Like any muscle heart muscle possesses: excitability, that is, the ability to respond with excitement to irritation, contractility. that is, the ability to contract, and conduction, that is, the ability to conduct excitation. In addition, the heart has the capacity for rhythmic automation.

Excitability... The heart muscle is capable of being stimulated by electrical, mechanical, thermal and chemical stimuli. Under the action of any of these stimuli, excitation and contraction of the heart muscle can occur. For this, however, it is necessary that the strength of the stimulation be equal to or exceed the threshold strength. Irritations weaker than the threshold do not cause excitation and contraction.

Excitation of the heart muscle... The excitation of muscle cells by the heart, like any other excitable tissue, can be judged by the change in the difference in electrical potentials that exists between the excited area and the unexcited one, or between the protoplasm of the cell and its external environment.

Refractoriness of the heart muscle... During excitation, the heart muscle loses its ability to respond with a second flash of excitement to artificial stimulation or to an impulse coming to it from the focus of automation. This state of non-excitability is called absolute refractoriness.

Contraction of the heart muscle... Excitation of the heart muscle causes its contraction, that is, an increase in its tension or a shortening of the length of the muscle fibers. The contraction of the heart muscle, like the excitation wave in it, lasts longer than the contraction and excitation of the skeletal muscle caused by one single stimulus, for example, by closing or opening a direct current. The period of contraction of individual muscle fibers of the heart roughly corresponds to the duration of the action potential. With a frequent rhythm of heart activity, both the duration of the action potential and the duration of contraction are shortened.

The mechanism and speed of conduction of excitement in the heart... Conduction of excitation in the myocardium is carried out electrically; the action potential, which has arisen in an excited muscle cell, serves as an irritant to neighboring cells.

The amplitude of the action potential in the muscle cells of the heart is 4-5 times higher than the threshold level of membrane depolarization required for a spreading action potential to arise in neighboring cells. Consequently, the action potential in its amplitude is over-sufficient to induce excitation in neighboring cells. Oto is an important device that ensures the reliability of the conduction of excitation through the conducting system and the myocardium of the atria and ventricles.

The rate of conduction of excitation in different parts of the heart is not the same. Excitation spreads along the atrial myocardium in warm-blooded animals at a speed of 0.8-1 m / sec. In the conduction system of the ventricles, consisting of Purkine fibers, the speed of excitation conduction is greater and reaches 2-4.2 m / sec. Excitation spreads along the ventricular myocardium at a speed of 0.8-0.9 m / sec.

With the transition of excitation from the muscle fibers of the atria to the cells of the atrioventricular node, there is a delay in the conduction of the impulse. Recent studies by Hoffman and Crenfield using microelectrode technology have shown that in a short section of 1 mm in the upper part of the atrioventricular node, the propagation of excitation slows down and it is carried out at a very low speed - 0.02-0.05 m / sec.

A delay in impulse conduction in the atrioventricular node causes a later onset of ventricular excitation compared to the atria. This is of great physiological importance for the coordinated work of the parts of the heart. That is why the excitation of the ventricles begins only but a lapse of 0.12-0.18 seconds after the excitation of the atria begins.

Myocardium- the heart muscle, is a thick part of the section of the heart wall and contains cardiomyocytes - the contractile cells of the heart. The myocardium is a unique muscle in the human body, there is nowhere else in humans of this type of muscle. The ability and strength of the heart to pump blood depends on the thickness of the myocardium.

Heart muscle properties

The myocardium is located between the outer layer of the epicardium and the inner layer of the endocardium.

The myocardium is a muscle that, unlike skeletal muscle adapted to be resistant to fatigue (fatigue). This is achieved due to the fact that cardiomyocytes have a large number of mitochondria, which helps to maintain a constant aerobic respiration... In addition, the myocardium has a large supply of blood compared to its size, providing it with a continuous flow of nutrients and oxygen, thereby removing metabolic waste much faster and more efficiently.

The main purpose of the myocardium is to organize the rhythmic movements of the heart, which consists in continuous automatic contractions and relaxation of muscle fibers.

Myocardial structure

In some characteristics, the myocardium has similarities with other muscles, but has many of its own characteristics. Cardiomyocytes are much shorter than their relatives - myocytes, have fewer nuclei. Each muscle fiber connects to the plasma membrane (sarcolemma) with special tubules (T-tubules). In these T-tubules, the sarcolemma is spiked with a large number of calcium channels, which allow calcium-ion exchange to proceed much faster than in the neuromuscular junction in skeletal muscles. The contraction of myocardial muscle cells occurs due to the stimulation of the action potential by the flow of calcium ions.

Like other muscles, the myocardium is composed of sarcomeres, which are the main contractile units of muscles. The sarcomere has a length of 1.6 to 2.2 µm. The sarcomere contains light and dark stripes. There is a dark stripe in the center, which has a constant length equal to 1.5 μm. Sarcomeres are made up of long, fibrous proteins that slide together as muscles contract and relax. The main two proteins found in sarcomeres are myosin forming thick threads, as well actin which forms fine threads. Anatomically, myosin has a long, wavy tail and a globular head that binds to actin. The myosin head also binds to ATP, which is an energy source for cellular metabolism, and is essential for cardiomyocytes to maintain their normal function. Together, myosin and actin form myofibrillar filaments, which are elongated, contractile filaments found in muscle tissue. Like skeletal muscle, the myocardium contains a protein called myoglobin, which stores oxygen.

Inside the heart, the myocardium is of varying thickness. Thus, heart chambers with a thicker myocardial layer are able to pump blood under greater pressure and force, compared to chambers with thinner myocardial layers. The thinnest layer of the myocardium is located in the atria, since these chambers are primarily filled with blood through passive blood flow. In the right ventricle, the myocardium is much thicker, since this part of the heart muscle must pump a large volume of blood that returns to the lungs for oxygenation. The thickest layer of the myocardium is located in the left ventricle, since this part of the heart must pump blood through the aorta throughout the circulatory system.

The thickness of the myocardium can also change in each person, due to previous diseases, it can be thicker and harder, or thinner and become flabby. For example, hypertension leads to hypertrophy of the heart muscle when myocardial cells increase an adaptive response due to high blood pressure. Hypertrophy of the heart muscle can eventually lead to cardiac arrest when the myocardium becomes so stiff that the heart can no longer pump blood. Flabby (weak) heart muscle myocardium becomes like this after infections and heart attacks. The heart muscle in this case becomes so weak, but cannot cope with pumping blood, heart failure develops.

Excitation of the heart muscle causes its contraction, i.e., an increase in its tension or shortening of the length of muscle fibers. Contraction of the heart muscle just like the excitation wave in it, it lasts longer than the contraction and excitation of skeletal muscle caused by one single stimulus, for example, by closing or opening a direct current. The period of contraction of individual muscle fibers of the heart roughly corresponds to the duration of the action potential. With a frequent rhythm of heart activity, both the duration of the action potential and the duration of contraction are shortened.

As a rule, any wave of excitement is accompanied by contraction. However, a break in the connection between arousal and contraction is also possible. So, with prolonged passage through an isolated heart of Ringer's solution, from which the calcium salt is excluded, rhythmic outbursts of excitation, and, consequently, action potentials, remain, and the contractions stop.

The structure of the human heart muscle, its properties and what processes take place in the heart

These and a number of other experiments show that calcium ions are necessary for the contractile process, but are not necessary for muscle excitation.

A break in the connection between excitation and contraction can also be observed in a dying heart: rhythmic oscillations of electrical potentials are still occurring, while the contractions of the heart have already stopped.

The direct supplier of energy expended at the first moment of contraction of the heart muscle, like that of skeletal muscle, are high-energy phosphorus-containing compounds - adenosine triphosphate and creatine phosphate. Resynthesis of these compounds occurs due to the energy of respiratory and glycolytic phosphorylation, i.e. due to the energy supplied by carbohydrates. In the heart muscle, aerobic processes using oxygen prevail over anaerobic processes, which occur much more intensively in the skeletal muscles.

The ratio between the initial length of the fibers of the heart muscle and the force of their contraction... If you increase the inflow of Ringer's solution to the isolated heart, i.e., increase the filling and stretching of the walls of the ventricles, then the force of contraction of the heart muscle increases. The same can be observed if you subject a strip of heart muscle cut from the wall of the heart to a slight stretch: when stretched, the force of its contraction increases.

On the basis of such facts, the dependence of the force of contraction of the fibers of the heart muscle on their length before the start of contraction was established. This dependence is also the basis for the "law of the heart" formulated by Starling. According to this empirically established law, which is true only for certain conditions, the force of contraction of the heart is the greater, the greater the stretching of muscle fibers in diastole.

Lectures 2nd semester.

Lecture number 1 Physiology of the cardiovascular system.

The circulatory system includes the heart and blood vessels - blood and lymph. The main importance of the circulatory system is the supply of blood to organs and tissues. The heart is a biological pump, due to the work of which blood moves through a closed vascular system. The human body has 2 circles of blood circulation.

A large circle of blood circulation begins with the aorta, which departs from the left ventricle, and ends with vessels flowing into the right atrium. The aorta gives rise to large, medium and small arteries. Arteries pass into arterioles, which end in capillaries.

Capillaries in a wide network penetrate all organs and tissues of the body. In the capillaries, the blood gives oxygen to the tissues and nutrients, and from them metabolic products, including carbon dioxide, enter the bloodstream.

Physiological properties of the heart muscle.

Capillaries pass into venules, blood from which enters small, medium and large veins. Blood from the upper part of the body enters the superior vena cava, from the lower - into the inferior vena cava. Both of these veins flow into the right atrium, where the systemic circulation ends

Small circle of blood circulation(pulmonary) begins with the pulmonary trunk, which departs from the right ventricle and carries venous blood into the lungs. The pulmonary trunk branches into two branches leading to the left and right lungs. In the lungs, the pulmonary arteries are divided into smaller arteries, arterioles, and capillaries. In the capillaries, the blood gives off carbon dioxide and is enriched with oxygen. The pulmonary capillaries pass into the venules, which then form the veins. Through four pulmonary veins, arterial blood enters the left atrium.

A heart- a hollow muscular organ. A solid vertical septum divides the heart into left and right halves. The horizontal partition, together with the vertical one, divides the heart into four chambers. The upper chambers are the atria, the lower ones are the ventricles.

The wall of the heart consists of three layers. The inner layer is represented by the endothelial membrane ( endocardium, lines the inner surface of the heart). Middle layer ( myocardium) consists of striated muscle. The outer surface of the heart is covered with a serous membrane ( epicardium), which is the inner layer of the pericardial sac - the pericardium. Pericardium(heart shirt) surrounds the heart like a bag and allows it to move freely.

Heart valves. Separates the left atrium from the left ventricle bivalve valve ... On the border between the right atrium and the right ventricle is tricuspid valve ... An aortic valve separates it from the left ventricle, and a pulmonary valve separates it from the right ventricle.

With atrial contraction ( systole) the blood from them enters the ventricles. When the ventricles contract, blood is forced into the aorta and pulmonary trunk. Relaxation ( diastole) of the atria and ventricles helps to fill the cavities of the heart with blood.

Valve apparatus value. During atrial diastole the atrioventricular valves are open, the blood coming from the corresponding vessels fills not only their cavities, but also the ventricles. During atrial systole the ventricles are completely filled with blood. This excludes the return of blood to the vena cava and pulmonary veins. This is due to the fact that, first of all, the musculature of the atria, which forms the mouth of the veins, contracts. As the cavities of the ventricles are filled with blood, the cusps of the atrioventricular valves close tightly and separate the atrial cavity from the ventricles.

As a result of contraction of the papillary muscles of the ventricles at the time of their systole, the tendon filaments of the atrioventricular valves are stretched and do not allow them to turn towards the atria.

By the end of the systole of the ventricles, the pressure in them becomes greater than the pressure in the aorta and pulmonary trunk. This facilitates the discovery semilunar valves of the aorta and pulmonary trunk , and the blood from the ventricles enters the corresponding vessels.

Thus, the opening and closing of the heart valves is associated with a change in the pressure in the cavities of the heart. The value of the valve apparatus is that it provides blood movement in the cavities of the heart in one direction.

Basic physiological properties of the heart muscle.

Excitability. The heart muscle is less excitable than the skeletal muscle. The reaction of the heart muscle does not depend on the strength of the stimulation applied. The heart muscle is reduced as much as possible for both threshold and stronger irritation.

Conductivity. Excitation along the fibers of the heart muscle spreads at a slower speed than along the fibers of the skeletal muscle. Excitation along the fibers of the muscles of the atria spreads at a speed of 0.8-1.0 m / s, along the fibers of the muscles of the ventricles - 0.8-0.9 m / s, along the conduction system of the heart - 2.0-4.2 m / s ...

Contractility. The contractility of the heart muscle has its own characteristics. The atrial muscles contract first, followed by the papillary muscles and the subendocardial layer of the ventricular muscles. In the future, the contraction also covers the inner layer of the ventricles, ensuring the movement of blood from the cavities of the ventricles to the aorta and the pulmonary trunk.

The physiological features of the heart muscle include an extended refractory period and automatism.

Refractory period. The heart has a significantly pronounced and prolonged refractory period. It is characterized by a sharp decrease in tissue excitability during the period of its activity. Due to the pronounced refractory period, which lasts longer than the systole period (0.1-0.3 s), the heart muscle is not capable of tetanic (prolonged) contraction and performs its work as a single muscle contraction.

Automatism. Outside the body, under certain conditions, the heart is able to contract and relax, maintaining the correct rhythm.

Consequently, the reason for the contractions of an isolated heart lies in itself. The ability of the heart to rhythmically contract under the influence of impulses arising in itself is called automatism.

The excitability of the heart muscle is not the same in all parts of the heart. The most excitable sinus node. The excitability of the His bundle is much less. Although during contraction, the heart muscle is excitable. But during this period, which almost coincides with systole, the strongest artificial stimuli of the heart do not cause a new contraction due to the "conflict of two strong excitations, too close to one another in time in the same substrate" (A. A. Ukhtomsky). This state of complete loss of excitability during the contraction of the heart is designated as absolute refractoriness. After that, during relaxation of the muscle of the heart when the heart is irritated by an induction electric current, due to a change in the time interval between two excitations and a change functional state heart, can be obtained out of turn, but a weaker contraction.

This second period of incomplete excitability during relaxation of the heart is referred to as relative refractoriness. Immediately after a period of relative refractoriness, a short-term increase in excitability is observed - the exaltation phase. The duration of absolute and relative refractoriness depends on the duration of the cardiac cycle. The period of absolute refractoriness of the sinusoatrial node in humans reaches 0.3 s, the atria - from 0.06 to 0.12 s, and the ventricles - from 0.3 to 0.4 s.

Due to prolonged refractoriness, the heart responds to prolonged stimulation with rhythmic contractions and, under normal conditions, cannot enter the tetanus state.

If irritation is applied to the heart ventricle of a cold-blooded animal before the arrival of the next automatic impulse, that is, in the period of relative refractoriness, then a premature heart contraction occurs - an extrasystole, followed by a compensatory pause longer than usual.

Extrasystoles occur with changes in the conducting system or in the heart muscle. The effect on the change in excitability is designated as batmotropic.

The contraction of the heart muscle does not increase with increasing irritation. If you directly apply irritation to the heart muscle, increasing the amount of irritation each time, then the following fact is revealed. At first, with weak stimuli, the muscle does not react to them by contraction, then with a slight increase in the magnitude of the irritation, it contracts. This is the maximum reduction. A further increase in the strength of irritation no longer increases the contraction of the heart muscle (G. Boudich, 1871).

However, this is only a special case, not a rule, since the height of contraction of the heart muscle ("everything") changes and depends on its excitability and lability, that is, on its functional state. “Nothing” also does not exist, since with subthreshold stimuli, excitement arises, which is summed up at a certain frequency of stimuli.

The magnitude of the greatest contraction of the heart muscle depends on the level of metabolism in it. The effect on the force of heart contractions is designated as inotropic.

In the process of phylogenesis, the ability of the heart muscle to increase the strength of its contractions was developed, depending on the increase in the amount of blood flowing to the heart and the increase in blood pressure in the arterial system.

An increase in blood flow to the heart and an increase in blood pressure under physiological conditions are caused by muscle work and certain emotions.

How does the heart increase the strength of its contractions with increased loads?

The force of contractions of the heart increases due to an increase in the initial length of muscle fibers (Starling, 1916).

Muscle fibers have a certain length during the diastole of the heart during rest, before the heart begins to contract (initial length). With an increase in blood flow to the heart and with difficulties in outflow caused by an increase in blood pressure, the heart in diastole stretches more strongly from the overflow of the cavity with blood, therefore, the initial length of the muscle fibers of the heart increases. The greater the flow of blood to the heart, or the greater the blood pressure that obstructs the flow of blood, the greater the initial stretch of the muscle fibers.

On the isolated muscles found that the contraction of the skeletal and cardiac muscles is directly proportional to the length of the muscle fibers. The longer the initial length of the fibers, the stronger the contraction. Therefore, with an increase in the initial length of the fibers of the heart, it contracts more strongly during systole, and due to this, the amount ejected increases.

Great importance has blood supply and nutrition to the heart muscle. The better the muscle is nourished, the less it is pre-stretched.

Under natural conditions, in the absence of additional stretching of the heart, an increase in contractions is the result of an increase in the heart muscle under the influence nervous system(trophic influence).

When the heart muscle gets tired, the heart falls and it stretches. The ability of the heart to perform the same work during fatigue depends on the degree of stretching of its muscle fibers.

The degree of stretching of the heart is determined by the thickness and condition of the heart muscle. The maximum heart can expand to the pericardium, which, thus, determines the maximum expansion of the heart.

STRUCTURE OF THE HEART WALL

The wall of the heart consists of three layers: inner - endocardium, middle - myocardium and outdoor - epicardium.

Endocardium lines the surface of the heart chambers from the inside, it is formed by a special type of epithelial tissue - endothelium... The endothelium has a very smooth, shiny surface that reduces friction as the blood moves through the heart.

Myocardium makes up the bulk of the heart wall.

He is educated transversely-striated cardiac muscle, the fibers of which, in turn, are arranged in several layers. The atrial myocardium is significantly thinner than the ventricular myocardium. The left ventricular myocardium is three times thicker than the right ventricular myocardium. The degree of development of the myocardium depends on the amount of work performed by the chambers of the heart. The myocardium of the atria and ventricles is separated by a layer of connective tissue (annulus fibrosus), which makes it possible to alternate contraction of the atria and ventricles.

Epicard- This is a special serous membrane of the heart, formed by connective and epithelial tissue.

CENTRAL BAG (PERICARD)

This is a kind of closed bag in which the heart is enclosed. The bag consists of two sheets. The inner layer grows together over the entire surface with the epicardium. The outer leaf seems to cover the top of the inner leaf. There is a slit cavity between the inner and outer leaf - pericardial cavity) filled with liquid. The bag itself and the liquid in it play a protective role and reduce the friction of the heart during its work. The bag helps to fix the heart in a certain position.

HEART VALVES

The work of the heart valves provides a one-way movement of blood in the heart.

The heart valves themselves include flap valves located on the border of the atria and ventricles. In the right half of the heart is telescopic valve, on the left - bicuspid (mitral). The butterfly valve consists of three elements: 1) sash dome-shaped and formed by dense connective tissue, 2) papillary muscle 3) tendon filaments stretched between the leaflet and the papillary muscle. When the ventricles contract, the leaflet valves close the lumen between the atrium and the ventricle. The mechanism of operation of these valves is as follows: when the pressure in the ventricles rises, blood rushes into the atria, raising the leaflets of the valves, and they close, interrupting the gap between the atrium and the ventricle; the leaflets do not turn towards the atria, because they are held by tendon threads, which are stretched due to the contraction of the papillary muscle.

On the border of the ventricles and the vessels extending from them (aorta and pulmonary trunk) are located semilunar valves consisting of crescent flaps ... The vessels named contain three such valves. Each lunar flap has the shape of a thin-walled pocket, the entrance to which is open towards the vessel. When blood is expelled from the ventricles, the semilunar valves are pressed against the walls of the vessel. During the relaxation of the ventricles, the blood rushes in the opposite direction, fills the "pockets", they move away from the walls of the vessel and close, blocking the lumen of the vessel, not allowing blood to flow into the ventricles. The semilunar valve, located on the border of the right ventricle and the pulmonary trunk, is called pulmonary valve, on the border of the left ventricle and the aorta - aortic valve.

Heart functions

The function of the heart is that the myocardium of the heart during contraction pumps blood from the venous to the arterial vascular bed. The source of energy necessary for the movement of blood through the vessels is the work of the heart. The contraction energy of the myocardium of the heart is converted into pressure, imparted to the portion of blood expelled from the heart during the contraction of the ventricles. Blood pressure- This is the force that is spent on overcoming the force of friction of blood against the walls of blood vessels. The difference in pressure in different parts of the vascular bed is the main reason for the movement of blood. Blood movement in cardiovascular system in one direction is provided by the work of the heart and vascular valves.

Heart muscle properties

The main properties of the heart muscle include automation, excitability, conductivity and contractility.

1. Automation- This is the ability to rhythmic contraction without any external influences under the influence of impulses arising in the heart. A striking manifestation of this property of the heart is the ability of the heart extracted from the body, when the necessary conditions are created, to contract for hours and even days. The nature of automation is still not fully understood. But it is unequivocally clear that the emergence of impulses is associated with activity atypical muscle fibers embedded in some parts of the myocardium. Inside atypical muscle cells, electrical impulses of a certain frequency are spontaneously generated, which then propagate throughout the myocardium. The first such area is located in the area of ​​the mouth of the vena cava and is called sinus, or sinoatrial node... In the atypical fibers of this node, impulses spontaneously arise with a frequency of 60-80 times per minute. It is the main center of the heart's automation. The second section is located in the thickness of the septum between the atria and ventricles and is called atrioventricular, or atrioventricular node... The third area is atypical fibers that make up bundle of His lying in the interventricular septum. Thin fibers of atypical tissue originate from the bundle of His - Purkinje fibers branching in the ventricular myocardium. All areas of atypical tissue are capable of generating impulses, but their frequency is highest in the sinus node, therefore it is called first-order pacemaker (first-order pacemaker), and all other centers of automation obey this rhythm.

The totality of all levels of atypical muscle tissue make up cardiac conduction system... Thanks to the conducting system, the excitation wave that occurs in the sinus node consistently spreads throughout the myocardium.

2. Excitability the heart muscle lies in the fact that under the influence of various stimuli (chemical, mechanical, electrical, etc.), the heart is able to come into a state of excitement. The excitation process is based on the appearance of a negative electrical potential on the outer surface of the membranes of cells exposed to the stimulus. As with any excitable tissue, the membrane of muscle cells (myocytes) is polarized. At rest, it is positively charged from the outside, negatively from the inside. The potential difference is determined by the different concentration of Na + and K + ions on both sides of the membrane. The action of the stimulus increases the membrane permeability for K + and Na + ions, the membrane potential is rearranged ( potassium - sodium pump), as a result, an action potential arises that spreads to other cells. Thus, there is a spread of excitement throughout the heart.

Impulses originating in the sinus node propagate through the atrial musculature. Having reached the atrioventricular node, the excitation wave propagates along the His bundle, and then along the Purkinje fibers. Thanks to the conducting system of the heart, a sequential contraction of parts of the heart is observed: first, the atria contract, then the ventricles (starting from the apex of the heart, the contraction wave propagates to their base). The peculiarity of the atrioventricular node is the conduction of the excitation wave in only one direction: from the atria to the ventricles.

3. Contractility is the ability of the myocardium to contract. It is based on the ability of the myocardial cells themselves to respond to excitation by contraction. This property of the heart muscle determines the ability of the heart to perform mechanical work. The work of the heart muscle obeys the law "all or nothing" The essence of this law is as follows: if an irritating action of various strengths is applied to the heart muscle, the muscle responds each time with a maximum contraction (" all "). If the strength of the stimulus does not reach the threshold value, then the heart muscle does not respond by contraction (" nothing ").

It is difficult to overestimate the work of the heart. After all, an organ the size of a fist fills the whole organism with vitality, oxygen. We will talk about how the heart works and what are the most important properties of the heart muscle in our article.

1 Inside view

If we look at the heart from the inside, we see a hollow, four-chambered organ. Moreover, the chambers are separated from each other by two perpendicularly located partitions, valves are provided for blood circulation in the heart chambers through which blood flows freely during heart shocks, at the same time the heart "doormen" - valves, do not allow blood to flow back and control its movement from the upper atrial chambers into the ventricles. The human heart has 3 layers, which are well studied and differentiated.

Let's consider them from external to internal:

Having examined the structure of the heart in layers, we will proceed to the study of the most important and mysterious muscle of the human body - the heart.

2 Meet the myocardium!

The heart muscle or myocardium belongs to the striated muscles, but, unlike others, it has its own characteristics. What does the striated muscle, for example, of the limbs look like? These are fibers made up of multinucleated cells, right? With the muscle of the heart, everything is different: it is not represented by fibers, but by a network of cells with one nucleus (cardiomyocytes), which are connected by bridges. Such a network in medicine has the complex name of pseudosynthesis.

There are 2 parts of the myocardium: the muscle layers of the atria and the muscle layers of the ventricles. The fibers of each of the two sections do not pass into each other, this allows the upper and lower heart chambers to participate in contraction independently of each other. In the upper heart chambers, the muscles form two layers: the superficial layer, which "embraces" both heart chambers, and the deep layer, which belongs separately to each atrium. Ventricular muscles and do have 3 layers:

• 1 - superficial. It is a thin layer of longitudinal fibers that envelop both lower heart chambers;
• 2 - the middle layer, unlike the outer one, does not pass from one chamber to another, but is independent for each ventricle;
• 3 - inner layer, it is formed as a result of bending of the outer layer under the middle, the so-called "curl".

The heart muscle has a rather complex structure, it is understandable, because its properties are not simple. Let us consider sequentially the properties of the heart muscle.

3 Automatic

The frog will help us explain this physiological property. How? Very simple! It just so happened that this animal was a classic for the study of the physiological properties of the heart muscle. Her prepared heart in saline can carry out spontaneous heart shocks for no less than a few hours! Why is this happening? The fact is that, unlike skeletal muscles, the heart does not need stimulating impulses from the outside.

In its thickness there is its own unique mechanism, called pacemaker, or pacemaker. He himself generates impulses that excite the myocardium. The main pacemaker is located in the sinoatrial node, right atrial. It is in this section that the emerging action potentials spread to the underlying sections and cause regular rhythmic contractions of the heart. So, the ability to produce impulses yourself and, under their influence, to carry out contractions - this is cardiac automation.

4 Conductivity

Another important property of the myocardium, without which it would not be possible to carry out strokes of the human "motor". A separate system is responsible for this property - conductive. It is represented by the following elements:

1. CA-node (described above), in which pacemaker cells generate pulses;
2. Atrial bundle and tracts. From the overlying department, the excitation passes to this bundle and tracts;
3. The AV node is located at the bottom of the upper right heart chamber, going into the interventricular septum. In this node, the excitation is somewhat slowed down;
4. A bundle of His and its two legs. The branches of the bundle branch into small, thin filaments - Purkinje fibers.

Although this system contains separate elements, it works harmoniously and clearly, ensuring that the excitation is carried out strictly "top-down", due to which the upper and then the lower chambers are first reduced. This system contributes to the fact that not a single cell of the main "motor" remains unexcited, and this is extremely important for its operation.

5 Contractility

Let's imagine that you have just learned extremely good news and your heart literally sang with happiness? Looking into it at the molecular level so you can observe? The sympathetic nerves run up to the heart and release some chemicals that help convey messages. And on the surface of the heart cells there are small receptors, when they interact with chemicals in the cell, a signal is produced, Ca enters the cell, combines with muscle proteins - a contraction occurs.

6 Excitability

The excitability of the heart muscle obeys two fundamental laws that medical students cram in the subject of "physiology". Let's get acquainted with these laws and we:

1. All or nothing. If the magnitude of the exciting stimulus is insufficient, the muscle tissue does not react to it, and immediately gives the maximum response to a sufficiently strong stimulus. And if you further increase the strength of the stimulus, this response does not change.
2. Frank Starling. The more the heart muscle is stretched, the higher the excitability and its contraction. If more blood enters the heart, the myocardium is proportionally more stretched, but the strength of the heart impulses will also increase.

When the muscle of the heart is in a state of excitement, it is unable to respond to other stimuli, this condition is called refractoriness.
It is difficult to clearly distinguish between these properties, since they are all interconnected very tightly, because all properties pursue one goal - to provide a constant normal ability for myocardial contraction and the expulsion of blood into the vessels.

7 How many grams?

Another important characteristic of a healthy heart is myocardial mass. The mass of the left ventricular myocardium is determined by echocardiography by certain methods: either according to the formulas, or a program is already driven into the apparatus, which, taking into account other data during the study, automatically calculates this indicator. You can calculate directly the mass, or the myocardial mass index.

These data are within the normal range, for men the values ​​are slightly higher than for women, which is understandable. On average, for men, the mass of the myocardium is 130-180 g, for women - 90-142 g., The index for men is 70-90 g / m2, the index for women is 70-88 g / m2. The given data are averaged, since the indicators can change upward in people actively involved in sports. In this category of persons, the heart "sways", building up muscle mass.

PHYSIOLOGY OF BLOOD CIRCULATION

Circulation- This is the process of blood movement along the vascular bed, ensuring the performance of its functions.

The physiological circulatory system is made up of the heart and blood vessels. The heart provides the energy needs of the system, and the vessels are the bloodstream. The heart pumps about 5 liters of blood per minute, 260 tons per year, and during life about 200 "000 tons of blood. The total length of the vessels is about 100" 000 km.

First Scientific research systems were produced by W. Garvey. In 1628 he published his work "Anatomical study of the movement of the heart and blood in animals." In 1653 the monk M. Serve described the pulmonary circulation, and in 1661 Malpighi discovered capillaries under a microscope.

The systemic circulation begins with the aorta extending from the left ventricle. With distance from the heart, it is divided into arteries of large, medium and small caliber, arterioles, precapillaries, capillaries. Capillaries are connected to post-capillaries, venules, then veins. Ends with a large circle of vena cava flowing into the right atrium. The pulmonary circulation begins with the pulmonary artery extending from the right ventricle. It also forks into arteries, arternoles and capillaries that penetrate the lungs. The capillaries combine to form venules and pulmonary veins. The latter flow into the left atrium.

A heart Is a hollow muscular organ. Its weight is 200-400 grams or 1/200 of body weight. The wall of the heart is formed by three layers: endocardium, myocardium and epicardium. It has the greatest thickness of 10-15 mm in the area of ​​the left ventricle. The wall thickness of the right is 5-8 mm, and the thickness of the atria is 2-3 mm. The myocardium consists of 2 types of muscle cells: contractile and atypical... Most of them are contractile cardiomyocytes.

The heart is divided by partitions into 4 chambers: 2 atria and 2 ventricles. The atria are connected to the ventricles by atrioventricular holes... They contain the leaflet atrioventricular valves. The right valve is tricuspid (tricuspid), and the left is bicuspid (mitral). Tendon threads are attached to the valve cusps. At the other end, these threads are connected to the papillary (papillary) muscles. At the beginning of the ventricular systole, these muscles contract and the threads are pulled. Thanks to this, there is no eversion of the valve leaflets into the atrial cavity and the reverse movement of blood - regurgitation... In the places where the aorta and pulmonary artery exit from the ventricles, the aortic and pulmonary valves are located. They look like crescent-shaped pockets. Therefore, they are called crescent. The function of the heart valve apparatus is to provide unilateral blood flow through the circulation. In the clinic, the function of the valve apparatus is investigated by such indirect methods as auscultation, phonocardiography, and radiography. Echocardiography allows visual observation of valve activity.

The cycle of the heart. Pressure in the cavities of the heart in different phases of cardiac activity

The contraction of the chambers of the heart is called systole, relaxation - diastole... Normally, the heart rate (HR) is 60-80 per minute. The heart cycle begins with atrial systole. However, in the physiology of the heart and in the clinic, the classic Wiggers scheme is used to describe it. It divides the cycle of cardiac activity into periods and phases. The duration of the cycle, at a frequency of 75 beats per minute, is 0.8 sec. The duration of the ventricular systole is 0.33 seconds. It includes 2 periods: a voltage period lasting 0.08 sec. and the expulsion period is 0.25 sec. The voltage period is divided into two phases: an asynchronous contraction phase with a duration of 0.05 sec and a phase isometric reduction 0.03 sec. In the phase of asynchronous contraction, non-simultaneous contraction occurs, i.e. asynchronous, contraction of the fibers of the myocardium of the interventricular septum. Then the contraction is synchronized and covers the entire myocardium. Ventricular pressure builds up and the atrioventricular valves close. However, its value is insufficient to open the semilunar valves. The isometric contraction phase begins. Those. during her muscle fibers are not shortened, but the force of their contractions and pressure in the cavities of the ventricles increases. When it reaches 120-130 mm Hg. in the left and 25-30 mm Hg. in the right, the semilunar valves open - aortic and pulmonary. The period of exile begins. It lasts 0.25 seconds. and includes a phase of fast and slow expulsion. The fast expulsion phase lasts 0.12 sec., The slow one - 0.13 sec. During the phase of rapid expulsion, the pressure in the ventricles is much higher than in the corresponding vessels, so blood comes out of them quickly. But as the pressure in the vessels increases, the release of blood slows down.

After the blood is expelled from the ventricles, ventricular diastole begins. Its duration is 0.47 sec. It includes the protodiastolic period, the isometric relaxation period, the filling period and the presystolic period. The duration of the protodiastolic period is 0.04 sec. During it, relaxation of the ventricular myocardium begins. The pressure in them becomes lower than in the aorta and pulmonary artery, so the semilunar valves close. After that, a period of isometric relaxation begins. Its duration is 0.08 sec. During this period, all valves are closed and relaxation occurs without changing the length of myocardial fibers. The pressure in the ventricles continues to decrease. When it decreases to 0, i.e. becomes lower than in the atria, atrioventricular valves open. The filling period begins, lasting 0.25 sec. It includes a fast filling phase, which lasts 0.08 seconds, and a slow filling phase, 0.17 seconds. After the ventricles are passively filled with blood, the presystolic period begins, during which atrial systole occurs. Its duration is 0.1 sec. During this period, additional blood is pumped into the ventricles. The pressure in the atria, during their systole, is 8-15 mm Hg in the left, and 3-8 mm Hg in the right. The length of time from the beginning of the protodiastolic period to the presystolic period, i.e. atrial systole is called a general pause. Its duration is 0.4 sec. At the time of the general pause, the semilunar valves are closed, and the atrioventricular valves open. Initially, the atria and then the ventricles are filled with blood. During a general pause, the energy reserves of cardiomyocytes are replenished, metabolic products, calcium and sodium ions are removed from them, and oxygenated. The shorter the total pause, the worse conditions work of the heart. The pressure in the cavities of the heart in the experiment is measured by puncturing, and in the clinic - by their catheterization.

Physiological properties of the heart muscle Automation of the heart

The heart muscle is characterized by excitability, conduction, contractility and automaticity. Excitability Is the ability of the myocardium to be excited by the action of an irritant, conductivity- to carry out excitement, contractility- shorten when excited. Special property - automatics Is the ability of the heart to contract spontaneously. Even Aristotle wrote that in the nature of the heart there is the ability to beat from the very beginning of life to its end, without stopping. In the last century, there were 3 main theories of the automaticity of the heart.

Prochaska and Müller put forward neurogenic theory, considering nerve impulses to be the cause of its rhythmic contractions. Gaskell and Engelman suggested myogenic theory, according to which impulses of excitation arise in the heart muscle itself. Existed heart hormone theory, which is produced in it and initiates its reduction.

The automaticity of the heart can be observed on an isolated heart according to Straub. In 1902, using this technique, the Tomsk professor A.A. Kulyabko for the first time revived the human heart.

At the end of the 19th century, in various parts of the myocardium of the atria and ventricles, accumulations of muscle cells of a peculiar structure were found, which were called atypical... These cells are larger in diameter than contractile cells, they have fewer contractile elements and more glycogen granules. IN last years it was found that the clusters are formed by P-cells (Purkine cells) or pacemaker (rhythm-conducting). In addition, they also contain transitional cells. They borrow intermediate position between contractile and pacemaker cardiomyocytes and serve to transmit excitation. These 2 types of cells form cardiac conduction system... The following nodes and paths are distinguished in it:

1. sinoatrial node(Case-Fleck). It is located at the mouth of the vena cava, i.e. in the venous sinuses;

2. internodal and interatrial pathways Bachmann, Wenckenbach and Torell. Pass through the atrial myocardium and interatrial septum;

3. atrioventricular node(Ashoffa-Tavara). Located in the lower part of the interatrial septum under the endocardium of the right atrium;

4. atrioventricular bundle or a bundle of His. It goes from the atrioventricular node along the upper part of the interventricular septum. Then it is divided into two legs - right and left. They form branches in the ventricular myocardium;

5. Purkine fibers... These are the terminal ramifications of the branches of the bundle branch. They form contacts with the cells of the contractile myocardium of the ventricles.

The sinoatrial node is formed mainly by P-cells. The remaining parts of the conducting system are transitional cardiomyocytes. However, a small number of pacemaker cells are also present in them, as well as in the contractile myocardium of the atria and ventricles. Contractile cardiomyocytes are connected to Purkinje fibers, as well as to each other nexus, i.e. intercellular contacts with low electrical resistance. Due to this and approximately the same excitability of cardiomyocytes, the myocardium is functional syncytium, i.e. the heart muscle reacts to irritation as a whole.

The role of various parts of the conducting system in the automation of the heart was first established by Stannius and Gaskell. Stannius applied ligatures (dressings) to various parts of the heart. First ligature superimposed between the venous sinus, where the sinoatrial node is located, and the right atrium. After that, the sinus continues to contract in a normal rhythm, i.e. with a frequency of 60-80 beats per minute, and the atria and ventricles stop. Second ligature superimposed on the border of the atria and ventricles. This causes the occurrence of ventricular contractions with a frequency approximately 2 times less than the frequency of sinus node automation, i.e. 30-40 per minute. The ventricles begin to contract due to mechanical irritation of the cells of the atrioventricular node. Third ligature superimposed on the middle of the ventricles. After that their top part decreases in the atrioventricular rhythm, and the lower one with a frequency 4 times less than the sinus rhythm, i.e. 15-20 per minute.

Gaskell caused local cooling of the nodes of the conducting system and found that the leading pacemaker of the heart is sinoatrial. Based on the experiments of Stannius and Gaskell, it was formulated principle of decreasing gradient of automation... It says that the farther the center of the heart's automation is located from its venous end and closer to the arterial, the less its ability to automate. Under normal conditions, the sinoatrial node suppresses the automaticity of the underlying ones, because the frequency of its spontaneous activity is higher. Therefore, the sinoatrial node is called the center of automation of the first order, the atrioventricular - the second, and the bundle of His and Purkinje fibers - the third.

The normal sequence of contractions of the parts of the heart is due to the peculiarities of the conduction of excitation along its conducting system. Excitation begins in the leading pacemaker - the sinoatrial node. From it, along the interatrial branches of the Bachmann bundle, excitation at a speed of 0.9-1.0 m / s spreads through the atrial myocardium. Their systole begins. At the same time, excitation from the sinus node along the inter-nodal pathways of Venckenbach and Torella reaches the atrioventricular node. In it, the conduction speed drops sharply to 0.02-0.05 m / s. There is an atrioventricular delay. Those. conduction of impulses to the ventricles is delayed by 0.02-0.04 sec. Due to this delay, blood during atrial systole enters the ventricles that have not yet begun to contract. From the atrioventricular node along the bundle of His, its legs and their branches, excitation proceeds at a speed of 2-4 m / s. Due to such a high speed, it simultaneously covers the interventricular septum and the myocardium of both ventricles. The speed of excitation through the ventricular myocardium is 0.8-0.9 m / s.