Contraction of the heart muscles 7 letters. Function of myocardial contractility. Contractions of the heart muscles. Features of muscle tissue are

Contractility function represents the ability of the heart muscle to respond by contraction to the resulting excitation. The force of contraction of the cardiac fiber is directly proportional to the degree of its elongation during diastole and depends on the height of blood pressure in the aorta, the amount of remaining blood in the cavities of the heart and the degree of myocardial damage.

Contractile function of the heart is the main one in its activity as a pump, carried out on the basis of coordination of individual muscle cells. By connecting to each other through intercalary discs in the longitudinal direction and forming lateral openings, the cells form muscle fibers. Intercalated disks are characterized by low electrical resistance, which facilitates the spread of excitation from cell to cell, and thereby their contraction.

IN heart Two main processes constantly occur: the conduction of an electrochemical impulse and the conversion of chemical energy into mechanical energy. The conduction of an electrochemical impulse from cell to cell is carried out using specialized areas of the cell surface - locking fascia. The conversion of chemical energy into mechanical energy occurs in sarcomeres (functional units of the contractile myocardium).

Every muscle myocardial contractile fiber consists of 200-500 contractile protein structures - myofibrils. They contain myosin and actin. Myosin has the specific ability to bind various ions, mainly calcium and magnesium. According to Draper and Hodge (1950), it also contains potassium. V. A. Engelhardt (1941) showed that myosin has enzymatic properties - the ability to catalyze the breakdown of adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and phosphate. ATP and creatinine phosphate (CP) are formed in mitochondria.

From the moment of cell depolarization before its reduction, a number of biochemical and biophysical changes occur in it. ATP, in the presence of myosin and actin, is gradually broken down into ADP and phosphate, releasing energy, but not in the form of heat, since this would release a large amount of heat, which would lead to denaturation of the protein. It is assumed that the energy during the breakdown of ATP is directly transferred to the contractile structures of the myocardium with the formation of ADP and phosphorylated protein and, without the intermediate formation of heat, is converted into electrical energy. Thus, ATP breakdown occurs in the heart during systole, resulting in contraction of the heart muscle. In a relaxed muscle there is a special relaxation factor that occurs in the presence of magnesium ions and ATP. During contraction, calcium temporarily inhibits the formation of the relaxation factor and neutralizes its effect.

Myocardium consists of two types of cells connected to each other through so-called intercalary signs. Most of the muscle cells of the heart perform a contractile function and are called contractile cells - cardiomyocytes.

Distinctive feature myocardium- the presence of intercalary plates that delimit muscle fibers, having a transverse scalariform arrangement in relation to the muscle fibers. Cardiac muscle fibers are much thinner than skeletal muscle fibers. They contain more sarcoplasm and less sarcolemma. Important distinguishing features of the heart muscle are a significant number of mitochondria and the absence of longitudinal propagation of excitation throughout the heart: the impulse jumps from one cell to another in the area of ​​the intercalary plates.

Increased content calcium increases the contractility of the heart muscle, and its absence makes it impossible to contract the myocardium. The effect of calcium on the contraction mechanism occurs throughout the entire period of membrane depolarization. A decrease in the concentration of extracellular sodium increases the contractility of the heart muscle, as it increases the rate of penetration of calcium into the cell. Increasing sodium concentration blocks calcium entry and eliminates contraction. Potassium is indirectly involved in the process of muscle activity - leaving the cell during the period of its excitation, it leaves free intracellular anion sites in the endoplasm, which are occupied by calcium, which ultimately causes contraction.

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Rhythmic contraction of the heart muscle, 7 letters, crossword puzzle

A word of 7 letters, the first letter is “S”, the second letter is “I”, the third letter is “S”, the fourth letter is “T”, the fifth letter is “O”, the sixth letter is “L”, the seventh letter is "A", the word starting with the letter "C", the last "A". If you don’t know a word from a crossword or scanword, then our site will help you find the most difficult and unfamiliar words.

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Systole

7 letter word, definitions in scanwords:

rhythmic contraction of the heart (see abbreviation 10 letters)

contraction of the atria and ventricles of the heart

contraction of the heart muscle

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Rhythmic contraction of the heart muscle

Answer for the question "Rhythmic contraction of the heart muscle", 7 letters:

Alternative crossword questions for the word systole

Contraction of the atria and ventricles of the heart, which forces blood into the arteries

Rhythmic contraction of the heart

Definition of the word systole in dictionaries

Wikipedia Meaning of the word in the Wikipedia dictionary

Systole is a phonetic phenomenon in ancient verse; shortening the long syllable metri causa (as required by the meter). Eg. in Greek instead of, instead of; in lat. donec instead of dōnec. The phenomenon of systole usually occurs when a long syllable occurs.

Encyclopedic Dictionary, 1998. The meaning of the word in the dictionary Encyclopedic Dictionary, 1998.

SYSTOL (from Greek systole - abbreviation) contraction of the atria and ventricles of the heart, during which blood is pumped into the arteries. Systole together with diastole (relaxation) of the atria and ventricles constitute the cycle of cardiac activity.

New explanatory dictionary of the Russian language, T. F. Efremova. The meaning of the word in the dictionary New explanatory dictionary of the Russian language, T. F. Efremova.

and. Contraction as one of the phases of the heart.

Great Soviet Encyclopedia Meaning of the word in the dictionary Great Soviet Encyclopedia

(from the Greek systole ≈ compression, contraction), contraction of the heart muscle, or myocardium; consists of separately but sequentially flowing S. atria and S. ventricles. Consecutive S. and diastole make up the cycle of cardiac activity. In humans, at frequency.

Dictionary Russian language. D.N. Ushakov The meaning of the word in the dictionary Explanatory Dictionary of the Russian Language. D.N. Ushakov

systole, plural no, w. (Greek systole - compression) (physiol.). Rhythmic contraction muscular organ(mainly about the heart).

Dictionary of medical terms Meaning of the word in the dictionary Dictionary of medical terms

phase of the cardiac cycle, consisting of sequential contractions of the myocardium of the atria and ventricles.

Examples of the use of the word systole in literature.

I also remember a whole series of references, filling with a single melody the remembered essence of getting them, taking them in battle, reaching out to them, begging them from officials, elderly doctors, elderly women of average height, extremely soulful, possessing some strange resolution and ability in the essence of the lens of my conversation with my parents about serving in the army, which had a foreskin, through which I saw something, the very ability to consider something, to distinguish, to recognize, to contemplate, to have something more or less advantageous, but always natural in my struggle for the will to power, prevailed a cacophony of sounds, the time of which alone merged, composed into a jubilant, festive, initially manifesting remnant of meaning, a spring sound sequence of certificates, stamps, paragraphs, headings, returning handwriting, the indistinguishability of my name in this schematic solidity and transparency, and another musical seven-part structure of the fundamentals of documentation , whose letters were produced by those dressed in her toga with neon

I also allowed a priori knowledge, as well as synthetic judgments a priori: after all, throughout my entire life, writing and observing, I acted either synthetically or analytically: systole and the diastole of the human spirit were like a second wind for me.

I SEE the seasons slowly changing systole and the diastoles of winter and summer, autumn and spring, fully delineated rhythms of heat and cold, drought and rain, sunlight, fog and darkness.

Three hundred years of diastole - and suddenly a quick and unexpected systole- as if clenching a fist!

And it was precisely the one who delved lovingly into the phenomena of Schelling and Hegel, who thanks to this was able to see, see with love, what could not arise through the philosophy of the West, who, for the sake of the West, had to strive for anthroposophy, for an anthroposophically oriented spiritual science, so that we have received something that draws from the spirit in the same way as the people of the East drew from the spirit through systole and diastole, and their interaction.

systole

Meaning of the word systole

Dictionary of foreign words

systole [gr. systole contraction, contraction] is a rhythmically repeated contraction of the heart muscle that occurs after its relaxation (diastole).

New explanatory and word-formative dictionary of the Russian language. Author T. F. Efremova.

systole Contraction as one of the phases of the heart.

systole s`istola, -s

Explanatory Dictionary of the Russian Language, ed. D. N. Ushakova

SYSTOL systole, plural. no, w. (Greek systole - compression) (physiol.). Rhythmic contraction of a muscle organ (mainly about the heart).

Modern explanatory dictionary

SYSTOL (from the Greek systole - abbreviation), contraction of the atria and ventricles of the heart, during which blood is pumped into the arteries. Systole together with diastole (relaxation) of the atria and ventricles constitute the cycle of cardiac activity.

7 letters in the word “systole”: a and l o s t.

Words formed from 7 letters of the word systole:

Properties of the heart muscle and its diseases

The cardiac muscle (myocardium) in the structure of the human heart is located in the middle layer between the endocardium and epicardium. It is this that ensures the uninterrupted operation of “distilling” oxygenated blood into all organs and systems of the body.

Any weakness affects blood flow and requires compensatory restructuring and coordinated functioning of the blood supply system. Insufficient ability to adapt causes a critical decrease in the performance of the heart muscle and its diseases.

Myocardial endurance is ensured by its anatomical structure and empowered.

Structural features

It is customary to judge the development of the muscle layer by the size of the heart wall, because the epicardium and endocardium are normally very thin membranes. The child is born with the same thickness of the right and left ventricles (about 5 mm). TO adolescence the left ventricle increases by 10 mm, and the right by only 1 mm.

In a healthy adult, in the relaxation phase, the thickness of the left ventricle ranges from 11 to 15 mm, the right - 5–6 mm.

Features of muscle tissue are:

  • striated striations formed by myofibrils of cardiomyocyte cells;
  • the presence of two types of fibers: thin (actin) and thick (myosin), connected by cross bridges;
  • connection of myofibrils into bundles, different lengths and directionality, which allows us to distinguish three layers (surface, internal and middle).

The structure of the heart muscle is different from skeletal and smooth muscle muscles, which provide movement and protection of internal organs

The morphological features of the structure provide a complex mechanism of heart contraction.

How does the heart contract?

Contractility is one of the properties of the myocardium, which consists in creating rhythmic movements atria and ventricles, which allow blood to be pumped into the vessels. The chambers of the heart constantly pass through 2 phases:

  • Systole - is caused by the combination of actin and myosin under the influence of ATP energy and the release of potassium ions from the cells, while thin fibers slide over thick ones and the bundles decrease in length. The possibility of wave-like movements has been proven.
  • Diastole - relaxation and separation of actin and myosin occurs, restoration of expended energy due to the synthesis of enzymes, hormones, and vitamins obtained through the “bridges”.

It has been established that the force of contraction is provided by calcium entering the myocytes.

The entire cycle of heart contraction, including systole, diastole and the general pause after them, with a normal rhythm fits into 0.8 seconds. Begins with atrial systole, the ventricles are filled with blood. Then the atria “rest”, moving into the diastole phase, and the ventricles contract (systole).

Calculating the time of “work” and “rest” of the heart muscle showed that per day the contraction state accounts for 9 hours 24 minutes, and relaxation - 14 hours 36 minutes.

The sequence of contractions, ensuring the physiological characteristics and needs of the body during stress and anxiety depend on the connection of the myocardium with the nervous and endocrine systems, the ability to receive and “decipher” signals, to actively adapt to human living conditions.

The spread of excitation from the sinus node can be traced by intervals and ECG waves

Cardiac mechanisms that provide contraction

The properties of the heart muscle have the following purposes:

  • support myofibril contraction;
  • ensure the correct rhythm for optimal filling of the cavities of the heart;
  • maintain the ability to push blood through any extreme conditions for the body.

For this, the myocardium has the following abilities.

Excitability - the ability of myocytes to respond to any incoming pathogens. Cells protect themselves from above-threshold stimulation by a state of refractoriness (loss of the ability to excite). In a normal contraction cycle, a distinction is made between absolute and relative refractoriness.

  • During the period of absolute refractoriness, for 200 to 300 msec, the myocardium does not respond even to extremely strong stimuli.
  • When relative, it is able to respond only to sufficiently strong signals.

This property prevents the heart muscle from “distracting” the contraction mechanism during the systole phase

Conductivity - the property of receiving and transmitting impulses to different parts of the heart. It is provided by a special type of myocytes that have processes very similar to neurons in the brain.

Automaticity - the ability to create its own action potential inside the myocardium and cause contractions even when isolated from the body. This property allows resuscitation in emergency cases and maintains blood supply to the brain. The importance of the located network of cells and their accumulation in nodes during transplantation of a donor heart.

The importance of biochemical processes in the myocardium

The viability of cardiomyocytes is ensured by the supply of nutrients, oxygen and the synthesis of energy in the form of adenosine triphosphoric acid.

All biochemical reactions occur maximally during systole. The processes are called aerobic because they are possible only with a sufficient amount of oxygen. The left ventricle consumes 2 ml of oxygen per minute per 100 g of mass.

To produce energy, the following are used in the blood:

  • glucose,
  • lactic acid,
  • ketone bodies,
  • fatty acid,
  • pyruvic and amino acids,
  • enzymes,
  • B vitamins,
  • hormones.

If the heart rate increases (physical activity, anxiety), the need for oxygen increases 40–50 times, and the consumption of biochemical components also increases significantly.

What compensatory mechanisms does the heart muscle have?

A person does not develop pathology as long as the compensation mechanisms work well. Regulation is carried out by the neuroendocrine system.

The sympathetic nerve delivers signals to the myocardium about the need for increased contractions. This is achieved by more intense metabolism and increased ATP synthesis.

A similar effect occurs with increased synthesis of catecholamines (adrenaline, norepinephrine). In such cases, increased work of the myocardium requires an increased supply of oxygen.

The vagus nerve helps reduce the frequency of contractions during sleep, during rest periods, and preserve oxygen reserves.

It is important to consider reflex adaptation mechanisms.

Tachycardia is caused by congestive stretching of the mouths of the vena cava.

Reflex slowing of the rhythm is possible with aortic stenosis. Wherein high blood pressure in the cavity of the left ventricle irritates the endings of the vagus nerve, promoting bradycardia and hypotension.

The duration of diastole increases. Favorable conditions are created for the functioning of the heart. Therefore, aortic stenosis is considered a well-compensated defect. It allows patients to live to an old age.

How to treat hypertrophy?

Typically, prolonged increased load causes hypertrophy. The thickness of the left ventricular wall increases by more than 15 mm. In the mechanism of education important point is a lag in the growth of capillaries deep into the muscle. In a healthy heart, the number of capillaries per mm2 of heart muscle tissue is about 4000, and with hypertrophy the figure drops to 2400.

Therefore, the condition is considered compensatory up to a certain point, but with significant thickening of the wall it leads to pathology. It usually develops in the part of the heart that must work hard to push blood through a narrowed hole or overcome a vascular obstruction.

Hypertrophied muscle is able to maintain blood flow for a long time in case of heart defects.

The muscle of the right ventricle is less developed; it works against a pressure of 15–25 mm Hg. Art. Therefore, compensation for mitral stenosis and cor pulmonale does not last long. But right ventricular hypertrophy has great importance in case of acute myocardial infarction, cardiac aneurysm in the area of ​​the left ventricle, relieves overload. The significant capabilities of the right sections in training during physical exercises have been proven.

Thickening of the left ventricle compensates for aortic valve defects and mitral insufficiency

Can the heart adapt to work in hypoxic conditions?

An important property of adaptation to work without sufficient oxygen supply is the anaerobic (oxygen-free) process of energy synthesis. A very rare occurrence for human organs. Turns on only in emergency situations. Allows the heart muscle to continue contracting.

Negative consequences are the accumulation of breakdown products and overwork of muscle fibrils. One cardiac cycle is not enough for energy resynthesis.

However, another mechanism is involved: tissue hypoxia reflexively causes the adrenal glands to produce more aldosterone. This hormone:

  • increases the amount of circulating blood;
  • stimulates an increase in the content of red blood cells and hemoglobin;
  • increases venous flow to the right atrium.

This means that it allows the body and myocardium to adapt to the lack of oxygen.

How myocardial pathology occurs, mechanisms of clinical manifestations

Myocardial diseases develop under the influence of various causes, but appear only when adaptation mechanisms fail.

Long-term loss of muscle energy, the impossibility of independent synthesis in the absence of components (especially oxygen, vitamins, glucose, amino acids) lead to a thinning of the actomyosin layer, breaking the connections between myofibrils, replacing them with fibrous tissue.

This disease is called dystrophy. It accompanies:

  • anemia,
  • avitaminosis,
  • endocrine disorders,
  • intoxications.

Arises as a consequence:

Patients experience the following symptoms:

At a young age, most common cause may be thyrotoxicosis, diabetes. At the same time, obvious symptoms of increasing thyroid gland not detected.

The inflammation of the heart muscle is called myocarditis. It accompanies both infectious diseases of children and adults, as well as those unrelated to infection (allergic, idiopathic).

It develops in focal and diffuse forms. Proliferation of inflammatory elements affects myofibrils, interrupts pathways, and changes the activity of nodes and individual cells.

As a result, the patient develops heart failure (usually right ventricular failure). Clinical manifestations consist of:

  • pain in the heart area;
  • rhythm interruptions;
  • shortness of breath;
  • dilation and pulsation of the neck veins.

Atrioventricular blocks of varying degrees are recorded on the ECG.

The most well-known disease caused by impaired blood flow to the heart muscle is myocardial ischemia. It proceeds in the form:

  • angina attacks,
  • acute heart attack,
  • chronic coronary insufficiency,
  • sudden death.

All forms of ischemia are accompanied by paroxysmal pain. They are figuratively called “the cry of the starving myocardium.” The course and outcome of the disease depends on:

  • speed of assistance;
  • restoration of blood circulation due to collaterals;
  • the ability of muscle cells to adapt to hypoxia;
  • formation of a strong scar.

A controversial drug included in the doping list because it gives extra energy heart muscle

How to help the heart muscle?

People involved in sports remain the most prepared for critical impacts. A clear distinction should be made between cardio training offered by fitness centers and therapeutic exercises. Any cardio program is designed for healthy people. Increased training can cause moderate hypertrophy of the left and right ventricles. When the work is done correctly, the person himself monitors the sufficiency of the load using his pulse.

Therapeutic exercise is indicated for people suffering from any diseases. If we talk about the heart, then it has the goal:

  • improve tissue regeneration after a heart attack;
  • strengthen the spinal ligaments and eliminate the possibility of pinching of the paravertebral vessels;
  • “boost” the immune system;
  • restore neuroendocrine regulation;
  • ensure the functioning of auxiliary vessels.

Exercise therapy is prescribed by doctors; it is better to master the complex under the supervision of specialists in a sanatorium or medical institution

Treatment with drugs is prescribed in accordance with their mechanism of action.

There is currently a sufficient arsenal of means for therapy:

  • relieving arrhythmias;
  • improving metabolism in cardiomyocytes;
  • enhancing nutrition by dilating the coronary vessels;
  • increasing resistance to hypoxia conditions;
  • suppressing unnecessary foci of excitability.

You can’t joke with your heart; experimenting on yourself is not recommended. Only a doctor can prescribe and select medications. In order to prevent pathological symptoms for as long as possible, proper prevention is needed. Everyone can help their heart by limiting their alcohol intake, fatty foods by quitting smoking. Regular physical exercise can solve many problems.

Hello, I am 41 years old, I did push-ups once in the morning and in the evening, now I have pain in the heart area after even the slightest physical activity or when lifting weights, please tell me what is wrong with my heart and how to treat it?

Emergency medicine

The continuous movement of blood through a closed system of vessels of the pulmonary and systemic circulation is carried out due to the contractile function of the heart. The systemic circulation supplies the body's organs with oxygen-rich blood and also collects venous blood and brings it to the heart. In the pulmonary circulation, the blood is enriched with oxygen.

Venous blood of the systemic circulation is sent through the right ventricle and pulmonary arteries to the lungs, and oxygenated blood enters through the pulmonary veins into the left atrium (Fig. 65). Due to the rhythmic contractions of the ventricles, blood is pushed from the left ventricle into the aorta, and from the right into the pulmonary arteries.

Contraction of the heart muscle occurs in a strict sequence, with a regular rhythm (Fig. 66). In the cardiac cycle, atrial systole is distinguished, which lasts at a contraction frequency of 75 times per minute, 0.04 - 0.07 s, ventricular systole (0.3 s), and ventricular diastole (0.5 s). 0.1 s before the end of ventricular diastole, atrial systole begins. Consequently, atrial diastole lasts 0.7 s.

The joint diastole of the atria and ventricles (pause) lasts 0.4 s. Of the total duration of the cardiac cycle, equal in the case considered to 0.9 s, the ventricles are in a state of contraction 1/3 of the time, and the atria - three times less. Both in ventricular systole and diastole, several phases are distinguished.

In the structure of ventricular contraction, the phases of asynchronous and isometric contraction, fast and slow expulsion. In the phase of asynchronous contraction of the ventricles, some of the mynthecal fibers contract, and some relax. The pressure in the ventricles does not change. The duration of this phase at the pulse frequency already discussed is about 0.05 s.

Asynchronous contraction is replaced by isometric contraction, in which the ventricles are strained and their shape changes. Intraventricular pressure remains constant. The duration of an isometric contraction is about 0.03 s. Throughout the tension phase, the aortic and atrioventricular heart valves remain closed.

The onset of the ejection phase is accompanied by a steep increase in pressure in the ventricles (rapid ejection). In the slow ejection phase, the pressure decreases, but remains higher than in the aorta. The end of the ejection phase - the protodiastolic interval - is characterized by equalization of pressure in the efferent vessels and in the ventricles. These three cycles last 0.3-0.4 s.

The phase of isometric relaxation of the ventricles following protodiastole is accompanied by a drop in pressure to zero. A drop in pressure in the ventricles causes the atrioventricular valves of the heart to open. Blood from the atria first quickly (within 0.06 - 0.08 s), and then slowly (within 0.15 - 0.18 s) fills the ventricles. These are the fast and slow filling phases. Then the described pattern of contraction and relaxation of the heart is repeated.

Rice. 65. Diagram of the structure of the heart and the direction of blood movement in the cardiac cavities: 1 - aortic arch; 2 - superior vena cava; 3 - right lung; 4 - semilunar valve; 5 - right atrium; 6 - coronary vein; 7 - inferior vena cava; 8 - tricuspid valve; 9 - remnant of the ductus arteriosus; 10 - pulmonary artery; 11 - left lung; 12 - pulmonary vein; 13 - left atrium; 14 - two-leaf valve; 15 - semilunar valve; 16 - tendon thread; 17 - left ventricle; 18 - cardiac muscle; 19 - aorta; 20 - right ventricle

Rice. 66. Schematic representation of the relationship between mechanical and electrical systole at rest. The upper curve is an electrocardiogram recording, the lower curve is a phonocardiogram recording

Automatic contractile function. The regular nature of the alternation of phases of cardiac contraction is due to the autonomous self-regulatory system of the heart, called the conduction system. The conduction system of the heart consists of atypical muscle tissue (glycogen-rich muscle fibers of the Pur-Kinje). A cluster of conduction system cells (pacemakers) are located in the area of ​​the sinoatrial node, atrioventricular septum, and in the thickness of the muscular walls of the left and right ventricles (His fiber bundles).

The primary pacemaker is the sinoatrial node, located at the mouth of the vena cava. The cells of this node have the highest rate of spontaneous depolarization. From the sinoatrial node, excitation spreads along the wall of the right atrium to the atrioventricular node - the secondary pacemaker.

The thick muscular bundle of His is directed from the atrioventricular node into the ventricular septum. The terminal branches of the cardiac conduction system are presented muscle fibers Purkinje, anastomosing with contractile fibers of the heart muscle. The cardiac conduction system regulates the rhythmic contraction of the isolated heart.

In specially created conditions, it is possible to maintain rhythmic contractions of even individual heart cells for a long time. Spontaneous rhythmic contraction of isolated heart cells is a strong argument in favor of the myogenic nature of heart automation.

Myocardial muscle cells - myocytes - are connected to each other using intercellular intercalary discs - nexuses. Dense packaging facilitates the conduction of excitation in the myocardium, the heart muscle itself contracts as a single whole. The cardiac muscle and cardiac conduction system is a functional syncytium. This point of view is confirmed in electrophysiological experiments.

A feature of the electrical activity of pacemakers is a gradual decrease in membrane potential after the end of systole (diastolic polarization). Having reached a critical level, depolarization is replaced by a sharp shift in the electrical charge of the cell - an action potential, indicating its excitation.

The wave of excitation spreads to neighboring cells of the pacemaker node. This automatic change in electrical potential is characteristic of all cells of the sinoatrial node of the conduction system.

Contraction of the heart muscle is accompanied by the appearance of tones that are clearly audible in various areas of the heart’s projection onto chest. The first tone is systolic - low-frequency, dull, long-lasting. It coincides with the slamming of the atrioventricular valves. The second tone is diastolic - high, short. It coincides with the closure of the semilunar valves after the end of systole.

Excitability and refractoriness of the heart muscle. The excitability of individual parts is not the same. The most excitable is the sinoatrial pacemaker - the Keith-Fluck node. The atrioventricular node and the fibers of atypical muscle tissue that are part of the bundles of His are less excitable. The excitability of the contractile muscles of the heart is significantly lower than the excitability of its conduction system.

During contraction, the heart muscle does not respond to stimulation, its excitability sharply decreases. This is the phase of absolute refractoriness of the heart. In the initial period of relaxation, the excitability of the heart muscle is restored, but does not reach its original value - this is relative refractoriness. At this moment, the heart can respond with an extraordinary contraction - an extrasystole - to additional irritation. Relative refractoriness is replaced by a phase of increased excitability - exaltation.

The duration of the absolute refractory phase determines the heart rate. At rest, the heart rate of an adult is between 50 and 75 beats per minute. With muscular and intense mental work, with emotional arousal, the refractoriness of the heart decreases, the pulse rate increases, reaching in some cases 200 or more beats per minute.

Subthreshold stimuli that are weak in strength do not cause heart contraction. When the critical (threshold) strength of the stimulus is reached, the heart responds with a maximum contractile act. The power of cardiac contraction does not depend on the strength of the stimulus: after reaching a threshold value, a further increase in the strength of the stimulus does not affect the power of cardiac output. This phenomenon is called the “all or nothing” law.

An obvious exception to this law is the Frank-Starling Law of the Heart. Stretched by increased blood flow, the heart muscle contracts with greater force (heterometric mechanism of increasing contraction force). This occurs when blood flow to the heart increases. In stretched cardiac muscle fibers, the area of ​​interaction between actin and myosin filaments increases. Consequently, the force of contraction increases. Increasing cardiac output power in this case has important adaptive significance, for example, with large physical activity the force of cardiac contraction also increases with increasing pressure in large arteries (homeometric effect).

Rice. 67. Schematic representation of the connection between areas of excitation of the heart muscle and individual waves of the electrocardiogram: I - excitation of the atria; II - excitation of the atrioventricular node; III - beginning of ventricular excitation; 1 - sinoatrial node; 2 - atrioventricular node (according to E.B. Babsky et al., 1972). Latin letters indicate ECG waves

Fomin A.F. Human physiology, 1995

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