dioxide. In the lungs, hemoglobin releases carbon dioxide and takes up oxygen. Oxygen is delivered to the tissues, and carbon dioxide is taken from them. Therefore, erythrocytes in the body carry out gas exchange.

Leukocytes develop in the red bone marrow lymph nodes and spleen and in a mature state enter the bloodstream. The number of leukocytes in the blood of an adult ranges from 6000 to 8000 in one cubic millimeter. Leukocytes are capable of active movement. Adhering to the wall of capillaries, they penetrate through the gap between endothelial cells into the surrounding loose connective tissue. The process by which leukocytes leave the bloodstream is called migration. Leukocytes contain a nucleus, the size, shape and structure of which are diverse. Based on the structural features of the cytoplasm, two groups of leukocytes are distinguished: non-granular leukocytes (lymphocytes and monocytes) and granular leukocytes (neutrophilic, basophilic and eosinophilic), containing granular inclusions in the cytoplasm.

One of the main functions of leukocytes is to protect the body from microbes and various foreign bodies, the formation of antibodies. The doctrine of protective function leukocytes was developed by I.I. Mechnikov. Cells that capture foreign particles or microbes have been called phagocytes, and the process of absorption - phagocytosis. The place of reproduction of granular leukocytes is the bone marrow, and lymphocytes - the lymph nodes.

platelets or platelets play an important role in blood coagulation in violation of the integrity of blood vessels. A decrease in their number in the blood causes its slow clotting. A sharp decrease in blood coagulation is observed in hemophilia, which is inherited through women, and only men are ill.

In plasma, blood cells are in certain quantitative ratios, which are usually called the blood formula (hemogram), and the percentage of leukocytes in peripheral blood is called the leukocyte formula. AT medical practice a blood test is of great importance for characterizing the state of the body and diagnosing a number of diseases. The leukocyte formula allows you to evaluate functional state those hematopoietic tissues that supply various types of leukocytes into the blood. Increase total number leukocytes in peripheral blood is called leukocytosis. It can be physiological and pathological. Physiological leukocytosis is transient, it is observed with muscle tension (for example, in athletes), with a rapid transition from a vertical position to a horizontal position, etc. Pathological leukocytosis is observed in many infectious diseases, inflammatory processes, especially purulent ones, after operations. Leukocytosis has a certain diagnostic and prognostic value for the differential diagnosis of a number of infectious diseases and various inflammatory processes, assessing the severity of the disease, the reactive ability of the body, the effectiveness of therapy. Non-granular leukocytes include lymphocytes, among which there are T- and B-lymphocytes. They participate in the formation of antibodies when a foreign protein (antigen) is introduced into the body and determine the body's immunity.

The blood vessels are represented by arteries, veins and capillaries. The science of vessels is called angiology. Blood vessels that run from the heart to the organs and carry blood to them are called arteries, and the vessels that carry blood from the organs to the heart - veins. Arteries depart from the branches of the aorta and go to the organs. Entering the organ, the arteries branch, passing into arterioles, which branch into precapillaries and capillaries. The capillaries continue into postcapillaries, venules and finally in veins, which leave the organ and flow into the superior or inferior vena cava, which carry blood to the right atrium. Capillaries are the thinnest-walled vessels that perform an exchange function.

Individual arteries supply entire organs or parts thereof. In relation to the organ, arteries are distinguished that go outside the organ, before entering into it - extraorganic (main) arteries and their extensions branching inside the organ - intraorganic or intraorgan arteries. Branches depart from the arteries, which (before disintegration into capillaries) can connect with each other, forming anastomoses.

Rice. 33. The structure of the walls of blood vessels.

The structure of the vessel wall(Fig. 33). arterial wall consists of three shells: inner, middle and outer.

Inner shell(intimacy) lines the vessel wall from the inside. They consist of an endothelium lying on an elastic membrane.

Middle shell (media) contains smooth muscle and elastic fibers. As they move away from the heart, the arteries divide into branches and become smaller and smaller. The arteries closest to the heart (the aorta and its large branches) perform the main function of conducting blood. In them, counteraction to the stretching of the vessel wall by a mass of blood, which is ejected by a cardiac impulse, comes to the fore. Therefore, mechanical structures are more developed in the wall of arteries, i.e. elastic fibers predominate. Such arteries are called elastic arteries. In the middle and small arteries, in which the inertia of the blood weakens and its own contraction of the vascular wall is required for further movement of the blood, the contractile function predominates. It is provided by a large development in the vascular wall of muscle tissue. Such arteries are called muscular arteries.

Outer shell (externa) represented by connective tissue that protects the vessel.

The last branches of the arteries become thin and small and are called arterioles. Their wall consists of endothelium lying on one layer muscle cells. Arterioles continue directly into the precapillary, from which numerous capillaries depart.

capillaries(Fig. 33) are the thinnest vessels that perform the metabolic function. In this regard, the capillary wall consists of a single layer of endothelial cells, which are permeable to substances and gases dissolved in the liquid. Anastomosing with each other, the capillaries form capillary networks passing into postcapillaries. Postcapillaries continue into venules that accompany arterioles. Venules form the initial segments of the venous bed and pass into the veins.

Vienna carry blood in the opposite direction to the arteries - from the organs to the heart. The walls of the veins are arranged in the same way as the walls of the arteries, however, they are much thinner and contain less muscle and elastic tissue (Fig. 33). Veins, merging with each other, form large venous trunks - the superior and inferior vena cava, flowing into the heart. The veins anastomose widely with each other, forming venous plexuses. Reverse flow of venous blood is prevented valves. They consist of a fold of endothelium containing a layer of muscle tissue. The valves face the free end towards the heart and therefore do not interfere with the flow of blood to the heart and keep it from returning back.

Factors contributing to the movement of blood through the vessels. As a result of ventricular systole, blood enters the arteries, and they stretch. Contracting due to its elasticity and returning from a state of stretching to its original position, the arteries contribute to a more even distribution of blood along the vascular bed. The blood in the arteries flows continuously, although the heart contracts and ejects blood in a jerky manner.

The movement of blood through the veins is carried out due to contractions of the heart and the suction action of the chest cavity, in which negative pressure is created during inspiration, as well as the contraction of skeletal muscles, smooth muscles of organs and the muscular membrane of the veins.

Arteries and veins usually go together, with small and medium-sized arteries accompanied by two veins, and large ones by one. The exception is the superficial veins, which run in the subcutaneous tissue and do not accompany the arteries.

The walls of blood vessels have their own thin arteries and veins serving them. They also contain numerous nerve endings (receptors and effectors) associated with the central nervous system, due to which the nervous regulation of blood circulation is carried out by the mechanism of reflexes. Blood vessels are extensive reflexogenic zones that play an important role in the neurohumoral regulation of metabolism.

The movement of blood and lymph in the microscopic part of the vascular bed is called microcirculation. It is carried out in the vessels of the microvasculature (Fig. 34). The microcirculatory bed includes five links:

1) arterioles ;

2) precapillaries, which ensure the delivery of blood to the capillaries and regulate their blood supply;

3) capillaries, through the wall of which there is an exchange between the cell and blood;

4) postcapillaries;

5) venules, through which blood flows into the veins.

capillaries make up the main part of the microcirculatory bed, they exchange between blood and tissues. Oxygen, nutrients, enzymes, hormones come from the blood to the tissues, and waste products of metabolism and carbon dioxide from the tissues into the blood. The capillaries are very long. If we decompose the capillary network of only one muscular system, then its length will be equal to 100,000 km. The diameter of the capillaries is small - from 4 to 20 microns (average 8 microns). The sum of the cross sections of all functioning capillaries is 600-800 times greater than the diameter of the aorta. This is due to the fact that the rate of blood flow in the capillaries is about 600-800 times less than the rate of blood flow in the aorta and is 0.3-0.5 mm/s. The average speed of blood movement in the aorta is 40 cm/s, in medium-sized veins - 6-14 cm/s, and in the vena cava it reaches 20 cm/s. The blood circulation time in humans is on average 20-23 seconds. Therefore, in 1 minute a complete circulation of blood is performed three times, in 1 hour - 180 times, and in a day - 4320 times. And this is all in the presence of 4-5 liters of blood in the human body.

Rice. 34. Microcirculatory bed.

Circumferential or collateral circulation is a blood flow not along the main vascular bed, but along the lateral vessels associated with it - anastomoses. At the same time, the roundabout vessels expand and acquire the character of large vessels. The property of formation of roundabout blood circulation is widely used in surgical practice during operations on organs. Anastomoses are most developed in the venous system. In some places, the veins have a large number of anastomoses, called venous plexuses. The venous plexuses are especially well developed in the internal organs located in the pelvic area (bladder, rectum, internal genital organs).

The circulatory system is subject to significant age-related changes. They consist in reducing the elastic properties of the walls of blood vessels and the appearance of sclerotic plaques. As a result of such changes, the lumen of the vessels decreases, which leads to a deterioration in the blood supply to this organ.

From the microcirculatory bed, blood enters through the veins, and lymph through the lymphatic vessels that flow into the subclavian veins.

Venous blood containing attached lymph flows into the heart, first into the right atrium, then into the right ventricle. From the latter, venous blood enters the lungs through the small (pulmonary) circulation.

Rice. 35. Small circle of blood circulation.

Scheme of blood circulation. Small (pulmonary) circulation(Fig. 35) serves to enrich the blood with oxygen in the lungs. It starts at right ventricle where does it come from pulmonary trunk. The pulmonary trunk, approaching the lungs, is divided into right and left pulmonary arteries. The latter branch in the lungs into arteries, arterioles, precapillaries and capillaries. In the capillary networks that braid the pulmonary vesicles (alveoli), the blood gives off carbon dioxide and receives oxygen in return. Oxygenated arterial blood flows from capillaries to venules and veins, which drain into four pulmonary veins exiting the lungs and entering left atrium. The pulmonary circulation ends in the left atrium.

Rice. 36. Systemic circulation.

Arterial blood entering the left atrium is directed to the left ventricle, where the systemic circulation begins.

Systemic circulation(Fig. 36) serves to deliver nutrients, enzymes, hormones and oxygen to all organs and tissues of the body and remove metabolic products and carbon dioxide from them.

It starts at left ventricle of the heart from which comes out aorta, bearing arterial blood, which contains the nutrients and oxygen necessary for the life of the body, and has a bright scarlet color. The aorta branches into arteries that go to all organs and tissues of the body and pass in their thickness into arterioles and capillaries. The capillaries are collected into venules and veins. Through the walls of the capillaries, metabolism and gas exchange occurs between the blood and body tissues. Arterial blood flowing in the capillaries gives off nutrients and oxygen and in return receives metabolic products and carbon dioxide (tissue respiration). Therefore, the blood entering the venous bed is poor in oxygen and rich in carbon dioxide and has a dark color - venous blood. The veins extending from the organs merge into two large trunks - superior and inferior vena cava that fall into right atrium where the systemic circulation ends.

Rice. 37. Vessels supplying the heart.

Thus, “from heart to heart” the systemic circulation looks like this: left ventricle - aorta - main branches of the aorta - arteries of medium and small caliber - arterioles - capillaries - venules - veins of medium and small caliber - veins extending from organs - upper and inferior vena cava - right atrium.

The addition to the great circle is third (cardiac) circulation serving the heart itself (Fig. 37). It originates from the ascending aorta right and left coronary arteries and ends veins of the heart, which merge into coronary sinus opening in right atrium.

The central organ of the circulatory system is the heart, the main function of which is to ensure continuous blood flow through the vessels.

Heart It is a hollow muscular organ that receives blood from the venous trunks flowing into it and drives the blood into the arterial system. Contraction of the heart chambers is called systole, relaxation is called diastole.

Rice. 38. Heart (front view).

The heart has the shape of a flattened cone (Fig. 38). It has a top and a base. Apex of the heart facing down, forward and to the left, reaching the fifth intercostal space at a distance of 8-9 cm to the left of the midline of the body. It is produced by the left ventricle. Base facing up, back and to the right. It is formed by the atria, and in front by the aorta and pulmonary trunk. The coronal sulcus, running transversely to the longitudinal axis of the heart, forms the boundary between the atria and ventricles.

In relation to the midline of the body, the heart is located asymmetrically: one third is on the right, two thirds on the left. On the chest, the borders of the heart are projected as follows:

§ apex of the heart determined in the fifth left intercostal space 1 cm medially from the midclavicular line;

§ upper bound (base of the heart) passes at the level of the upper edge of the third costal cartilage;

§ right border goes from the 3rd to the 5th ribs 2-3 cm to the right from the right edge of the sternum;

§ bottom line goes transversely from the cartilage of the 5th right rib to the apex of the heart;

§ left border- from the apex of the heart to the 3rd left costal cartilage.

Rice. 39. Human heart (opened).

cavity of the heart consists of 4 chambers: two atria and two ventricles - right and left (Fig. 39).

The right chambers of the heart are separated from the left by a solid partition and do not communicate with each other. The left atrium and left ventricle together make up the left or arterial heart (according to the property of the blood in it); the right atrium and right ventricle make up the right or venous heart. Between each atrium and ventricle is the atrioventricular septum, which contains the atrioventricular orifice.

Right and left atrium shaped like a cube. The right atrium receives venous blood from the systemic circulation and the walls of the heart, the left - arterial blood from the pulmonary circulation. On the back wall of the right atrium there are openings of the superior and inferior vena cava and coronary sinus, in the left atrium there are openings of 4 pulmonary veins. The atria are separated from each other by the interatrial septum. Above, both atria continue into processes, forming the right and left ears, which cover the aorta and pulmonary trunk at the base.

The right and left atria communicate with the corresponding ventricles through the atrioventricular openings located in the atrioventricular septa. The holes are limited by the annulus fibrosus, so they do not collapse. Along the edge of the holes are valves: on the right - tricuspid, on the left - bicuspid or mitral (Fig. 39). The free edges of the valves face the cavity of the ventricles. On the inner surface of both ventricles there are papillary muscles protruding into the lumen and tendon chords, from which tendinous filaments stretch to the free edge of the valve cusps, preventing the valve cusps from eversion into the atrial lumen (Fig. 39). In the upper part of each ventricle, there is one more opening: in the right ventricle, the opening of the pulmonary trunk, in the left - aorta, equipped with semilunar valves, the free edges of which are thickened due to small nodules (Fig. 39). Between the walls of the vessels and the semilunar valves are small pockets - the sinuses of the pulmonary trunk and aorta. The ventricles are separated from each other by the interventricular septum.

During atrial contraction (systole), the cusps of the left and right atrioventricular valves are open towards the ventricular cavities, they are pressed against their wall by the blood flow and do not prevent the passage of blood from the atria to the ventricles. Following the contraction of the atria, the contraction of the ventricles occurs (at the same time, the atria are relaxed - diastole). When the ventricles contract, the free edges of the valve cusps close under blood pressure and close the atrioventricular orifices. In this case, blood from the left ventricle enters the aorta, from the right - into the pulmonary trunk. The semilunar flaps of the valves are pressed against the walls of the vessels. Then the ventricles relax, and a general diastolic pause occurs in the cardiac cycle. At the same time, the sinuses of the valves of the aorta and the pulmonary trunk are filled with blood, due to which the valve flaps close, closing the lumen of the vessels and preventing the return of blood to the ventricles. Thus, the function of the valves is to allow blood flow in one direction or to prevent back flow of blood.

Wall of the heart consists of three layers (shells):

ü internal - endocardium lining the cavity of the heart and forming valves;

ü medium - myocardium, which makes up most of the wall of the heart;

ü external - epicardium, which is the visceral layer of the serous membrane (pericardium).

The inner surface of the cavities of the heart is lined endocardium. It consists of a layer of connective tissue with a large number of elastic fibers and smooth muscle cells covered with an inner endothelial layer. All heart valves are duplication (doubling) of the endocardium.

Myocardium formed by striated muscle tissue. It differs from skeletal muscle in its fiber structure and involuntary function. The degree of development of the myocardium in various departments heart is determined by the function they perform. In the atria, the function of which is to expel blood into the ventricles, the myocardium is most poorly developed and is represented by two layers. The ventricular myocardium has a three-layer structure, and in the wall of the left ventricle, which provides blood circulation in the vessels of the systemic circulation, it is almost twice as thick as the right ventricle, the main function of which is to ensure blood flow in the pulmonary circulation. The muscle fibers of the atria and ventricles are isolated from each other, which explains their separate contraction. First, both atria contract simultaneously, then both ventricles (the atria are relaxed during ventricular contraction).

An important role in the rhythmic work of the heart and in the coordination of the activity of the muscles of individual chambers of the heart is played by conducting system of the heart , which is represented by specialized atypical muscle cells that form special bundles and nodes under the endocardium (Fig. 40).

sinus node located between the right ear and the confluence of the superior vena cava. It is associated with the muscles of the atria and is important for their rhythmic contraction. The sinoatrial node is functionally associated with atrioventricular node located at the base of the interatrial septum. From this node to the interventricular septum stretches atrioventricular bundle (bundle of His). This bundle is divided into right and left leg, going to the myocardium of the corresponding ventricles, where it branches into Purkinje fibers. Due to this, the regulation of the rhythm of heart contractions is established - first the atria, and then the ventricles. Excitation from the sinoatrial node is transmitted through the atrial myocardium to the atrioventricular node, from which it spreads along the atrioventricular bundle to the ventricular myocardium.

Rice. 40. Conducting system of the heart.

Outside, the myocardium is covered epicardium representing the serous membrane.

Blood supply to the heart carried out by the right and left coronary or coronary arteries (Fig. 37), extending from the ascending aorta. The outflow of venous blood from the heart occurs through the veins of the heart, which flow into the right atrium both directly and through the coronary sinus.

Innervation of the heart carried out by the cardiac nerves extending from the right and left sympathetic trunks, and by the cardiac branches of the vagus nerves.

Pericardium. The heart is located in a closed serous sac - the pericardium, in which two layers are distinguished: external fibrous and internal serous.

The inner layer is divided into two sheets: visceral - epicardium (outer layer of the heart wall) and parietal, fused with the inner surface of the fibrous layer. Between the visceral and parietal sheets is the pericardial cavity containing serous fluid.

The activity of the circulatory system and, in particular, the heart, is influenced by numerous factors, including systematic sports. With increased and prolonged muscular work, increased demands are placed on the heart, as a result of which certain structural changes. First of all, these changes are manifested by an increase in the size and mass of the heart (mainly the left ventricle) and are called physiological or working hypertrophy. The greatest increase in the size of the heart is observed in cyclists, rowers, marathon runners, the most enlarged hearts in skiers. In runners and swimmers for short distances, in boxers and football players, an increase in the heart is found to a lesser extent.

VESSELS OF THE SMALL (PULMONARY) CIRCULATION

The pulmonary circulation (Fig. 35) serves to enrich the blood flowing from the organs with oxygen and remove carbon dioxide from it. This process is carried out in the lungs, through which all the blood circulating in the human body passes. Venous blood through the superior and inferior vena cava enters the right atrium, from it into the right ventricle, from which it exits pulmonary trunk. It goes to the left and up, crosses the aorta lying behind and at the level of 4-5 thoracic vertebrae is divided into the right and left pulmonary arteries, which go to the corresponding lung. In the lungs, the pulmonary arteries divide into branches that carry blood to the corresponding lung lobes. The pulmonary arteries accompany the bronchi along their entire length and, repeating their branching, the vessels divide into ever smaller intrapulmonary vessels, passing at the level of the alveoli into capillaries that braid the pulmonary alveoli. Gas exchange takes place through the walls of capillaries. The blood gives off excess carbon dioxide and is saturated with oxygen, as a result of which it becomes arterial and acquires a scarlet color. Oxygenated blood is collected into small, and then large veins that follow the course of arterial vessels. Blood flowing from the lungs is collected in four pulmonary veins that exit the lungs. Each pulmonary vein opens into the left atrium. The vessels of the small circle do not participate in the blood supply of the lung.

ARTERIES OF THE GREAT CIRCULATION

Aorta represents the main trunk of the arteries of the systemic circulation. It carries blood out of the left ventricle of the heart. As the distance from the heart increases, the cross-sectional area of ​​the arteries increases, i.e. the bloodstream becomes wider. In the area of ​​the capillary network, its increase is 600-800 times compared to the cross-sectional area of ​​the aorta.

The aorta is divided into three sections: the ascending aorta, the aortic arch, and the descending aorta. At the level of the 4th lumbar vertebra, the aorta divides into the right and left common iliac arteries (Fig. 41).

Rice. 41. Aorta and its branches.

Branches of the ascending aorta are right and left coronary arteries, supplying the wall of the heart (Fig. 37).

From the aortic arch depart from right to left: brachiocephalic trunk, left common carotid and left subclavian arteries (Fig. 42).

Shoulder head trunk located in front of the trachea and behind the right sternoclavicular joint, it is divided into the right common carotid and right subclavian arteries (Fig. 42).

Branches of the aortic arch supply blood to the organs of the head, neck and upper limbs. Projection of the aortic arch- in the middle of the handle of the sternum, brachiocephalic trunk - from the aortic arch to the right sternoclavicular joint, common carotid artery - along the sternocleidomastoid muscle to the level of the upper edge of the thyroid cartilage.

Common carotid arteries(right and left) go up both sides of the trachea and esophagus and at the level of the upper edge of the thyroid cartilage are divided into external and internal carotid arteries. The common carotid artery is pressed against the tubercle of the 6th cervical vertebra to stop bleeding.

The blood supply to the organs, muscles and skin of the neck and head is carried out due to the branches external carotid artery, which at the level of the neck of the lower jaw is divided into its final branches - the maxillary and superficial temporal artery. Branches of the external carotid artery supply blood to the external integuments of the head, face and neck, mimic and masticatory muscles, salivary glands, teeth of the upper and lower jaws, tongue, pharynx, larynx, hard and soft palate, palatine tonsils, sternocleidomastoid muscle and other muscles of the neck located above the hyoid bone.

Internal carotid artery(Fig. 42), starting from the common carotid artery, rises to the base of the skull and penetrates into the cranial cavity through the carotid canal. It does not give branches in the neck area. The artery supplies blood to the dura mater eyeball and its muscles, nasal mucosa, brain. Its main branches are ophthalmic artery, anterior and middle cerebral artery and posterior communicating artery(Fig. 42).

subclavian arteries(Fig. 42) depart left from the aortic arch, right from the brachiocephalic trunk. Both arteries exit through the upper opening of the chest to the neck, lie on the 1st rib and penetrate into the axillary region, where they receive the name axillary arteries. The subclavian artery supplies blood to the larynx, esophagus, thyroid and goiter glands, and back muscles.

Rice. 42. Branches of the aortic arch. Vessels of the brain.

Branches off the subclavian artery vertebral artery, blood supply to the brain and spinal cord, deep muscles of the neck. In the cranial cavity, right and left vertebral arteries merge together to form basilar artery, which at the anterior edge of the bridge (brain) is divided into two posterior cerebral arteries (Fig. 42). These arteries, together with the branches of the carotid artery, are involved in the formation of the arterial circle of the cerebrum.

The continuation of the subclavian artery is axillary artery. It lies deep in the armpit, passes along with the axillary vein and trunks of the brachial plexus. The axillary artery supplies blood shoulder joint, skin and muscles of the girdle of the upper limb and chest.

The continuation of the axillary artery is brachial artery, which supplies blood to the shoulder (muscles, bone and skin with subcutaneous tissue) and elbow joint. It reaches the elbow and at the level of the neck radius is divided into terminal branches - radial and ulnar arteries. These arteries feed with their branches the skin, muscles, bones and joints of the forearm and hand. These arteries anastomose widely with each other and form two networks in the area of ​​the hand: dorsal and palmar. On the palmar surface there are two arcs - superficial and deep. They are an important functional device, because. due to the diverse function of the hand, the vessels of the hand are often subjected to compression. With a change in blood flow in the superficial palmar arch, the blood supply to the hand does not suffer, since blood delivery occurs in such cases through the arteries of the deep arch.

It is important to know the projection of large arteries on the skin of the upper limb and the places of their pulsation when stopping bleeding and applying tourniquets in cases of sports injuries. The projection of the brachial artery is determined in the direction of the medial groove of the shoulder to the cubital fossa; radial artery - from the cubital fossa to the lateral styloid process; ulnar artery - from the ulnar fossa to the pisiform bone; superficial palmar arch - in the middle of the metacarpal bones, and deep - at their base. The place of pulsation of the brachial artery is determined in its medial sulcus, radial - in the distal forearm on the radius.

descending aorta(continuation of the aortic arch) runs along the left spinal column from the 4th thoracic to the 4th lumbar vertebrae, where it divides into its terminal branches - the right and left common iliac arteries (Fig. 41, 43). The descending aorta is divided into thoracic and abdominal parts. All branches of the descending aorta are divided into parietal (parietal) and visceral (visceral).

Parietal branches of the thoracic aorta: a) 10 pairs of intercostal arteries running along the lower edges of the ribs and supplying the muscles of the intercostal spaces, the skin and muscles of the lateral sections of the chest, back, upper sections of the anterior abdominal wall, the spinal cord and its membranes; b) superior phrenic arteries (right and left), supplying the diaphragm.

To the organs of the chest cavity (lungs, trachea, bronchi, esophagus, pericardium, etc.) visceral branches of the thoracic aorta.

To parietal branches abdominal aorta include the lower phrenic arteries and 4 lumbar arteries, which supply blood to the diaphragm, lumbar vertebrae, spinal cord, muscles and skin of the lumbar region and abdomen.

Visceral branches of the abdominal aorta(Fig. 43) are divided into paired and unpaired. Paired branches go to paired organs abdominal cavity: to the adrenal glands - the middle adrenal artery, to the kidneys - renal artery, to the testicles (or ovaries) - testicular or ovarian arteries. The unpaired branches of the abdominal aorta go to the unpaired organs of the abdominal cavity, mainly the organs of the digestive system. These include the celiac trunk, superior and inferior mesenteric arteries.

Rice. 43. Descending aorta and its branches.

celiac trunk(Fig. 43) departs from the aorta at the level of the 12th thoracic vertebra and is divided into three branches: the left gastric, common hepatic and splenic arteries, supplying the stomach, liver, gallbladder, pancreas, spleen, duodenum.

superior mesenteric artery departs from the aorta at the level of the 1st lumbar vertebra, it gives off branches to the pancreas, small intestine and early parts of the colon.

Inferior mesenteric artery departs from the abdominal aorta at the level of the 3rd lumbar vertebra, it supplies blood to lower divisions large intestine.

At the level of the 4th lumbar vertebra, the abdominal aorta divides into right and left common iliac arteries(Fig. 43). When bleeding from the underlying arteries, the trunk of the abdominal aorta is pressed against the spinal column in the navel, which is located above its bifurcation. At the superior edge of the sacroiliac joint, the common iliac artery divides into the external and internal iliac arteries.

internal iliac artery descends into the pelvis, where it gives off parietal and visceral branches. Parietal branches go to the muscles lumbar region, gluteal muscles, spinal column and spinal cord, muscles and skin of the thigh, hip joint. The visceral branches of the internal iliac artery supply blood to the pelvic organs and external genital organs.

Rice. 44. External iliac artery and its branches.

External iliac artery(Fig. 44) goes outwards and downwards, passes under inguinal ligament through a vascular lacuna to the thigh, where it is called the femoral artery. The external iliac artery gives branches to the muscles of the anterior wall of the abdomen, to the external genital organs.

Its continuation is femoral artery, which runs in the groove between the iliopsoas and pectineus muscles. Its main branches supply blood to the muscles of the abdominal wall, the ilium, the muscles of the thigh and femur, the hip and partly the knee joints, and the skin of the external genital organs. The femoral artery enters the popliteal fossa and continues into the popliteal artery.

Popliteal artery and its branches supply blood to the lower thigh muscles and the knee joint. She comes from rear surface knee joint to the soleus muscle, where it divides into the anterior and posterior tibial arteries, which feed the skin and muscles of the anterior and posterior muscle groups of the lower leg, knee and ankle joints. These arteries pass into the arteries of the foot: the anterior - into the dorsal (dorsal) artery of the foot, the posterior - into the medial and lateral plantar arteries.

The projection of the femoral artery on the skin of the lower limb is shown along the line connecting the middle of the inguinal ligament with the lateral epicondyle of the thigh; popliteal - along the line connecting the upper and lower corners of the popliteal fossa; anterior tibial - along the anterior surface of the lower leg; posterior tibial - from the popliteal fossa in the middle of the posterior surface of the lower leg to the inner ankle; dorsal artery of the foot - from the middle ankle joint to the first interosseous space; lateral and medial plantar arteries - along the corresponding edge of the plantar surface of the foot.

VEINS OF THE GREAT CIRCULATION

The venous system is a system of blood vessels through which blood returns to the heart. Venous blood flows through the veins from organs and tissues, excluding the lungs.

Most veins go along with arteries, many of them have the same names as arteries. The total number of veins is much greater than arteries, so the venous bed is wider than the arterial one. Each large artery, as a rule, is accompanied by one vein, and the middle and small arteries by two veins. In some parts of the body, for example in the skin, the saphenous veins run independently without arteries and are accompanied by cutaneous nerves. The lumen of the veins is wider than the lumen of the arteries. In the wall of internal organs that change their volume, veins form venous plexuses.

The veins of the systemic circulation are divided into three systems:

1) the system of the superior vena cava;

2) the system of the inferior vena cava, including both the portal vein system and

3) the system of veins of the heart, forming the coronary sinus of the heart.

The main trunk of each of these veins opens with an independent opening into the cavity of the right atrium. The superior and inferior vena cava anastomose with each other.

Rice. 45. Superior vena cava and its tributaries.

Superior vena cava system. superior vena cava 5-6 cm long is located in the chest cavity in anterior mediastinum. It is formed as a result of the confluence of the right and left brachiocephalic veins behind the connection of the cartilage of the first right rib with the sternum (Fig. 45). From here, the vein descends along the right edge of the sternum and joins the right atrium at the level of the 3rd rib. The superior vena cava collects blood from the head, neck, upper limbs, walls and organs of the chest cavity (except the heart), partly from the back and abdominal wall, i.e. from those areas of the body that are supplied with blood by the branches of the aortic arch and the thoracic part of the descending aorta.

Each brachiocephalic vein is formed as a result of the confluence of the internal jugular and subclavian veins (Fig. 45).

Internal jugular vein collects blood from the organs of the head and neck. On the neck, it goes as part of the neurovascular bundle of the neck along with the common carotid artery and vagus nerve. The tributaries of the internal jugular vein are outdoor and anterior jugular vein collecting blood from the integuments of the head and neck. The external jugular vein is clearly visible under the skin, especially when straining or in head-down positions.

subclavian vein(Fig. 45) is a direct continuation of the axillary vein. It collects blood from the skin, muscles and joints of the entire upper limb.

Veins of the upper limb(Fig. 46) are divided into deep and superficial or subcutaneous. They form numerous anastomoses.

Rice. 46. ​​Veins of the upper limb.

Deep veins accompany the arteries of the same name. Each artery is accompanied by two veins. The exceptions are the veins of the fingers and the axillary vein, formed as a result of the fusion of two brachial veins. All deep veins upper limbs have numerous tributaries in the form of small veins that collect blood from the bones, joints and muscles of the areas in which they pass.

The saphenous veins include (Fig. 46) include lateral saphenous vein arms or cephalic vein(begins in the radial section of the rear of the hand, goes along the radial side of the forearm and shoulder and flows into the axillary vein); 2) medial saphenous vein of the arm or main vein(begins on the ulnar side of the back of the hand, goes to the medial section of the anterior surface of the forearm, passes to the middle of the shoulder and flows into the brachial vein); and 3) intermediate vein of the elbow, which is an oblique anastomosis connecting the main and head veins in the elbow area. This vein is of great practical importance, as it serves as a place for intravenous infusions medicinal substances, blood transfusion and taking it for laboratory research.

Inferior vena cava system. inferior vena cava- the thickest venous trunk in the human body, located in the abdominal cavity to the right of the aorta (Fig. 47). It is formed at the level of the 4th lumbar vertebra from the confluence of two common iliac veins. The inferior vena cava goes up and to the right, passes through a hole in the tendon center of the diaphragm in chest cavity and enters the right atrium. The tributaries flowing directly into the inferior vena cava correspond to the paired branches of the aorta. They are divided into parietal veins and veins of the viscera (Fig. 47). To parietal veins include the lumbar veins, four on each side, and the inferior phrenic veins.

To veins of the viscera include testicular (ovarian), renal, adrenal and hepatic veins (Fig. 47). hepatic veins, flowing into the inferior vena cava, carry blood from the liver, where it enters through portal vein and the hepatic artery.

Portal vein(Fig. 48) is a thick venous trunk. It is located behind the head of the pancreas, its tributaries are the splenic, superior and inferior mesenteric veins. At the gates of the liver, the portal vein is divided into two branches, which go to the liver parenchyma, where they break up into many small branches that braid the hepatic lobules; numerous capillaries penetrate the lobules and eventually form into the central veins, which are collected in 3-4 hepatic veins, which flow into the inferior vena cava. Thus, the portal venous system, unlike other veins, is inserted between two networks of venous capillaries.

Rice. 47. Inferior vena cava and its tributaries.

Portal vein collects blood from all unpaired organs of the abdominal cavity, with the exception of the liver - from the organs of the gastrointestinal tract, where nutrients are absorbed, the pancreas and spleen. Blood flowing from the organs of the gastrointestinal tract enters the portal vein to the liver for neutralization and deposition in the form of glycogen; insulin comes from the pancreas, which regulates sugar metabolism; from the spleen - the breakdown products of blood elements enter, used in the liver to produce bile.

Common iliac veins, right and left, merging with each other at the level of the 4th lumbar vertebra, form the inferior vena cava (Fig. 47). Each common iliac vein at the level of the sacroiliac joint, it is composed of two veins: the internal iliac and the external iliac.

Internal iliac vein lies behind the artery of the same name and collects blood from the pelvic organs, its walls, external genital organs, from the muscles and skin of the gluteal region. Its tributaries form a number of venous plexuses (rectal, sacral, vesical, uterine, prostatic), anastomosing with each other.

Rice. 48. Portal vein.

As well as on the upper limb, veins of the lower limb divided into deep and superficial or subcutaneous, which pass independently of the arteries. The deep veins of the foot and lower leg are double and accompany the arteries of the same name. Popliteal vein, which is composed of all the deep veins of the lower leg, is a single trunk located in the popliteal fossa. Passing to the thigh, the popliteal vein continues into femoral vein, which is located medially from the femoral artery. Numerous muscular veins flow into the femoral vein, draining blood from the muscles of the thigh. After passing under the inguinal ligament, the femoral vein passes into external iliac vein.

Superficial veins form a rather dense subcutaneous venous plexus, into which blood is collected from the skin and superficial layers of the muscles of the lower extremities. The largest superficial veins are small saphenous vein of the leg(starts on the outside of the foot, goes along the back of the leg and flows into the popliteal vein) and great saphenous vein of the leg(starts at thumb foot, goes along its inner edge, then along the inner surface of the lower leg and thigh and flows into the femoral vein). The veins of the lower extremities have numerous valves that prevent the backflow of blood.

One of the important functional adaptations of the body, associated with the high plasticity of blood vessels and ensuring uninterrupted blood supply to organs and tissues, is collateral circulation. Collateral circulation refers to lateral, parallel blood flow through the lateral vessels. It occurs with temporary difficulties in blood flow (for example, with squeezing of blood vessels at the time of movement in the joints) and with pathological conditions(with blockage, wounds, ligation of blood vessels during operations). Lateral vessels are called collaterals. If the blood flow through the main vessels is obstructed, the blood rushes along the anastomoses to the nearest lateral vessels, which expand and their wall is rebuilt. As a result, impaired blood circulation is restored.

Track systems venous outflow blood are connected kava caval(between the inferior and superior vena cava) and port-cavalry(between portal and vena cava) anastomoses, which provide a roundabout flow of blood from one system to another. Anastomoses are formed by branches of the superior and inferior vena cava and the portal vein, where the vessels of one system communicate directly with another (for example, the venous plexus of the esophagus). Under normal conditions of the body's activity, the role of anastomoses is small. However, if the outflow of blood through one of the venous systems is obstructed, anastomoses take an active part in the redistribution of blood between the main outflow highways.

PATTERNS OF DISTRIBUTION OF ARTERIES AND VEINS

The distribution of blood vessels in the body has certain patterns. The arterial system reflects in its structure the laws of the structure and development of the body and its individual systems (P.F. Lesgaft). By supplying blood to various organs, it corresponds to the structure, function and development of these organs. Therefore, the distribution of arteries in the human body is subject to certain patterns.

Extraorgan arteries. These include arteries that go outside the organ before entering it.

1. Arteries are located along the neural tube and nerves. So, parallel to the spinal cord is the main arterial trunk - aorta, each segment of the spinal cord corresponds to segmental arteries. Arteries are initially laid down in connection with the main nerves, so in the future they go along with the nerves, forming neurovascular bundles, which also include veins and lymphatic vessels. There is a relationship between nerves and vessels, which contributes to the implementation of a single neurohumoral regulation.

2. According to the division of the body into organs of plant and animal life, the arteries are divided into parietal(to the walls of body cavities) and visceral(to their contents, i.e. to the insides). An example is the parietal and visceral branches of the descending aorta.

3. One main trunk goes to each limb - to the upper limb subclavian artery, to the lower limb - external iliac artery.

4. Most of the arteries are located according to the principle of bilateral symmetry: paired arteries of the soma and viscera.

5. Arteries run according to the skeleton, which is the basis of the body. So, along the spinal column is the aorta, along the ribs - the intercostal arteries. AT proximal parts limbs that have one bone (shoulder, thigh) are located in one main vessel (brachial, femoral arteries); in the middle sections, which have two bones (forearm, lower leg), there are two main arteries (radial and ulnar, large and small tibial).

6. Arteries follow the shortest distance, giving off branches to nearby organs.

7. Arteries are located on the flexion surfaces of the body, since when unbending, the vascular tube stretches and collapses.

8. The arteries enter the organ on a concave medial or internal surface facing the source of nutrition, therefore all the gates of the viscera are on a concave surface directed towards the midline, where the aorta lies, sending them branches.

9. The caliber of the arteries is determined not only by the size of the organ, but also by its function. Thus, the renal artery is not inferior in diameter to the mesenteric arteries that supply blood to the long intestine. This is due to the fact that it carries blood to the kidney, the urinary function of which requires a large blood flow.

Intraorganic arterial bed corresponds to the structure, function and development of the organ in which these vessels branch. This explains that in different bodies the arterial bed is built differently, and in similar ones it is approximately the same.

Patterns of distribution of veins:

1. In veins, blood flows in most of the body (torso and limbs) against the direction of gravity and therefore more slowly than in arteries. Its balance in the heart is achieved by the fact that the venous bed in its mass is much wider than the arterial one. The greater width of the venous bed compared to the arterial bed is provided by the large caliber of the veins, the paired accompaniment of the arteries, the presence of veins that do not accompany the arteries, a large number of anastomoses, and the presence of venous networks.

2. The deep veins accompanying the arteries, in their distribution, obey the same laws as the arteries they accompany.

3. Deep veins are involved in the formation of neurovascular bundles.

4. Superficial veins lying under the skin accompany the cutaneous nerves.

5. In humans, due to the vertical position of the body, a number of veins have valves, especially in the lower extremities.

FEATURES OF BLOOD CIRCULATION IN THE FETUS

On the early stages development, the embryo receives nutrients from the vessels of the yolk sac (auxiliary extraembryonic organ) - yolk circulation. Up to 7-8 weeks of development, the yolk sac also performs the function of hematopoiesis. Further develops placental circulation Oxygen and nutrients are delivered to the fetus from the mother's blood through the placenta. It happens in the following way. Enriched with oxygen and nutrients arterial blood flows from the mother's placenta to umbilical vein, which enters the body of the fetus in the navel and goes up to the liver. At the level of the hilum of the liver, the vein divides into two branches, one of which flows into the portal vein, and the other into the inferior vena cava, forming the venous duct. The branch of the umbilical vein, which flows into the portal vein, delivers pure arterial blood through it, this is due to the hematopoietic function necessary for the developing organism, which predominates in the fetus in the liver and decreases after birth. After passing through the liver, the blood flows through the hepatic veins into the inferior vena cava.

Thus, all blood from the umbilical vein enters the inferior vena cava, where it mixes with venous blood flowing through the inferior vena cava from the lower half of the fetal body.

Mixed (arterial and venous) blood flows through the inferior vena cava into the right atrium and through the oval hole located in the atrial septum enters the left atrium, bypassing the still non-functioning pulmonary circle. From the left atrium, mixed blood enters the left ventricle, then into the aorta, along the branches of which it goes to the walls of the heart, head, neck and upper limbs.

The superior vena cava and the coronary sinus also drain into the right atrium. Venous blood entering through the superior vena cava from the upper half of the body then enters the right ventricle, and from the latter into the pulmonary trunk. However, due to the fact that in the fetus the lungs do not yet function as a respiratory organ, only a small part of the blood enters the lung parenchyma and from there through the pulmonary veins to the left atrium. Most of the blood from the pulmonary trunk enters directly into the aorta through batallov duct which connects the pulmonary artery to the aorta. From the aorta, along its branches, blood enters the organs of the abdominal cavity and lower extremities, and through two umbilical arteries, which pass as part of the umbilical cord, it enters the placenta, carrying metabolic products and carbon dioxide with it. Top part body (head) receives blood richer in oxygen and nutrients. The lower half feeds worse than the upper half and lags behind in its development. This explains the small size of the pelvis and lower extremities of the newborn.

The act of birth is a leap in the development of the organism, in which there are fundamental qualitative changes in vital processes. The developing fetus passes from one environment (the uterine cavity with its relatively constant conditions: temperature, humidity, etc.) to another ( external world with its changing conditions), as a result of which metabolism, ways of eating and breathing change. Nutrients previously received through the placenta now come from the digestive tract, and oxygen begins to come not from the mother, but from the air due to the work of the respiratory organs. With the first breath and stretching of the lungs, the pulmonary vessels greatly expand and fill with blood. Then the batallian duct collapses and obliterates during the first 8-10 days, turning into a batallian ligament.

umbilical arteries overgrow during the first 2-3 days of life, umbilical vein- after 6-7 days. The flow of blood from the right atrium to the left through the foramen ovale stops immediately after birth, as the left atrium is filled with blood from the lungs. Gradually, this hole closes. In cases of non-closure of the foramen ovale and the batallian duct, they speak of the development in the child birth defect heart, which is the result of abnormal formation of the heart during the prenatal period.

CIRCULATORY SYSTEM

The circulatory system is the system of blood vessels and cavities

which the blood circulates. Through the circulatory system of the cell

and tissues of the body are supplied with nutrients and oxygen and

released from metabolic products. Therefore, the circulatory system

sometimes referred to as a transport or distribution system.

The heart and blood vessels form a closed system through which

blood moves due to contractions of the heart muscle and myocytes of the walls

vessels. The blood vessels are the arteries that carry blood from

heart, veins through which blood flows to the heart, and microcirculatory

a channel consisting of arterioles, capillaries, postcopillary venules and

arteriovenular anastomoses.

As you move away from the heart, the caliber of the arteries gradually decreases.

down to the smallest arterioles, which in the thickness of the organs pass into the network

capillaries. The latter, in turn, continue into small, gradually

enlarge

veins that carry blood to the heart. Circulatory system

divided into two circles of blood circulation large and small. The first one starts at

left ventricle and ends in the right atrium, the second begins in

right ventricle and ends in the left atrium. Blood vessels

are absent only in the epithelial cover of the skin and mucous membranes, in

hair, nails, cornea and articular cartilage.

Blood vessels get their name from the organs they

blood supply (renal artery, splenic vein), places of their discharge from

larger vessel (superior mesenteric artery, inferior mesenteric

artery), the bone to which they are attached (ulnar artery), directions

(medial artery surrounding the thigh), depth of occurrence (superficial

or deep artery). Many small arteries are called branches, and veins are

tributaries.

Depending on the area of ​​branching, the arteries are divided into parietal

(parietal), blood-supplying walls of the body, and visceral

(visceral), blood supply to internal organs. Before artery entry

into an organ it is called organ, having entered an organ it is called intraorgan. Last

branches within and supplies its individual structural elements.

Each artery splits into smaller vessels. At the main

type of branching from the main trunk - the main artery, the diameter of which

side branches gradually decrease. With tree type

branching artery immediately after its discharge is divided into two or

several terminal branches, while resembling the crown of a tree.

Blood, tissue fluid and lymph form the internal environment. It retains the relative constancy of its composition - physical and chemical properties (homeostasis), which ensures the stability of all body functions. Preservation of homeostasis is the result of neuro-humoral self-regulation. Each cell needs a constant supply of oxygen and nutrients, and the removal of metabolic products. Both of these things happen through the blood. The cells of the body do not directly come into contact with blood, since the blood moves through the vessels of a closed circulatory system. Each cell is washed by a liquid that contains the substances necessary for it. It is intercellular or tissue fluid.

Between the tissue fluid and the liquid part of the blood - plasma, through the walls of the capillaries, the exchange of substances is carried out by diffusion. Lymph is formed from tissue fluid that enters the lymphatic capillaries, which originate between tissue cells and pass into the lymphatic vessels that flow into the large veins of the chest. Blood is a liquid connective tissue. It consists of a liquid part - plasma and individual shaped elements: red blood cells - erythrocytes, white blood cells - leukocytes and platelets - platelets. Formed elements of blood are formed in the hematopoietic organs: in the red bone marrow, liver, spleen, lymph nodes. 1 mm cube blood contains 4.5-5 million erythrocytes, 5-8 thousand leukocytes, 200-400 thousand platelets. The cellular composition of the blood of a healthy person is fairly constant. Therefore, its various changes occurring in diseases can be of great diagnostic value. Under certain physiological conditions of the body, the qualitative and quantitative composition of the blood often changes (pregnancy, menstruation). However, slight fluctuations occur throughout the day, influenced by food intake, work, and the like. To eliminate the influence of these factors, blood for repeated analyzes should be taken at the same time and under the same conditions.

The human body contains 4.5-6 liters of blood (1/13 of its body weight).

Plasma makes up 55% of the blood volume, and formed elements - 45%. The red color of blood is given by red blood cells containing a red respiratory pigment - hemoglobin, which attaches oxygen in the lungs and gives it to the tissues. Plasma is a colorless transparent liquid composed of inorganic and organic matter(90% water, 0.9% various mineral salts). Plasma organic matter includes proteins - 7%, fats - 0.7%, 0.1% - glucose, hormones, amino acids, metabolic products. Homeostasis is maintained by the activity of the organs of respiration, excretion, digestion, etc., the influence of the nervous system and hormones. In response to influences from the external environment, responses automatically arise in the body that prevent strong changes in the internal environment.

The vital activity of body cells depends on the salt composition of the blood. And the constancy of the salt composition of the plasma ensures the normal structure and function of blood cells. Blood plasma performs the following functions:

1) transport;

2) excretory;

3) protective;

4) humoral.

Blood, continuously circulating in a closed system of blood vessels, performs various functions in the body:

1) respiratory - carries oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs;

2) nutritional (transport) - delivers nutrients to cells;

3) excretory - takes out junk food metabolism;

4) thermoregulatory - regulates body temperature;

5) protective - produces substances necessary to fight microorganisms

6) humoral - connects various organs and systems, transferring substances that are formed in them.

Hemoglobin, the main component of erythrocytes (red blood cells), is a complex protein consisting of heme (the iron-containing part of Hb) and globin (the protein part of Hb). The main function of hemoglobin is to carry oxygen from the lungs to the tissues, as well as to remove carbon dioxide (CO2) from the body and regulate the acid-base state (ACS)

Erythrocytes - (red blood cells) - the most numerous formed elements of blood containing hemoglobin, transporting oxygen and carbon dioxide. Formed from reticulocytes after their release from the bone marrow. Mature erythrocytes do not contain a nucleus, have the shape of a biconcave disc. The average life span of erythrocytes is 120 days.

Leukocytes are white blood cells that differ from erythrocytes in the presence of a nucleus, large size and the ability to amoeboid movement. The latter makes possible the penetration of leukocytes through the vascular wall into the surrounding tissues, where they perform their functions. The number of leukocytes in 1 mm3 of the peripheral blood of an adult is 6-9 thousand and is subject to significant fluctuations depending on the time of day, the state of the body, and the conditions in which it resides. The sizes of various forms of leukocytes range from 7 to 15 microns. The duration of stay of leukocytes in the vascular bed is from 3 to 8 days, after which they leave it, passing into the surrounding tissues. Moreover, leukocytes are only transported by blood, and their main functions - protective and trophic - are performed in tissues. The trophic function of leukocytes consists in their ability to synthesize a number of proteins, including enzyme proteins, which are used by tissue cells for building (plastic) purposes. In addition, some proteins released as a result of the death of leukocytes can also serve to carry out synthetic processes in other cells of the body.

The protective function of leukocytes lies in their ability to free the body from genetically alien substances (viruses, bacteria, their toxins, mutant cells of one's own body, etc.), while maintaining and maintaining the genetic constancy of the internal environment of the body. The protective function of white blood cells can be carried out either

By phagocytosis ("devouring" genetically alien structures),

By damaging the membranes of genetically foreign cells (which is provided by T-lymphocytes and leads to the death of foreign cells),

Production of antibodies (substances of a protein nature that are produced by B-lymphocytes and their descendants - plasma cells and are able to specifically interact with foreign substances (antigens) and lead to their elimination (death))

The production of a number of substances (for example, interferon, lysozyme, components of the complement system), which are capable of exerting a nonspecific antiviral or antibacterial effect.

Platelets (platelets) are fragments of large cells of the red bone marrow - megakaryocytes. They are non-nuclear, oval-rounded (in not active state have a disc-shaped shape, and in the active - spherical) and differ from other blood cells in the smallest sizes (from 0.5 to 4 microns). The number of platelets in 1 mm3 of blood is 250-450 thousand. The central part of platelets is granular (granulomere), and the peripheral part does not contain granules (hyalomer). They perform two functions: trophic in relation to the cells of the vascular walls (angiotrophic function: as a result of the destruction of platelets, substances are released that are used by the cells for their own needs) and participate in blood clotting. The latter is their main function and is determined by the ability of platelets to cluster and stick together into a single mass at the site of damage to the vascular wall, forming a platelet plug (thrombus), which temporarily clogs the gap in the vessel wall. In addition, according to some researchers, platelets are able to phagocytize foreign bodies from the blood and, like other uniform elements, fix antibodies on their surface.

Blood clotting is a protective reaction of the body, aimed at preventing the loss of blood from damaged vessels. The mechanism of blood clotting is very complex. It involves 13 plasma factors, designated by Roman numerals in the order of their chronological discovery. In the absence of damage to the blood vessels, all blood clotting factors are in an inactive state.

The essence of the enzymatic process of blood coagulation is the transition of the soluble plasma protein fibrinogen into insoluble fibrous fibrin, which forms the basis of a blood clot - a thrombus. The chain reaction of blood coagulation is started by the enzyme thromboplastin, which is released when tissues, vascular walls, or platelets are damaged (stage 1). Together with certain plasma factors and in the presence of Ca2 "ions, it converts the inactive enzyme prothrombin, formed by liver cells in the presence of vitamin K, into the active thrombin enzyme (stage 2). At the 3rd stage, fibrinogen is converted to fibrin with the participation of thrombin and Ca2+ ions

According to the generality of some antigenic properties of erythrocytes, all people are divided into several groups, called blood groups. Belonging to a certain blood group is congenital and does not change throughout life. The most important is the division of blood into four groups according to the "AB0" system and into two groups - according to the "Rhesus" system. Compliance with blood compatibility for these groups is of particular importance for safe blood transfusion. However, there are other, less significant, blood types. You can determine the probability of a child having a particular blood type, knowing the blood types of his parents.

Each individual person has one of four possible blood types. Each blood group differs in the content of specific proteins in plasma and red blood cells. In our country, the population is distributed according to blood types approximately as follows: group 1 - 35%, 11 - 36%, III - 22%, group IV - 7%.

The Rh factor is a special protein found in the red blood cells of most people. They are classified as Rh-positive. If such people are transfused with human blood with the absence of this protein (Rh-negative group), then serious complications are possible. To prevent them, gamma globulin, a special protein, is additionally administered. Each person needs to know their Rh factor and blood type and remember that they do not change throughout life, this is a hereditary trait.

The heart is the central organ of the circulatory system, which is a hollow muscular organ that functions as a pump and ensures the movement of blood in the circulatory system. The heart is a muscular hollow cone-shaped organ. In relation to the midline of a person (the line dividing the human body into left and right halves), the human heart is located asymmetrically - about 2/3 - to the left of the middle line of the body, about 1/3 of the heart - to the right of the midline of the human body. The heart is located in the chest, enclosed in a pericardial sac - the pericardium, located between the right and left pleural cavities containing the lungs. The longitudinal axis of the heart goes obliquely from top to bottom, from right to left and from back to front. The position of the heart is different: transverse, oblique or vertical. The vertical position of the heart most often occurs in people with a narrow and long chest , transverse - in people with a wide and short chest. Distinguish the base of the heart, directed anteriorly, downwards and to the left. At the base of the heart are the atria. From the base of the heart exit: the aorta and the pulmonary trunk, into the base of the heart enter: the superior and inferior vena cava, right and left pulmonary veins. Thus, the heart is fixed on the large vessels listed above. With its posterior surface, the heart is adjacent to the diaphragm (a bridge between the chest and abdominal cavities), and with its sternocostal surface, it faces the sternum and costal cartilages. Three grooves are distinguished on the surface of the heart - one coronal; between the atria and ventricles and two longitudinal (anterior and posterior) between the ventricles. The length of the heart of an adult varies from 100 to 150 mm, the width at the base is 80–110 mm, and the anteroposterior distance is 60–85 mm. The weight of the heart on average in men is 332 g, in women - 253 g. In newborns, the weight of the heart is 18-20 g. The heart consists of four chambers: right atrium, right ventricle, left atrium, left ventricle. The atria are located above the ventricles. The atrial cavities are separated from each other by the interatrial septum, and the ventricles are separated by the interventricular septum. The atria communicate with the ventricles through openings. The right atrium has a capacity of 100–140 ml in an adult, and a wall thickness of 2–3 mm. The right atrium communicates with the right ventricle through the right atrioventricular orifice, which has a tricuspid valve. Behind, the superior vena cava flows into the right atrium above, below - the inferior vena cava. The mouth of the inferior vena cava is limited by a flap. The coronary sinus of the heart, which has a valve, flows into the posterior-lower part of the right atrium. The coronary sinus of the heart collects venous blood from the heart's own veins. The right ventricle of the heart has the shape of a trihedral pyramid, with its base facing up. The capacity of the right ventricle in adults is 150-240 ml, the wall thickness is 5-7 mm. The weight of the right ventricle is 64-74 g. Two parts are distinguished in the right ventricle: the ventricle itself and the arterial cone located in the upper part of the left half of the ventricle. The arterial cone passes into the pulmonary trunk - a large venous vessel that carries blood to the lungs. Blood from the right ventricle enters the pulmonary trunk through the tricuspid valve. The left atrium has a capacity of 90-135 ml, a wall thickness of 2-3 mm. On the back wall of the atrium are the mouths of the pulmonary veins (vessels carrying oxygen-enriched blood from the lungs), two on the right and two on the left. the left ventricle has a conical shape; its capacity is from 130 to 220 ml; wall thickness 11 - 14 mm. The weight of the left ventricle is 130-150 g. There are two openings in the cavity of the left ventricle: the atrioventricular (left and front), equipped with a bicuspid valve, and the opening of the aorta (the main artery of the body), equipped with a tricuspid valve. In the right and left ventricles there are numerous muscular protrusions in the form of crossbars - trabeculae. The valves are controlled by the papillary muscles. The wall of the heart consists of three layers: the outer one - the epicardium, the middle one - the myocardium (muscle layer), and the inner one - the endocardium. Both the right and left atrium have small protruding parts on the sides - ears. The source of innervation of the heart is the cardiac plexus - part of the general thoracic vegetative plexus. In the heart itself there are many nerve plexuses and ganglions that regulate the frequency and strength of heart contractions, the work of heart valves. The blood supply to the heart is carried out by two arteries: the right coronary and the left coronary, which are the first branches of the aorta. The coronary arteries divide into smaller branches that enclose the heart. The diameter of the mouths of the right coronary artery ranges from 3.5 to 4.6 mm, the left - from 3.5 to 4.8 mm. Sometimes, instead of two coronary arteries, there may be one. The outflow of blood from the veins of the walls of the heart mainly occurs in the coronary sinus, which flows into the right atrium. Lymphatic fluid flows through the lymphatic capillaries from the endocardium and myocardium to the lymph nodes located under the epicardium, and from there the lymph enters the lymphatic vessels and nodes of the chest. The work of the heart as a pump is the main source of mechanical energy for the movement of blood in the vessels, which maintains the continuity of metabolism and energy in the body. The activity of the heart occurs due to the conversion of chemical energy into mechanical energy of myocardial contraction. In addition, the myocardium has the property of excitability. Excitation impulses arise in the heart under the influence of the processes occurring in it. This phenomenon is called automation. There are centers in the heart that generate impulses leading to excitation of the myocardium with its subsequent contraction (i.e., the process of automation is carried out with subsequent excitation of the myocardium). Such centers (nodes) provide rhythmic contraction in the required order of the atria and ventricles of the heart. The contractions of both atria, and then both ventricles, are carried out almost simultaneously. Inside the heart, due to the presence of valves, the blood moves in one direction. In the diastole phase (expansion of the cavities of the heart associated with relaxation of the myocardium), blood flows from the atria into the ventricles. In the systole phase (consecutive contractions of the atrial myocardium, and then the ventricles), blood flows from the right ventricle to the pulmonary trunk, from the left ventricle to the aorta. In the diastolic phase of the heart, the pressure in its chambers is close to zero; 2/3 of the volume of blood entering in the diastolic phase flows due to positive pressure in the veins outside the heart and 1/3 is pumped into the ventricles in the atrial systole phase. The atria are a reservoir for incoming blood; atrial volume may increase due to the presence of atrial lugs. A change in pressure in the chambers of the heart and the vessels departing from it causes the movement of the heart valves, the movement of blood. During contraction, the right and left ventricles expel 60-70 ml of blood each. Compared to other organs (with the exception of the cerebral cortex), the heart absorbs oxygen most intensively. In men, the size of the heart is 10-15% larger than in women, and the heart rate is 10-15% lower. Physical activity causes an increase in blood flow to the heart due to its displacement from the veins of the extremities during muscle contraction and from the veins of the abdominal cavity. This factor acts mainly under dynamic loads; static loads insignificantly change venous blood flow. An increase in the flow of venous blood to the heart leads to an increase in the work of the heart. With maximum physical activity, the value of the energy costs of the heart can increase by 120 times compared to the state of rest. Prolonged exposure to physical activity causes an increase in the reserve capacity of the heart. Negative emotions cause the mobilization of energy resources and increase the release of adrenaline (hormone of the adrenal cortex) into the blood - this leads to an increase in heart rate (normal heart rate is 68-72 per minute), which is an adaptive reaction of the heart. The heart is also affected by environmental factors. So, in conditions of high mountains, with a low oxygen content in the air, oxygen starvation of the heart muscle develops with a simultaneous reflex increase in blood circulation as a response to this oxygen starvation. Sharp fluctuations in temperature, noise, ionizing radiation, magnetic fields, electromagnetic waves, infrasound, many chemicals (nicotine, alcohol, carbon disulfide, organometallic compounds, benzene, lead) have a negative effect on the activity of the heart.

The circulatory system (cardiovascular system) performs a transport function - the transfer of blood to all organs and tissues of the body. The circulatory system consists of the heart and blood vessels. Heart (cor)- a muscular organ that pumps blood around the body. The heart and blood vessels form a closed system through which blood moves due to contractions of the heart muscle and vessel walls. The contractile activity of the heart, as well as the pressure difference in the vessels, determine the movement of blood through the circulatory system. The circulatory system forms - large and small.

Heart function The function of the heart is based on the alternation of relaxation (diastole) and contraction (systole) of the ventricles of the heart. Contractions and relaxation of the heart occur due to the work myocardium (myocardium)- the muscular layer of the heart. During diastole, blood from the organs of the body through the vein (A in the figure) enters the right atrium (atrium dextrum) and through the open valve into the right ventricle (ventriculus dexter). At the same time, blood from the lungs through the artery (B in the figure) enters the left atrium (atrium sinistrum) and through the open valve into the left ventricle (ventriculus sinister). The valves of vein B and artery A are closed. During diastole, the right and left atria contract and the right and left ventricles fill with blood. During systole, due to ventricular contraction, pressure increases and blood is pushed into vein B and artery A, while the valves between the atria and ventricles are closed, and the valves along vein B and artery A are open. Vein B transports blood to the pulmonary (pulmonary) circulation, and artery A to the systemic circulation. In the pulmonary circulation, the blood, passing through the lungs, is cleared of carbon dioxide and enriched with oxygen. The main purpose of the systemic circulation is to supply blood to all tissues and organs. human body. With each contraction, the heart ejects about 60 - 75 ml of blood (determined by the volume of the left ventricle). Peripheral resistance to blood flow in the vessels of the pulmonary circulation is approximately 10 times less than in the vessels of the systemic circulation. Therefore, the right ventricle works less intensively than the left. The alternation of systole and diastole is called the heart rate. Normal heart rate (a person does not experience serious mental or physical stress) 55 - 65 beats per minute. The frequency of the heart's own rhythm is calculated: 118.1 - (0.57 * age).

The heart is surrounded by a pericardial sac pericardium(from peri... and Greek kardia heart) containing pericardial fluid. This bag allows the heart to contract and expand freely. The pericardium is strong, it consists of connective tissue and has a two-layer structure. Pericardial fluid is contained between the layers of the pericardium and, acting as a lubricant, allows them to slide freely over each other as the heart expands and contracts.
The contraction and relaxation of the heart is set by the pacemaker, the sinoatrial node (pacemaker), a specialized group of cells in the heart in vertebrates, which spontaneously contracts, setting the rhythm for the beating of the heart itself.

In the heart, the role of the pacemaker is performed by sinus node(Sinoatrial Node, Sa Node) located at the junction of the superior vena cava with the right atrium. It generates impulses of excitation, leading to the beating of the heart.
Atrioventricular Node- part of the conduction system of the heart; located in the interatrial septum. The impulse enters it from the sinoatrial node through atrial cardiomyocytes, and then is transmitted through the atrioventricular bundle to the ventricular myocardium.
Bundle Of His atrioventricular bundle (AV bundle) - a bundle of cells of the cardiac conduction system, coming from the atrioventricular node through the atrioventricular septum towards the ventricles. At the top of the interventricular septum, it branches into right and left pedicles that run to each ventricle. The legs branch in the thickness of the myocardium of the ventricles into thin bundles of conductive muscle fibers. Through the bundle of His, excitation is transmitted from the atrioventricular (atrioventricular) node to the ventricles.

If the sinus node is not doing its job, it can be replaced with an artificial pacemaker, an electronic device that stimulates the heart with weak electrical signals, in order to maintain a normal heart rhythm. The rhythm of the heart is regulated by hormones that enter the bloodstream, that is, the work and the difference in the concentration of electrolytes inside and outside the blood cells, as well as their movement and create an electrical impulse of the heart.

Vessels. The largest vessels (both in diameter and length) of a person are veins and arteries. The largest of them, the artery going to the systemic circulation is the aorta. As they move away from the heart, the arteries pass into arterioles and then into capillaries. Similarly, veins pass into venules and further into capillaries. The diameter of the veins and arteries coming out of the heart reaches 22 millimeters, and the capillaries can only be seen through a microscope. Capillaries form an intermediate system between arterioles and venules - a capillary network. It is in these networks that, under the action of osmotic forces, oxygen and nutrients pass into individual cells of the body, and in return, the products of cellular metabolism enter the bloodstream.

All vessels are arranged in the same way, except that the walls of large vessels, such as the aorta, contain more elastic tissue than the walls of smaller arteries, which are dominated by muscle tissue. According to this tissue feature, the arteries are divided into elastic and muscular.
Endothelium- gives to an internal surface of a vessel the smoothness facilitating a blood-groove.
Basement membrane - (Membrana basalis) A layer of intercellular substance that delimits the epithelium, muscle cells, lemmocytes and endothelium (except for the endothelium of the lymphatic capillaries) from the underlying tissue; Possessing selective permeability, the basement membrane is involved in interstitial metabolism.
Smooth muscles- spirally oriented smooth muscle cells. Provide return of the vascular wall to its original state after its stretching by a pulse wave.
The outer elastic membrane and the inner elastic membrane allow muscles to glide when they contract or relax.
Outer sheath (adventitia)- consists of an external elastic membrane and loose connective tissue. The latter contains nerves, lymphatics and own blood vessels.
To ensure proper blood supply to all parts of the body during both phases of the cardiac cycle, a certain level of blood pressure is needed. Normal blood pressure averages 100 - 150 mmHg during systole and 60 - 90 mmHg during diastole. The difference between these indicators is called pulse pressure. For example, a person with a blood pressure of 120/70 mmHg has a pulse pressure of 50 mmHg.
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What is a "shell of the heart"?How many red blood cells are in a drop of blood?

How many kilometers of blood vessels are in my body?

This is a classic SWOT. The circulatory system consists of veins, arteries and capillaries. Its length is approximately 100,000 kilometers, and the area is more than half a hectare, and all this is in the body of one adult. According to Dave Williams, most of the length of the circulatory system is in "capillary miles." " Each capillary is very short, but we have an extremely large number of them.» 7 .

If you are in relatively good health, you will survive even if you lose about a third of your blood.

People living above sea level have a relatively large volume of blood compared to those living at sea level. Thus, the body adapts to an environment with a lack of oxygen.

If your kidneys are healthy, they filter about 95 milliliters of blood per minute.

If you stretch all your arteries, veins and blood vessels in length, you can wrap them around the Earth twice.

Blood travels throughout your body, starting from one side of the heart and returning to the other at the end of a full circle. Your blood travels 270,370 kilometers per day.

The distribution of blood throughout the human body is carried out due to the work of the cardiovascular system. Its main organ is the heart. Each of his blows contributes to the fact that the blood moves and nourishes all organs and tissues.

System Structure

There are different types of blood vessels in the body. Each of them has its own purpose. So, the system includes arteries, veins and lymphatic vessels. The first of them are designed to ensure that blood enriched with nutrients enters the tissues and organs. It is saturated with carbon dioxide and various products released during the life of the cells, and through the veins returns back to the heart. But before entering this muscular organ, the blood is filtered in the lymphatic vessels.

The total length of the system consisting of circulatory and lymphatic vessels, in the body of an adult is about 100 thousand km. And the heart is responsible for its normal functioning. It is it that pumps about 9.5 thousand liters of blood every day.

Principle of operation

The circulatory system is designed to support the entire body. If there are no problems, then it functions as follows. Oxygenated blood exits the left side of the heart through the largest arteries. It spreads throughout the body to all cells through wide vessels and the smallest capillaries, which can only be seen under a microscope. It is the blood that enters the tissues and organs.

The place where the arterial and venous systems connect is called the capillary bed. The walls of the blood vessels in it are thin, and they themselves are very small. This allows you to fully release oxygen and various nutrients through them. The waste blood enters the veins and returns through them to the right side of the heart. From there, it enters the lungs, where it is enriched again with oxygen. Passing through the lymphatic system, the blood is cleansed.

Veins are divided into superficial and deep. The first are close to the surface of the skin. Through them, blood enters the deep veins, which return it to the heart.

The regulation of blood vessels, heart function and general blood flow is carried out by the central nervous system and local chemicals released in the tissues. This helps control the flow of blood through the arteries and veins, increasing or decreasing its intensity depending on the processes taking place in the body. For example, it increases with physical activity and decreases with injury.

How does blood flow

The spent "depleted" blood through the veins enters the right atrium, from where it flows into the right ventricle of the heart. With powerful movements, this muscle pushes the incoming fluid into the pulmonary trunk. It is divided into two parts. The blood vessels of the lungs are designed to enrich the blood with oxygen and return them to the left ventricle of the heart. Each person has this part of him more developed. After all, it is the left ventricle that is responsible for how the entire body will be supplied with blood. It is estimated that the load that falls on it is 6 times greater than that to which the right ventricle is subjected.

The circulatory system includes two circles: small and large. The first of them is designed to saturate the blood with oxygen, and the second - for its transportation throughout the orgasm, delivery to every cell.

Requirements for the circulatory system

In order for the human body to function normally, a number of conditions must be met. First of all, attention is paid to the state of the heart muscle. After all, it is she who is the pump that drives the necessary biological fluid through the arteries. If the work of the heart and blood vessels is impaired, the muscle is weakened, then this can cause peripheral edema.

It is important that the difference between the areas of low and high pressure is observed. It is necessary for normal blood flow. So, for example, in the region of the heart, the pressure is lower than at the level of the capillary bed. This allows you to comply with the laws of physics. Blood moves from an area of ​​higher pressure to an area where it is lower. If a number of diseases occur, due to which the established balance is disturbed, then this is fraught with congestion in the veins, swelling.

The ejection of blood from the lower extremities is carried out thanks to the so-called musculo-venous pumps. This is what the calf muscles are called. With each step they contract and push blood against natural force attraction towards the right atrium. If this function is disturbed, for example, as a result of injury and temporary immobilization of the legs, then edema occurs due to a decrease in venous return.

Another important link responsible for ensuring that the human blood vessels function normally are venous valves. They are designed to support the fluid flowing through them until it enters the right atrium. If this mechanism is disturbed, and this is possible as a result of injuries or due to valve wear, abnormal blood collection will be observed. As a result, this leads to an increase in pressure in the veins and squeezing out the liquid part of the blood into the surrounding tissues. A striking example of a violation of this function is varicose veins in the legs.

Vessel classification

To understand how the circulatory system works, it is necessary to understand how each of its components functions. So, the pulmonary and hollow veins, the pulmonary trunk and the aorta are the main ways of moving the necessary biological fluid. And all the rest are able to regulate the intensity of the inflow and outflow of blood to the tissues due to the ability to change their lumen.

All vessels in the body are divided into arteries, arterioles, capillaries, venules, veins. All of them form a closed connecting system and serve a single purpose. Moreover, each blood vessel has its own purpose.

arteries

The areas through which blood moves are divided depending on the direction in which it moves in them. So, all arteries are designed to carry blood from the heart throughout the body. They are elastic, muscular and muscular-elastic type.

The first type includes those vessels that are directly connected with the heart and exit from its ventricles. This is the pulmonary trunk, pulmonary and carotid arteries, aorta.

All of these vessels of the circulatory system consist of elastic fibers that are stretched. This happens with every heartbeat. As soon as the contraction of the ventricle has passed, the walls return to their original form. Due to this, normal pressure is maintained for a period until the heart fills with blood again.

Blood enters all tissues of the body through the arteries that depart from the aorta and pulmonary trunk. At the same time, different organs need different amounts of blood. This means that the arteries must be able to narrow or expand their lumen so that the fluid passes through them only in the required doses. This is achieved due to the fact that smooth muscle cells work in them. Such human blood vessels are called distributive. Their lumen is regulated by the sympathetic nervous system. The muscular arteries include the artery of the brain, radial, brachial, popliteal, vertebral and others.

Other types of blood vessels are also isolated. These include muscular-elastic or mixed arteries. They can contract very well, but at the same time they have high elasticity. This type includes the subclavian, femoral, iliac, mesenteric arteries, celiac trunk. They contain both elastic fibers and muscle cells.

Arterioles and capillaries

As blood moves along the arteries, their lumen decreases and the walls become thinner. Gradually they pass into the smallest capillaries. The area where arteries end is called arterioles. Their walls consist of three layers, but they are weakly expressed.

The thinnest vessels are the capillaries. Together, they represent the longest part of the entire circulatory system. It is they who connect the venous and arterial channels.

A true capillary is a blood vessel that is formed as a result of branching of arterioles. They can form loops, networks that are located in the skin or synovial bags, or vascular glomeruli that are located in the kidneys. The size of their lumen, the speed of blood flow in them and the shape of the networks formed depend on the tissues and organs in which they are located. So, for example, in the skeletal muscles, lungs and nerve sheaths are the most thin vessels- their thickness does not exceed 6 microns. They form only flat networks. In mucous membranes and skin, they can reach 11 microns. In them, the vessels form a three-dimensional network. The widest capillaries are found in the hematopoietic organs, endocrine glands. Their diameter in them reaches 30 microns.

The density of their placement is also not the same. The highest concentration of capillaries is noted in the myocardium and brain, for every 1 mm 3 there are up to 3,000 of them. skeletal muscle there are only up to 1000 of them, and even less in bone tissue. It is also important to know that in an active state, under normal conditions, blood does not circulate in all capillaries. About 50% of them are in an inactive state, their lumen is compressed to a minimum, only plasma passes through them.

Venules and veins

Capillaries, which receive blood from arterioles, unite and form more large vessels. They are called postcapillary venules. The diameter of each such vessel does not exceed 30 µm. Folds form at the transition points, which perform the same functions as the valves in the veins. Elements of blood and plasma can pass through their walls. Postcapillary venules unite and flow into collecting venules. Their thickness is up to 50 microns. Smooth muscle cells begin to appear in their walls, but often they do not even surround the lumen of the vessel, but they outer shell already clearly expressed. The collecting venules become muscle venules. The diameter of the latter often reaches 100 microns. They already have up to 2 layers of muscle cells.

The circulatory system is designed in such a way that the number of vessels that drain blood is usually twice the number of those through which it enters the capillary bed. In this case, the liquid is distributed as follows. Up to 15% of the total amount of blood in the body is in the arteries, up to 12% in the capillaries, and 70-80% in the venous system.

By the way, fluid can flow from arterioles to venules without entering the capillary bed through special anastomoses, the walls of which include muscle cells. They are found in almost all organs and are designed to ensure that blood can be discharged into the venous bed. With their help, pressure is controlled, the transition of tissue fluid and blood flow through the organ is regulated.

Veins are formed after the confluence of venules. Their structure directly depends on the location and diameter. The number of muscle cells is affected by the place of their localization and the factors under the influence of which fluid moves in them. Veins are divided into muscular and fibrous. The latter include the vessels of the retina, spleen, bones, placenta, soft and hard shells of the brain. The blood circulating in the upper part of the body moves mainly under the force of gravity, as well as under the influence of the suction action during inhalation of the chest cavity.

The veins of the lower extremities are different. Each blood vessel in the legs must resist the pressure that is created by the fluid column. And if the deep veins are able to maintain their structure due to the pressure of the surrounding muscles, then the superficial ones have a harder time. They have a well-developed muscle layer, and their walls are much thicker.

Also, a characteristic difference between the veins is the presence of valves that prevent the backflow of blood under the influence of gravity. True, they are not in those vessels that are in the head, brain, neck and internal organs. They are also absent in the hollow and small veins.

The functions of blood vessels differ depending on their purpose. So, veins, for example, serve not only to move fluid to the region of the heart. They are also designed to reserve it in separate areas. The veins are activated when the body is working hard and needs to increase the volume of circulating blood.

The structure of the walls of the arteries

Each blood vessel is made up of several layers. Their thickness and density depend solely on what type of veins or arteries they belong to. It also affects their composition.

So, for example, elastic arteries contain a large number of fibers that provide stretching and elasticity of the walls. The inner shell of each such blood vessel, which is called the intima, is about 20% of the total thickness. It is lined with endothelium, and under it is loose connective tissue, intercellular substance, macrophages, muscle cells. The outer layer of the intima is limited by an internal elastic membrane.

The middle layer of such arteries consists of elastic membranes, with age they thicken, their number increases. Between them are smooth muscle cells that produce intercellular substance, collagen, elastin.

The outer shell of the elastic arteries is formed by fibrous and loose connective tissue, elastic and collagen fibers are located longitudinally in it. It also contains small vessels and nerve trunks. They are responsible for the nutrition of the outer and middle shells. It is the outer part that protects the arteries from ruptures and overstretching.

The structure of blood vessels, which are called muscular arteries, is not much different. They also have three layers. The inner shell is lined with endothelium, it contains the inner membrane and loose connective tissue. In small arteries, this layer is poorly developed. The connective tissue contains elastic and collagen fibers, they are located longitudinally in it.

The middle layer is formed by smooth muscle cells. They are responsible for the contraction of the entire vessel and for pushing blood into the capillaries. Smooth muscle cells are connected to the intercellular substance and elastic fibers. The layer is surrounded by a kind of elastic membrane. Fibers located in muscle layer, are connected to the outer and inner shells of the layer. They seem to form an elastic frame that prevents the artery from sticking together. And muscle cells are responsible for regulating the thickness of the lumen of the vessel.

The outer layer consists of loose connective tissue, in which collagen and elastic fibers are located, they are located obliquely and longitudinally in it. Nerves, lymphatic and blood vessels pass through it.

The structure of mixed-type blood vessels is an intermediate link between muscular and elastic arteries.

Arterioles also consist of three layers. But they are rather weakly expressed. The inner shell is the endothelium, a layer of connective tissue and an elastic membrane. The middle layer consists of 1 or 2 layers of muscle cells that are arranged in a spiral.

The structure of the veins

In order for the heart and blood vessels called arteries to function, it is necessary that blood can rise back up, bypassing the force of gravity. For these purposes, venules and veins, which have a special structure, are intended. These vessels consist of three layers, as well as arteries, although they are much thinner.

The inner shell of the veins contains endothelium, it also has a poorly developed elastic membrane and connective tissue. The middle layer is muscular, it is poorly developed, there are practically no elastic fibers in it. By the way, precisely because of this, the cut vein always subsides. The outer shell is the thickest. It consists of connective tissue, it contains a large number of collagen cells. It also contains smooth muscle cells in some veins. They help push blood towards the heart and prevent its reverse flow. The outer layer also contains lymph capillaries.