Organs of the circulatory system: structure and functions. Science Technology Technology Are all blood vessels the same size?

The circulatory system is a single anatomical and physiological formation, the main function of which is blood circulation, that is, the movement of blood in the body.
Thanks to blood circulation, gas exchange occurs in the lungs. During this process, carbon dioxide is removed from the blood, and oxygen from the inhaled air enriches it. Blood delivers oxygen and nutrients to all tissues, removing metabolic (decay) products from them.
The circulatory system is also involved in the processes of heat transfer, ensuring the vital activity of the body in different environmental conditions. Also, this system is involved in the humoral regulation of the activity of organs. Hormones are secreted by the endocrine glands and delivered to susceptible tissues. So the blood unites all parts of the body into a single whole.

Parts of the vascular system

The vascular system is heterogeneous in morphology (structure) and function. It can be divided into the following parts with a small degree of conventionality:

• aortoarterial chamber;
• vessels of resistance;
• exchange vessels;
• arteriovenular anastomoses;
• capacitive vessels.

The aortoarterial chamber is represented by the aorta and large arteries (common iliac, femoral, brachial, carotid, and others). Muscle cells are also present in the wall of these vessels, but elastic structures predominate, preventing their collapse during cardiac diastole. The vessels of the elastic type maintain the constancy of the blood flow velocity, regardless of pulse shocks.
Resistance vessels are small arteries, in the wall of which muscle elements predominate. They are able to quickly change their lumen, taking into account the needs of an organ or muscle for oxygen. These vessels are involved in maintaining blood pressure. They actively redistribute blood volumes between organs and tissues.
Exchange vessels are capillaries, the smallest branches of the circulatory system. Their wall is very thin, gases and other substances easily penetrate through it. Blood can flow from the smallest arteries (arterioles) into venules, bypassing capillaries, through arteriovenular anastomoses. These "connecting bridges" play a large role in heat transfer.
Capacitance vessels are so called because they are able to hold much more blood than arteries. These vessels include venules and veins. Through them, blood flows back to the central organ of the circulatory system - the heart.

Circles of blood circulation

Circulatory circles were described as early as the 17th century by William Harvey.
The aorta emerges from the left ventricle and begins the systemic circulation. The arteries that carry blood to all organs are separated from it. Arteries are divided into ever smaller branches, covering all the tissues of the body. Thousands of tiny arteries (arterioles) break up into a huge number of the smallest vessels - capillaries. Their walls are characterized by high permeability, so gas exchange occurs in the capillaries. Here, arterial blood is transformed into venous blood. Venous blood enters the veins, which gradually unite and eventually form the superior and inferior vena cava. The mouths of the latter open into the cavity of the right atrium.
In the pulmonary circulation, blood passes through the lungs. It gets there through the pulmonary artery and its branches. In the capillaries surrounding the alveoli, gas exchange with air occurs. Oxygenated blood flows through the pulmonary veins to the left side of the heart.
Some important organs (brain, liver, intestines) have blood supply features - regional blood circulation.

The structure of the vascular system

The aorta, leaving the left ventricle, forms the ascending part, from which the coronary arteries are separated. Then it bends, and vessels depart from its arc, directing blood to the arms, head, and chest. Then the aorta goes down along the spine, where it divides into vessels that carry blood to the organs of the abdominal cavity, pelvis, and legs.

The veins accompany the arteries of the same name.
Separately, it is necessary to mention the portal vein. It carries blood away from the digestive organs. In addition to nutrients, it may contain toxins and other harmful agents. The portal vein delivers blood to the liver, where toxic substances are removed.

The structure of the vascular walls

Arteries have outer, middle and inner layers. The outer layer is connective tissue. In the middle layer there are elastic fibers that support the shape of the vessel, and muscle. Muscle fibers can contract and change the lumen of the artery. From the inside, the arteries are lined with endothelium, which ensures a smooth flow of blood without obstruction.

The walls of veins are much thinner than those of arteries. They have very little elastic tissue, so they stretch and fall off easily. The inner wall of the veins forms folds: venous valves. They prevent the downward movement of venous blood. The outflow of blood through the veins is also ensured by the movement of skeletal muscles, "squeezing out" the blood when walking or running.

Regulation of the circulatory system

The circulatory system almost instantly responds to changes in external conditions and the internal environment of the body. Under stress or stress, it responds with an increase in heart rate, an increase in blood pressure, an improvement in blood supply to the muscles, a decrease in the intensity of blood flow in the digestive organs, and so on. During rest or sleep, the reverse processes occur.

The regulation of the function of the vascular system is carried out by neurohumoral mechanisms. The highest level regulatory centers are located in the cerebral cortex and in the hypothalamus. From there, the signals go to the vasomotor center, which is responsible for vascular tone. Through the fibers of the sympathetic nervous system, impulses enter the walls of blood vessels.

In the regulation of the function of the circulatory system, the feedback mechanism is very important. In the walls of the heart and blood vessels there are a large number of nerve endings that perceive changes in pressure (baroreceptors) and the chemical composition of the blood (chemoreceptors). Signals from these receptors go to higher regulatory centers, helping the circulatory system quickly adapt to new conditions.

Humoral regulation is possible with the help of the endocrine system. Most human hormones in one way or another affect the activity of the heart and blood vessels. The humoral mechanism involves adrenaline, angiotensin, vasopressin and many other active substances.

Blood- a liquid tissue that circulates in the human circulatory system and is an opaque red liquid consisting of pale yellow plasma and cells suspended in it - red blood cells (erythrocytes), white blood cells (leukocytes) and red platelets (platelets). The share of suspended cells (shaped elements) accounts for 42–46% of the total blood volume.

The main function of blood is the transport of various substances within the body. It carries respiratory gases (oxygen and carbon dioxide) in both physically dissolved and chemically bound form. Blood has this ability due to hemoglobin, a protein contained in red blood cells. In addition, the blood carries nutrients from the organs where they are absorbed or stored to where they are consumed; the metabolites formed here (metabolic products) are transported to the excretory organs or to those structures where their further use can take place. Purposefully, hormones, vitamins and enzymes are also transferred to target organs by blood. Due to the high heat capacity of its main component - water (1 liter of plasma contains 900–910 g of water), blood ensures the distribution of heat generated during metabolism and its release into the external environment through the lungs, respiratory tract and skin surface.

The proportion of blood in an adult is approximately 6-8% of the total body weight, which corresponds to 4-6 liters. A person's blood volume can undergo significant and long-term fluctuations depending on the degree of fitness, climatic and hormonal factors. So, in some athletes, the volume of blood as a result of training can exceed 7 liters. And after a long period of bed rest, it can become below normal. Short-term changes in blood volume are observed during the transition from a horizontal to a vertical position of the body and during muscle exercise.

Blood can perform its functions only when it is in constant motion. This movement is carried out through the system of vessels (elastic tubules) and is provided by the heart. Thanks to the vascular system of the body, blood is available to all corners of the human body, every cell. The heart and blood vessels (arteries, capillaries, veins) form cardiovascular system (Fig. 2.1).

The movement of blood through the vessels of the lungs from the right heart to the left heart is called pulmonary circulation (small circle). It begins with the right ventricle, which ejects blood into the pulmonary trunk. Then the blood enters the vascular system of the lungs, which in general terms has the same structure as the systemic circulation. Further, through four large pulmonary veins, it enters the left atrium (Fig. 2.2).

It should be noted that arteries and veins differ not in the composition of the blood moving in them, but in the direction of movement. So, through the veins, blood flows to the heart, and through the arteries, it flows away from it. In the systemic circulation, oxygenated (oxygenated) blood flows through the arteries, and in the pulmonary circulation, through the veins. Therefore, when blood saturated with oxygen is called arterial, only the systemic circulation is meant.

Heart is a hollow muscular organ divided into two parts - the so-called "left" and "right" heart, each of which includes an atrium and a ventricle. Partially deoxygenated blood from the organs and tissues of the body enters the right heart, pushing it to the lungs. In the lungs, the blood is saturated with oxygen, partially deprived of carbon dioxide, then returns to the left heart and again enters the organs.

The pumping function of the heart is based on the alternation of contraction (systole) and relaxation (diastole) of the ventricles, which is possible due to the physiological characteristics of the myocardium (the muscle tissue of the heart, which makes up the bulk of its mass) - automaticity, excitability, conduction, contractility and refractoriness. During diastole the ventricles fill with blood, and during systole they throw it into large arteries (aorta and pulmonary trunk). At the outlet of the ventricles, valves are located that prevent the return of blood from the arteries to the heart. Before filling the ventricles, blood flows through large veins (caval and pulmonary) into the atria.

Rice. 2.1. Human cardiovascular system

Atrial systole precedes ventricular systole; thus, the atria serve as an auxiliary pump, contributing to the filling of the ventricles.

Rice. 2.2. The structure of the heart, small (pulmonary) and large circles of blood circulation

The blood supply to all organs (except the lungs) and the outflow of blood from them is called the systemic circulation (large circle). It begins with the left ventricle, which ejects blood into the aorta during systole. Numerous arteries depart from the aorta, through which the blood flow is distributed to several parallel regional vascular networks that supply blood to individual organs and tissues - the heart, brain, liver, kidneys, muscles, skin, etc. The arteries divide, and as their number grows the diameter of each of them decreases. As a result of the branching of the smallest arteries (arterioles), a capillary network is formed - a dense interlacing of small vessels with very thin walls. It is here that the main two-way exchange of various substances between blood and cells occurs. When the capillaries merge, venules are formed, which are then combined into veins. Ultimately, only two veins enter the right atrium - the superior vena cava and the inferior vena cava.

Of course, in fact, both circles of blood circulation constitute a single bloodstream, in two parts of which (the right and left heart) the blood is supplied with kinetic energy. Although there is a fundamental functional difference between them. The volume of blood ejected into a large circle should be distributed over all organs and tissues, the need for blood supply to which is different and depends on their condition and activity. Any changes are instantly registered by the central nervous system (CNS), and the blood supply to the organs is regulated by a number of control mechanisms. As for the vessels of the lungs, through which a constant amount of blood passes, they make relatively constant demands on the right heart and perform mainly the functions of gas exchange and heat transfer. Therefore, the system of regulation of pulmonary blood flow is less complex.

In an adult, approximately 84% of all blood is contained in the systemic circulation, 9% in the pulmonary circulation, and the remaining 7% directly in the heart. The largest volume of blood is contained in the veins (approximately 64% of the total blood volume in the body), i.e., the veins play the role of blood reservoirs. At rest, blood circulates in only about 25-35% of all capillaries. The main hematopoietic organ is the bone marrow.

The requirements imposed by the body on the circulatory system vary significantly, so its activity varies widely. So, at rest in an adult, 60-70 ml of blood (systolic volume) is ejected into the vascular system with each contraction of the heart, which corresponds to 4-5 liters of cardiac output (the amount of blood ejected by the ventricle in 1 min). And with heavy physical exertion, the minute volume increases to 35 liters and above, while the systolic blood volume can exceed 170 ml, and the systolic blood pressure reaches 200–250 mm Hg. Art.

In addition to blood vessels in the body, there is another type of vessel - lymphatic.

Lymph- a colorless liquid formed from blood plasma by filtering it into the interstitial spaces and from there into the lymphatic system. Lymph contains water, proteins, fats and metabolic products. Thus, the lymphatic system forms an additional drainage system, through which tissue fluid flows into the bloodstream. All tissues, with the exception of the superficial layers of the skin, central nervous system and bone tissue, are penetrated by many lymphatic capillaries. These capillaries, unlike blood capillaries, are closed at one end. Lymphatic capillaries are collected in larger lymphatic vessels, which flow into the venous bed in several places. Therefore, the lymphatic system is part of the cardiovascular system.

Among the main systems that make up the human body, a special place is occupied by the circulatory system. How the circulatory system works until the 16th century remained a mystery to scientists. Such outstanding thinkers as Aristotle, Galen, Harvey and many others worked on its solution. All their discoveries are summarized in a coherent system of anatomical and physiological concepts.

History reference

A special role in the formation of correct ideas about what organs the human circulatory system consists of was played by the Spanish scientist Servetus and the English naturalist William Garvey. The first managed to prove that blood from the right ventricle can enter the left atrium only through the network of lungs. Harvey discovered the so-called large circle (closed) circulation. Thus, an end was put to the question of whether the blood moves strictly in a closed system, or not. The circulatory system of humans and mammals is closed.

It is also necessary to recall the works of the Italian physician Malpighi, who discovered the capillary circulation. Thanks to his research, it became clear how it turns into venous and vice versa. How does anatomy consider this question? The human circulatory system is a collection of organs such as the heart, blood vessels and auxiliary organs - red bone marrow, spleen and liver.

The heart is the main organ of the human circulatory system.

Since ancient times, in all cultures without exception, the heart has been assigned a central role not only as an organ of the physical body, but also as a spiritual receptacle of a person's personality. In the expressions “friend of the heart”, “with all my heart”, “sadness in my heart”, people showed the role of this organ in the formation of emotions and feelings.

Liquid tissue in the human body

The functions of transporting oxygen and nutrients, removing toxins and toxins, as well as producing antibodies are performed by the circulatory system. Blood, the structure of which can be represented as a mixture of cells (leukocytes, erythrocytes and platelets) and plasma (the liquid part), ensures the above tasks.

In the human body, there are hematopoietic tissues, one of which is myeloid. It is leading in the red bone marrow, is located in the diaphysis and contains the precursors of erythrocytes, leukocytes and platelets.

Features of the structure of blood

The red color of blood is due to the presence of the pigment hemoglobin. It is he who is responsible for the transport of gases dissolved in the blood - oxygen and carbon monoxide. It can have two forms: oxyhemoglobin and carboxyhemoglobin. 90% consists of water.

The remaining substances are proteins (albumin, fibrinogen, gamma globulin) and mineral salts, the main of which is sodium chloride. The formed elements of the blood perform the following functions:

• erythrocytes - carry oxygen;
• leukocytes, or white blood cells (neutrophils, eosinophils, T-lymphocytes, etc.), are involved in the formation of immunity;
• platelets - help stop bleeding in case of violation of the integrity of the walls of blood vessels (responsible for blood clotting).

The human circulatory system, due to the various functions of the blood, is the most important in maintaining the homeostasis of the body.

Vessels of the body: arteries, veins, capillaries

To understand what organs the human circulatory system consists of, you need to imagine it as a network of tubes with different diameters and wall thicknesses. Arteries have a powerful muscular wall, as the blood moves through them at high speed and high pressure. Therefore, arterial bleeding is very dangerous, as a result of which a person loses a large amount of blood in a short time. This can have fatal consequences.

The veins have soft walls richly supplied with semilunar valves. They ensure the movement of blood in the vessels in only one direction - to the main muscular organ of the circulatory system. Since the venous blood has to overcome gravity to rise to the heart, and the pressure in the veins is low, these valves do not allow the blood to move backward, that is, away from the heart.

A network of capillaries with a microscopic wall diameter performs the main function of gas exchange. It is in them that carbon dioxide (carbon dioxide) and toxins from tissue cells enter, and capillary blood, in turn, gives the cells the oxygen necessary for their vital activity. In total, there are more than 150 billion capillaries in the body, the total length of which in an adult is about 100 thousand km.

A special functional adaptation of the human body, which provides a constant supply of organs and tissues with the necessary substances, is which can be observed both in physiologically normal conditions and in complex violations of the system (for example, blockage of a vessel by a thrombus).

Systemic circulation

Let us return to the question of what organs the human circulatory system consists of. Recall that the vicious circle of blood circulation, discovered by Harvey, originates in the left ventricle and ends in the right atrium.

The aorta, as the main artery in the body and the beginning of the systemic circulation, carries oxygenated blood from the left ventricle. Through the system of vessels extending from the aorta and branching throughout the human body, blood enters all parts of the body and organs, saturating them with oxygen, performing the functions of exchanging and transporting nutrients.

From the upper part of the body (head, shoulders, chest, upper limbs), venous blood saturated with carbon dioxide is collected in and from the lower half of the body - into the inferior vena cava. Both vena cava flow into the right atrium, closing the systemic circulation.

Small circle of blood circulation

The circulatory system - the heart, the circulatory system - are also included in the so-called small (pulmonary) circulation. It was he who was discovered by Miguel Servet in the middle of the 16th century. This circle starts from the right ventricle and ends in the left atrium.

Venous blood through the right atrioventricular opening from the right atrium enters the right ventricle. From it, along the pulmonary trunk, and then along two pulmonary arteries - left and right - it enters the lungs. And despite the fact that these vessels are called arteries, the blood flows through them venous. It enters the right and left lungs, in which there are capillaries that braid the alveoli (pulmonary vesicles that make up the lung parenchyma). Gas exchange occurs between the oxygen of the alveoli and the connective tissue through the thinnest walls of the capillaries. It is in this part of the body that venous blood turns into arterial blood. Then it enters the postcapillary venules, which are enlarged to 4 pulmonary veins. Through them, arterial blood enters the left atrium, where the pulmonary circulation ends.

Blood circulation in all vessels occurs simultaneously, without stopping or interrupting for a second.

coronary circulation

What is an autonomous circulatory system, what organs does it consist of and what are the features of its functioning, were studied by such scientists as Shumlyansky, Bowman, Gis. They found that the most important in this system is the coronary or coronary blood circulation, which is carried out by special blood vessels that braid the heart and extend from the aorta. These are such vessels as the left coronary artery with the main branches, namely: the anterior interventricular, envelope branch and atrial branches. And also this is the right coronary artery with such branches: the right coronary and posterior interventricular.

Blood without oxygen returns back to the muscular organ in three ways: through the coronary sinus, the veins entering the atrial cavity, and the smallest vascular branches that flow into the right half of the heart without even showing up on its epicardium.

Portal vein circle

Since the circulatory system is very important in ensuring the internal constancy of the environment, what organs the portal vein circle consists of, natural scientists studied in the process of considering the systemic circulation. It was found that from the gastrointestinal tract, spleen and pancreas, blood accumulates in the inferior and superior mesenteric veins, which subsequently, when combined, form the portal (portal vein).

The portal vein, together with the hepatic artery, enters the gate of the liver. Arterial and venous blood in hepatocytes (liver cells) undergoes a thorough cleaning and then enters the right atrium. Thus, blood purification occurs due to the barrier function of the liver, which is also provided by the circulatory system.

What organs does the accessory system consist of?

The auxiliary organs include the red bone marrow, the spleen and the aforementioned liver. Since blood cells do not live long, approximately 60-90 days, it becomes necessary to utilize old spent blood cells and synthesize young ones. It is these processes that provide auxiliary organs of the circulatory system.

In the red bone marrow containing myeloid tissue, precursors of formed elements are synthesized.

The spleen, in addition to the function of depositing part of the blood that is not used in circulation, destroys old red blood cells and partially compensates for their loss.

The liver also disposes of dead white blood cells, red blood cells and platelets and stores blood that is not currently involved in the circulatory system.

The article examined in detail the circulatory system, what organs it consists of and what functions it performs in the human body.

If the circulatory system of an ordinary person is laid out in a straight line, its length will be more than 95,000 kilometers.

The human heart beats at a rate of about 70 times per minute, the heart of a shrew about 600 times, the heart of a hummingbird 1,300 times, and the heart of a blue whale 10 times per minute.

In a year, your heart beats about 42,075,900 times, and in your average lifespan, about 3 billion, give or take a few million.

By driving blood through the entire body, the human heart creates such pressure that it can release a jet of blood more than 9 meters.

It is estimated that it takes 1,120,000 mosquitoes to suck all the blood out of an average adult.

The largest vein in the human body is the inferior vena cava. This vessel returns blood from the lower body to the heart.

Your bone marrow produces 3 million blood cells every second and breaks down the same amount within 1 second.

Every second, 25 billion cells pass through our circulatory system.

A column of 500 blood cells will be only 1.02 millimeters high.

For unknown reasons, a woman's body is more likely to reject a transplanted heart than a man's.

Your heart beats faster during a brisk walk or a heated argument than during intimacy.

Blood is denser than fresh water, but the same density as sea blood.

We all know that human blood is red. But most of the blood of the individuals of this world belongs to insects, and it is light green.

Women with type B blood live longer than women with type O blood. Men with type O blood live longer than men with type B blood. These statistics are not explained in any way.

Men have about 10% more red blood cells than women.

The kidneys are organs that purify the blood. Almost all of us have two kidneys, but most will survive with one. In 1954, a patient of Dr. J. Hartwell Harrison and Dr. Joseph Murray was very ill as both of his kidneys failed. He urgently needed a transplant of the most similar organ - one that would not reject the body. Fortunately, he had a twin brother. In the first truly successful transplant of its kind, the patient's twin donated one of his kidneys and saved his brother's life.

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

During our lifetime, the heart pumps approximately 150 million liters of blood.

In about 25 days, the heart can fill a swimming pool with it, if you have extra supplies for this. In general, your body contains more than 4 liters of blood.

It takes 60 seconds for the blood to make one complete circle throughout the body.

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 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.

In a year, 3,152,715 liters of blood passes through your heart.

Every 17 minutes, all of your body's blood passes through your thyroid gland.

The average woman's heart is about 20% smaller than the average man's heart.

Your heart beats at a rate of 100,800 beats per day. In a year, it made a staggering number of strokes - 36,792,000.

When young cells of the heart muscle develop in the laboratory, it begins to pulsate on its own, without any external stimulus. This is a prime example of genetic memory.

People do not have the ability to change the rhythm of the heartbeat, but insects can.

Some heart attacks are not accompanied by chest pain.

On Monday, the risk of a heart attack is 33% higher than on any other day.

Typically, a person's blood pressure is higher in the right hand than in the left.

Talking raises blood pressure.

Taxi drivers and bus drivers very often have high blood pressure. The reasons for this are understandable, but there is something else - they are constantly forced to put off going to the toilet. It also raises the pressure.

It is not known why people with high blood pressure rarely develop cancer.

It is a rather complex structure. At first glance, it is associated with an extensive network of roads that allows vehicles to run. However, the structure of blood vessels at the microscopic level is quite complex. The functions of this system include not only the transport function, the complex regulation of the tone of blood vessels and the properties of the inner membrane allows it to participate in many complex processes of adaptation of the body. The vascular system is richly innervated and is under the constant influence of blood components and instructions coming from the nervous system. Therefore, in order to have a correct idea of ​​how our body functions, it is necessary to consider this system in more detail.

Some interesting facts about the circulatory system

Structure of blood vessels

In smaller vessels, there is a gradual predominance of muscle tissue. The fact is that these vessels are actively involved in the regulation of blood pressure, carry out the redistribution of blood flow, depending on external and internal conditions. Muscle tissue envelops the vessel and regulates the diameter of its lumen.

3. outer shell vessel ( adventitia) - provides a connection between the vessels and the surrounding tissues, due to which the mechanical fixation of the vessel to the surrounding tissues occurs.

What are the blood vessels?

According to the nature of the blood Vessels are divided into veins and arteries. Through the arteries, blood flows from the heart to the periphery, through the veins it flows back - from tissues and organs to the heart.
arteries have a more massive vascular wall, have a pronounced muscle layer, which allows you to regulate the flow of blood to certain tissues and organs, depending on the needs of the body.
Vienna have a fairly thin vascular wall, as a rule, in the lumen of large-caliber veins there are valves that prevent the reverse flow of blood.

According to the caliber of the artery can be divided into large, medium caliber and small
1. Large arteries- aorta and vessels of the second, third order. These vessels are characterized by a thick vascular wall - this prevents their deformation when the heart pumps blood under high pressure, at the same time, some compliance and elasticity of the walls can reduce pulsating blood flow, reduce turbulence and ensure continuous blood flow.

2. Vessels of medium caliber- take an active part in the distribution of blood flow. In the structure of these vessels there is a fairly massive muscle layer, which, under the influence of many factors ( chemical composition of blood, hormonal effects, immune reactions of the body, effects of the autonomic nervous system), changes the diameter of the lumen of the vessel during contraction.

3. smallest vessels These vessels are called capillaries. Capillaries are the most branched and long vascular network. The lumen of the vessel barely passes one erythrocyte - it is so small. However, this lumen diameter provides the maximum area and duration of contact of the erythrocyte with the surrounding tissues. When blood passes through the capillaries, erythrocytes line up one at a time and move slowly, simultaneously exchanging gases with surrounding tissues. Gas exchange and exchange of organic substances, the flow of liquid and the movement of electrolytes occur through the thin wall of the capillary. Therefore, this type of vessel is very important from a functional point of view.
So, gas exchange, metabolism occurs precisely at the level of capillaries - therefore, this type of vessel does not have an average ( muscular) shell.

What are small and large circles of blood circulation?

The beginning of a large circle of blood circulation takes from the aorta, then the blood moves through the vessels of the next order. The branches of the main vessels direct blood to the internal organs, to the brain, limbs. It does not make sense to list the names of these vessels, but it is important to regulate the distribution of blood flow pumped by the heart to all tissues and organs of the body. Upon reaching the blood-supplying organ, there is a strong branching of the vessels and the formation of a circulatory network from the smallest vessels - microvasculature. At the level of capillaries, metabolic processes take place and the blood, which has lost oxygen and part of the organic substances necessary for the functioning of organs, is enriched with substances formed as a result of the work of the cells of the organ and carbon dioxide.

As a result of such continuous work of the heart, small and large circles of blood circulation, continuous metabolic processes occur throughout the body - the integration of all organs and systems into a single organism is carried out. Thanks to the circulatory system, it is possible to supply oxygen to organs distant from the lung, remove and neutralize ( liver, kidneys) decay products and carbon dioxide. The circulatory system allows hormones to be distributed throughout the body in the shortest possible time, to reach any organ and tissue with immune cells. In medicine, the circulatory system is used as the main drug-distributing element.

Distribution of blood flow in tissues and organs