Minute volume of blood, the formula by which this indicator is calculated, as well as others important points must certainly be in the baggage of knowledge of any medical student, and even more so of persons already engaged in medical practice. What is this indicator, how does it affect human health, why is it important for doctors, and what depends on it - every young man or girl who wants to enter medical school is looking for answers to these questions. educational institution. It is these questions that are covered in this article.

Heart function

The main function of the heart is the delivery to organs and tissues of a certain volume of blood per unit of time (volume of blood per minute), due to the state of the heart itself and the conditions of work in the circulatory system. This most important mission of the heart is studied in school years. Most anatomy textbooks, unfortunately, don't talk much about this function. Cardiac output is the derivative of stroke volume and heart rate.

MO (SV) \u003d HR x SV

Cardiac index

Stroke volume - an indicator that determines the size and amount of blood expelled by the ventricles in one contraction, its value is approximately 70 ml.Cardiac index- size of 60-second volume converted to surface area human body. At rest, its normal value is about 3 l / min / m 2.

Normally, the minute volume of human blood depends on the size of the body. For example, the cardiac output of a female weighing 53 kg will undoubtedly be significantly lower than that of a male weighing 93 kg.

Normally, in a man weighing 72 kg, the minute volume of the heart pumped per minute is 5 l / min., Under load, this figure can grow up to 25 l / min.

What affects cardiac output?

These are a few indicators:

• systolic volume of blood entering the right atrium and the ventricle ("right heart"), and the pressure it creates is preload.
• the resistance experienced by the heart muscle at the time of the release of the next volume of blood from the left ventricle - afterload.
• the period and speed of heart contractions and myocardial contractility, which change under the influence of sensitive and parasympathetic nervous system.

Contractility - the ability to generate force by the heart muscle at any length of the muscle fiber. The combination of all these characteristics, of course, affects the minute volume of blood, speed and rhythm, as well as other cardiac indicators.

How is this process regulated in the myocardium?

Contraction of the heart muscle occurs if the concentration of calcium inside the cell becomes more than 100 mmol, the susceptibility of the contractile apparatus to calcium is less important.

In the resting period of the cell, calcium ions make their way into the cardiomyocyte through the L-channels of the membrane, and are also released inside the cell itself into its cytoplasm from the sarcoplasmic reticulum. Due to the double route of intake of this trace element, its concentration rapidly increases, and this is the beginning of the contraction of the cardiac myocyte. Such a double path of "ignition" is characteristic only for the heart. If there is no intake of extracellular calcium, then there will be no contraction of the heart muscle.

The hormone norepinephrine, which is released from sympathetic nerve endings, increases the rate and contractility of the heart, thus increasing cardiac output. This substance belongs to physiological inotropic agents. Digoxin is a medicinal inotropic drug that is used in certain cases to treat heart failure.

Stroke volume and filling pressure

The minute volume of blood in which is formed at the end of diastole and the base of systole depends on the elasticity muscle tissue and end diastolic pressure. in the right parts of the heart is associated with the pressure of the venous system.

When the end rises diastolic pressure, increases the strength of subsequent contractions and stroke volume. That is, the strength of the contraction is related to the degree of stretching of the muscle.

The shock blood from both ventricles is presumably equal. If the output from the right ventricle exceeds the output from the left for some time, pulmonary edema may develop. However, there are protective mechanisms, during the course of which, reflexively, due to an increase in stretching muscle fibers in the left ventricle, the amount of blood expelled from it increases. This increase in cardiac output prevents pressure buildup in the pulmonary circulation and restores balance.

By the same mechanism, there is an increase in the release of blood volume during exercise.

This mechanism is amplification heart contraction when a muscle fiber is stretched - is called the Frank-Starling law. It is an important compensatory mechanism in heart failure.

Afterload action

With an increase in blood pressure or an increase in afterload, the volume of ejected blood can also increase. This property was documented and experimentally confirmed many years ago, which made it possible to make appropriate corrections to the calculations and formulas.

If blood is ejected from the left ventricle under conditions of increased resistance, then for some time the volume of residual blood in the left ventricle will increase, the extensibility of myofibrils increases, this increases the stroke volume, and as a result, the minute volume of blood increases in accordance with the Frank-Starling rule. After several such cycles, the blood volume returns to its original value.
The autonomic nervous system is the external regulator of cardiac output.

Ventricular filling pressure, change, and contractility can change stroke volume. Central venous pressure and the autonomic nervous system are factors that control cardiac output.

So, we have considered the concepts and definitions named in the preamble of this article. We hope that the information presented above will be useful to all people interested in the topic voiced.

The main physiological function of the heart is to pump blood into the vascular system.

The amount of blood ejected by the ventricle of the heart per minute is one of the most important indicators functional state hearts and is called minute volume of blood flow or minute volume of the heart. It is the same for the right and left ventricles. When a person is at rest, the minute volume averages 4.5-5.0 liters. By dividing the minute volume by the number of heartbeats per minute, you can calculate systolic volume blood flow. With a heart rate of 70-75 per minute, the systolic volume is 65-70 ml of blood. Determination of the minute volume of blood flow in humans is used in clinical practice.

The most accurate method for determining the minute volume of blood flow in humans was proposed by Fick (1870). It consists in an indirect calculation of the minute volume of the heart, which is produced knowing: 1) the difference between the oxygen content in arterial and venous blood; 2) the volume of oxygen consumed by a person per minute. Let's say
that in 1 minute 400 ml of oxygen entered the blood through the lungs, every
100 ml of blood absorb 8 ml of oxygen in the lungs; therefore, in order to understand everything
the amount of oxygen that entered through the lungs into the blood per minute (in our
at least 400 ml), it is necessary that 100 * 400 / 8 = 5000 ml of blood pass through the lungs. This

the amount of blood and is the minute volume of blood flow, which in this case is equal to 5000 ml.

When using the Fick method, it is necessary to take venous blood from the right side of the heart. IN last years venous blood is taken from the right half of the heart with a probe inserted into the right atrium through the brachial vein. This method of taking blood is not widely used.

A number of other methods have been developed to determine the minute, and hence the systolic volume. Currently, some paints and radioactive substances are widely used. The substance injected into the vein passes through the right heart, the pulmonary circulation, left heart and enters the arteries of a large circle, where its concentration is determined. It first rises in waves and then falls. After some time, when the portion of blood containing the maximum amount of it passes through the left heart for the second time, its concentration in arterial blood slightly increases again (the so-called recirculation wave). The time from the moment the substance is administered to the start of recirculation is noted and a dilution curve is drawn, i.e., changes in the concentration (increase and decrease) of the test substance in the blood. Knowing the amount of the substance introduced into the blood and contained in the arterial blood, as well as the time required for the passage of the entire amount of the introduced substance through the circulatory system, it is possible to calculate the minute volume (MO) of blood flow in l/min using the formula:

where I is the amount of the administered substance in milligrams; C - its average concentration in milligrams per 1 liter, calculated from the dilution curve; T- duration of the first wave of circulation in seconds.

At present, a method has been proposed integral rheography. Rheography (impendance) is a method of recording the electrical resistance of human body tissues. electric current passed through the body. In order not to cause tissue damage, ultra-high frequency currents and very low strength are used. The resistance of the blood is much less than the resistance of the tissues, therefore, an increase in the blood supply to the tissues significantly reduces their electrical resistance. If we register the total electrical resistance chest in several directions, then periodic sharp decreases in it occur at the time of ejection of the systolic blood volume by the heart into the aorta and pulmonary artery. In this case, the magnitude of the decrease in resistance is proportional to the magnitude of the systolic ejection.

Keeping this in mind and using formulas that take into account the size of the body, the features of the constitution, etc., it is possible to determine the value of the systolic blood volume from the rheographic curves, and by multiplying it by the number of heartbeats, we can obtain the value of the minute volume of the heart.

Systolic volume is the amount of blood that enters the circulation during one ventricular contraction. Minute volume is the amount of blood that flows through the aorta in one minute. The systolic volume is determined in the clinic in such a way that the minute volume is measured and divided by the number of heartbeats per minute. Under physiological conditions, the systolic and minute volumes of the right and left ventricles are almost the same. The value of minute volume in healthy individuals is primarily determined by the body's need for oxygen. Under pathological conditions, the body's need for oxygen should also be satisfied, but it often cannot be satisfied even with a significant increase in minute volume.

In healthy individuals, the minute volume at rest for a long time is almost constant and is proportional to the body surface, expressed in square meters. The number indicating the minute volume per m2 of body surface is called the “cardiac indicator”. As a cardiac indicator for a long time, the value of 2.2 liters established by Grollmann was used. The rate calculated by Kurnan based on the data obtained by cardiac catheterization is higher: 3.12 liters per minute per 1 m2 of body surface. In the future, we use the Kurnan heart rate. If we want to determine the ideal minute volume of a child, then we determine the surface of the body from the Dubois table and multiply the resulting value by 3.12 and, thus, we get the minute volume in liters.

Previously, minute volume was compared with body weight. The wrongness of this approach, especially in pediatrics, is clear, because the surface of the body of infants and small children is large compared to their weight, and, accordingly, their minute volume is relatively large.
Body surface (in m2) of healthy children different ages, pulse beats per minute, minute volume, systolic volume, and age-appropriate mean blood pressure value are shown in Table 2. These tables are averages, and there are many individual deviations in life. It turns out that the minute volume of a newborn of average weight, which is 560 ml, in an adult increases almost tenfold. In the case of average development during the same time, the surface of the body also increases tenfold, and the two magnitudes are thus parallel. The weight of the human body during this time increases by 23 times. The table shows that in parallel with the increase in minute volume, the number of heartbeats per minute decreases. Thus, in the course of growth, the systolic volume by necessity increases to a greater extent than the minute volume, which increases in proportion to the increase in body surface. The body surface and minute volume of an average newborn increase 10 times in an adult, while the systolic volume increases 17 times.

With individual contractions of the heart, the blood in the ventricles is not completely expelled, and the amount of blood remaining there can, under normal circumstances, reach the amount of systolic volume. Under pathological conditions, the ventricles can remain significantly large quantity blood than which is expelled during systole. A number of attempts have been made to determine the amount of residual blood, partly by means of x-ray examination partly by the use of paints. According to research by Harmon and Nyulin, there is a close relationship between the time of circulation and the amount of blood remaining in systole in the ventricles.

Minute volume healthy person and under physiological conditions depends on a number of factors. Muscular work increases it by 4-5 times, in extreme cases on a short time 10 times. Approximately 1 hour after a meal, the minute volume becomes 30-40% more than it was before, and only after about 3 hours does it reach its original value. Fear, fright, excitement - probably due to the development a large number adrenaline - increase minute volume. At low temperatures, cardiac activity is more economical than at higher temperatures. high temperature. Temperature fluctuations of 26 ° C do not have a significant effect on the minute volume. At temperatures up to 40 ° C, it increases slowly, and above 40 ° C - very quickly. The position of the body also affects the minute volume. At lying position it decreases, and in standing it increases. Other data on the increase and decrease in minute volume are given partly in the chapter on decompensation, partly in the chapters considering individual pathological conditions.

The heart is able to increase the minute volume in three ways: 1. by increasing the number of pulse beats with the same systolic volume, 2. by increasing the systolic volume with the same number of pulse beats, 3. by simultaneously increasing the systolic volume and pulse rate.

With an increase in pulse rate, minute volume increases only if venous blood flow increases accordingly, otherwise the ventricle contracts after insufficient filling, and thus, due to a decrease in systolic volume, minute volume does not increase. With a very strong tachycardia, filling may be so imperfect (for example, with acute insufficiency coronary circulation, with paroxysmal tachycardia), that, despite the high pulse rate, the minute volume decreases.

The heart of a child is able to safely increase the number of contractions per minute from 100 to a maximum of 150-200. With an unchanged systolic volume, the minute volume can thus increase only 1.5-2 times. If a greater increase is needed, cardiac output is increased by simultaneous dilatation of the heart.

If, as a result of an abundant influx of venous blood in large veins and in the atria, there is enough blood to fill the ventricles, then during diastole, more blood enters the ventricles, and more high pressure in the ventricles increases systolic volume according to Starling's law. Thus, the minute volume increases without increasing the pulse rate. In humans, this phenomenon is observed mainly with hypertrophy of the heart muscle, it is rare in childhood. A small heart is not able to contain more than a certain amount of blood, especially since an increase in atrial pressure very soon causes an increase in the pulse rate through the Bainbridge reflex. In infancy and childhood there is already a greater tendency to tachycardia, and thus tachycardia plays a greater role in increasing minute volume than increasing dilatation. The ratio of these two factors is determined individual features, where the greatest role, of course, belongs to the influences from the nervous and hormonal systems. Hamilton's paper and West and Taylor's review paper very well set out physiological changes minute volume and external and internal factors affecting it.

If the body's need for oxygen cannot be satisfied by an increase in minute volume, the tissues absorb more oxygen from the blood than usual.

The main physiological function of the heart is the ejection of blood into the vascular system. Therefore, the amount of blood expelled from the ventricle is one of the most important indicators of the functional state of the heart.

The amount of blood ejected by the ventricle of the heart in 1 minute is called the minute volume of blood. It is the same for the right and left ventricles. When a person is at rest, the minute volume averages about 4.5-5 liters.

By dividing the minute volume by the number of heartbeats per minute, you can calculate systolic blood volume. With a heart rate of 70-75 per minute, the systolic volume is 65-70 ml of blood.

Definition minute volume of blood in humans is used in clinical practice.

The most accurate method for determining the minute volume of blood in humans was proposed by Fick. It consists in an indirect calculation of the minute volume of the heart, which is produced knowing:

1. the difference between the oxygen content in arterial and venous blood;

2. the volume of oxygen consumed by a person in 1 minute. Let us assume that in 1 minute 400 ml of oxygen entered the blood through the lungs and that the amount of oxygen in arterial blood is 8 vol.% more than in venous blood. This means that every 100 ml of blood absorbs 8 ml of oxygen in the lungs, therefore, in order to absorb the entire amount of oxygen that entered through the lungs into the blood in 1 minute, i.e. 400 ml in our example, 400/8=5000 ml of blood. This amount of blood is the minute volume of blood, which in this case is equal to 5000 ml.

When using this method, it is necessary to take mixed venous blood from the right half of the heart, since the blood of peripheral veins has an unequal oxygen content depending on the intensity of the body's organs. In recent years, mixed venous blood has been taken from a person directly from the right half of the heart using a probe inserted into the right atrium through the brachial vein. However, for obvious reasons, this method of blood sampling is not widely used.

A number of other methods have been developed to determine the minute and, consequently, the systolic volume of blood. Many of them are based on the methodological principle proposed by Stuart and Hamilton. It consists in determining the dilution and circulation rate of a substance introduced into a vein. Currently, some paints and radioactive substances are widely used for this. The substance introduced into the vein passes through the right heart, the pulmonary circulation, the left heart and enters the arteries of the large circle, where its concentration is determined.

The last wavy sleep parastays, and then falls. Against the background of a decrease in the concentration of the analyte, after some time, when the portion of blood containing the maximum amount of it passes through the left heart for the second time, its concentration in the arterial blood again slightly increases (this is the so-called recirculation wave) ( rice. 28). The time from the moment the substance is administered to the start of recirculation is noted and a dilution curve is drawn, i.e., changes in the concentration (increase and decrease) of the test substance in the blood. Knowing the amount of the substance introduced into the blood and contained in the arterial blood, as well as the time required for the passage of the entire amount through the entire circulatory system, it is possible to calculate the minute volume of blood using the formula: minute volume in l / min \u003d 60 I / C T, where I is the amount of the injected substance in milligrams; C - its average concentration in mg / l, calculated from the dilution curve; T is the duration of the first wave of circulation in seconds.

Cardiopulmonary drug. Influence various conditions the value of the systolic volume of the heart can be investigated in an acute experiment using the technique of a cardiopulmonary preparation developed by I. II. Pavlov and N. Ya. Chistovich and later improved by E. Starling.

With this technique, the animal's systemic circulation is turned off by ligation of the aorta and vena cava. The coronal circulation, as well as the circulation through the lungs, i.e., the small circle, is kept intact. Cannulas are inserted into the aorta and vena cava, which are connected to a system of glass vessels and rubber tubes. The blood ejected by the left ventricle into the aorta flows through this artificial system, enters the vena cava and then into the right atrium and right ventricle. From here, the blood is sent to the pulmonary circle. Having passed through the capillaries of the lungs, which rhythmically inflate with furs, the blood, enriched with oxygen and given off carbon dioxide, just as in normal conditions, returns to the left heart, from where it again flows into an artificial large circle of glass and rubber tubes.

By means of a special device, it is possible, by changing the resistance encountered by blood in an artificial big circle increase or decrease blood flow to the right atrium. Thus, the cardiopulmonary preparation makes it possible to change the workload of the heart at will.

The dependence of the strength of heart contractions and the magnitude of the systolic volume on the blood filling of the ventricles in diastole, and, consequently, on the stretching of their muscle fibers, is observed in a number of cases of pathology.

With insufficiency of the aortic semilunar valve, when there is a defect in this valve, the left ventricle during diastole receives blood not only from the atrium, but also from the aorta, since part of the blood ejected into the aorta returns to the ventricle back through the hole in the valve. The ventricle is therefore overstretched by excess blood; accordingly, but according to Starling's law, the strength of heart contractions increases. As a result, due to the increased systole, despite the aortic valve defect and the return of part of the blood to the ventricle from the aorta, the blood supply to the organs remains at a normal level.

Changes in minute blood volume during work. Systolic and minute volumes of blood are not constant values, on the contrary, they are very variable depending on the conditions in which the body is located and what work it does. During muscular work, there is a very significant increase in minute volume (up to 25-30 liters). This may be due to increased heart rate and increased systolic volume. In untrained people, an increase in minute volume usually occurs due to an increase in the heart rate.

For trained people at work moderate there is an increase in systolic volume and a much smaller increase in heart rate than in untrained people. At very great job, for example, in demanding sports competitions, even in well-trained athletes, along with an increase in systolic volume, there is also an increase in heart rate. increased frequency heart rate in combination with an increase in systolic volume, it causes a very large increase in minute volume, and, consequently, an increase in blood supply to working muscles, which creates conditions that ensure greater performance. The number of heartbeats in trained people can reach 200 or more per minute with a very heavy load.

Arterial pressure

Arterial or systemic blood pressure (BP) is the pressure which exerts blood on the walls arterial vessels. Pressure is an important parameter of the life of the organism, in the event of death, the pressure value drops to zero.

There are several types of system pressure:

in the arteries - arterial (this is the indicator that is most often measured);

in capillaries - capillary;

Systolic and minute blood volumes

The amount of blood ejected by the ventricle of the heart into the arteries per minute is an important indicator of the functional state of the heart. vascular system(SSS) and is called minute volume blood (IOC). It is the same for both ventricles and at rest is 4.5-5 liters. If we divide the IOC by the heart rate per minute, we get systolic volume (CO) of blood flow. With a contraction of the heart equal to 75 beats per minute, it is 65-70 ml, during work it increases to 125 ml. In athletes at rest, it is 100 ml, during work it increases to 180 ml. The definition of IOC and CO is widely used in the clinic, which can be done by calculating by indirect indicators (according to the Starr formula, see Workshop on Normal Physiology).

The volume of blood in the cavity of the ventricle, which it occupies before its systole is end-diastolic volume (120-130 ml).

The volume of blood remaining in the chambers after systole at rest is reserve and residual volumes. The reserve volume is realized with an increase in CO at loads. Normally, it is 15-20% of the end-diastolic.

The volume of blood in the cavities of the heart, remaining with the full implementation of the reserve volume, at maximum systole is residual volume. Normally, it is 40-50% of the end-diastolic. CO and IOC values ​​are not constant. With muscular activity, the IOC increases to 30-38 liters due to the increase in heart contractions and an increase in COC.

The IOC value divided by the body surface area in m 2 is defined as cardiac index(l / min / m 2). It is an indicator of the pumping function of the heart. Normally, the cardiac index is 3-4 l / min / m 2. If IOC and aortic BP are known (or pulmonary artery) can determine the external work of the heart

P \u003d MO x AD

P is the work of the heart in minutes in kilogram meters (kg / m).

MO - minute volume (l).

BP is the pressure in meters of water column.

At physical rest outside work of the heart is 70-110 J, during work it increases to 800 J, for each ventricle separately. The whole complex of manifestations of the activity of the heart is recorded using various physiological methods - cardiography: ECG, electrokymography, ballistocardiography, dynamocardiography, apical cardiography, ultrasound cardiography, etc.

Diagnostic method for the clinic is the electrical registration of the movement of the contour of the heart shadow on the screen of the X-ray machine. A photocell connected to an oscilloscope is applied to the screen at the edges of the heart contour. When the heart moves, the illumination of the photocell changes. This is recorded by the oscilloscope in the form of a curve of contraction and relaxation of the heart. This technique is called electrokymography.

Apical cardiogram is registered by any system that captures small local displacements. The sensor is fixed in the 5th intercostal space above the site of the cardiac impulse. Characterizes all phases cardiac cycle. But it is not always possible to register all phases: the cardiac impulse is projected differently, part of the force is applied to the ribs. Entry at different persons and in one person it may differ, depending on the degree of development of the fat layer, etc.

Research methods based on the use of ultrasound are also used in the clinic - ultrasound cardiography.

Ultrasonic vibrations at a frequency of 500 kHz and above penetrate deeply through tissues being formed by ultrasound emitters applied to the surface of the chest. Ultrasound is reflected from tissues of various densities - from the outer and inner surface heart, from vessels, from valves. The time of reaching the reflected ultrasound to the catching device is determined.

If the reflective surface moves, then the return time of the ultrasonic vibrations changes. This method can be used to record changes in the configuration of the structures of the heart during its activity in the form of curves recorded from the screen of a cathode ray tube. These techniques are called non-invasive.

Invasive techniques include:

Cardiac catheterization. An elastic probe-catheter is inserted into the central end of the opened brachial vein and pushed to the heart (into its right half). A probe is inserted into the aorta or left ventricle through the brachial artery.

Ultrasound Scan - the source of ultrasound is introduced into the heart using a catheter.

Angiography is a study of the movements of the heart in the field of x-rays, etc.

Thus, the work of the heart is determined by 2 factors:

1. The amount of blood flowing to it.

2. Vascular resistance during expulsion of blood into the arteries (aorta and pulmonary artery). When the heart cannot pump all the blood into the arteries with a given vascular resistance, heart failure occurs.

There are 3 types of heart failure:

Insufficiency from overload, when excessive demands are placed on the heart with normal contractility in case of defects, hypertension.

Heart failure in case of myocardial damage: infections, intoxication, beriberi, impaired coronary circulation. This reduces the contractile function of the heart.

A mixed form of insufficiency - with rheumatism, dystrophic changes in the myocardium, etc.

5. Regulation of cardiac activity

Adaptation of the activity of the heart to the changing needs of the body is carried out with the help of regulatory mechanisms:

Myogenic autoregulation.

neural mechanism regulation.

Humoral mechanism of regulation.

Myogenic autoregulation. The mechanisms of myogenic autoregulation are determined by the properties of the muscle fibers of the heart. Distinguish intracellular regulation. In each cardiomyocyte, there are mechanisms for regulating protein synthesis. With an increase in the load on the heart, there is an increase in the synthesis of myocardial contractile proteins and structures that ensure their activity. In this case, physiological myocardial hypertrophy occurs (for example, in athletes).

Intercellular regulation. Related to the nexus function. Here, impulses are transmitted from one cardiomyocyte to another, the transport of substances, the interaction of myofibrils. Part of the mechanisms of self-regulation is associated with reactions that occur when the initial length of myocardial fibers changes - heterometric regulation and reactions not associated with a change in the initial length of myocardial fibers - homeometric regulation.

The concept of heterometric regulation was formulated by Frank and Starling. It was found that the more the ventricles stretch during diastole (up to a certain limit), the stronger their contraction in the next systole. Increased filling of the heart with blood, caused by an increase in its inflow, or a decrease in the ejection of blood into the vessels, leads to stretching of the myocardial fibers and an increase in the strength of contractions.

Homeometric regulation includes effects associated with a change in pressure in the aorta (Anrep effect) and a change in the rhythm of heart contractions (the Bowditch effect or ladder). Anrep effect is that an increase in pressure in the aorta leads to a decrease in systolic ejection and an increase in the residual volume of blood in the ventricle. The incoming new volume of blood leads to stretching of the fibers, heterometric regulation is activated, which leads to an increase in the contraction of the left ventricle. The heart is freed from excess residual blood. The equality of venous inflow and cardiac output is established. At the same time, the heart, throwing out the same volume of blood against the increased resistance in the aorta, as with less pressure in the aorta, performs increased work. With a constant frequency of contractions, the power of each systole increases. Thus, the force of contraction of the ventricular myocardium increases in proportion to the increase in resistance in the aorta - the Anrep effect. Hetero- and homeometric regulation (both mechanisms) are interconnected. Bowditch effect is that the strength of myocardial contractions depends on the rhythm of contractions. If an isolated, stopped frog's heart is subjected to rhythmic stimulation, with an ever-increasing frequency, then the amplitude of contractions for each subsequent stimulus gradually increases. The increase in the strength of contractions for each subsequent stimulus (up to a certain value) was called the "phenomenon" (ladder) of Bowditch.

Intracardiac peripheral reflexes are closed in the intramural (intraorgan) ganglia of the myocardium. This system includes:

1. Afferent neurons form mechanoreceptors on myocytes and caronary vessels.

2. Intercalary neurons.

3. Efferent neurons. Innervate the myocardium and coronary vessels. These links form intracardiac reflex arcs. So, with an increase in the stretching of the right atrium (if the blood flow to the heart increases), the left ventricle is intensely reduced. The ejection of blood is accelerated, a place is made for the newly flowing blood. These reflexes are formed in ontogeny early before the appearance of central reflex regulation.

extracardiac nervous regulation. Most high level adaptation of the activity of the cardiovascular system is achieved neurohumoral regulation. Nervous regulation carried out by the central nervous system through the sympathetic and vagus nerves.

Influence vagus nerve . From the nucleus of the vagus nerve, located in the medulla oblongata, axons depart as part of the right and left nerve trunks, approach the heart and form synapses on the motor neurons of the intramural ganglia. The fibers of the right vagus nerve are distributed mainly in the right atrium: they innervate the myocardium, coronary vessels, SA node. The fibers of the left innervate mainly the AV node, affect the conduction of excitation. The studies of the Weber brothers (1845) established the inhibitory effect of these nerves on the activity of the heart.

When the peripheral end of the cut vagus nerve was irritated, the following changes were revealed:

1. Negative chronotropic effect (slowing the rhythm of contractions).

2. Negative inotropic the effect is a decrease in the amplitude of contractions.

3. Negative bathmotropic effect - lowering the excitability of the myocardium.

4. Negative dromotropic the effect is a decrease in the rate of excitation in cardiomyocytes.

Irritation of the vagus nerve can cause complete cardiac arrest, complete blockade conducting excitation in the AV node. However, with continued stimulation, the heart again restores contractions, there is escape heart from under the influence of the vagus nerve.

Influences of the sympathetic nerve. The first neurons of the sympathetic nerves are located in the lateral horns of the 5 upper segments. thoracic spinal cord. The second neurons from the cervical and upper thoracic sympathetic nodes go mainly to the ventricular myocardium and the conduction system. Their influence on the heart was studied by I.F. Zion. (1867), I.P. Pavlov, W. Gaskell. Their opposite effect on the activity of the heart was established:

1. Positive chronotropic effect (increased heart rate).

2. Positive inotropic effect (increase in the amplitude of contractions).

3. Positive bathmotropic effect (increased myocardial excitability).

4. Positive dromotropic effect (increase in the speed of excitation). Pavlov identified sympathetic branches that selectively increase the force of contraction of the heart. Through their stimulation, it is possible to remove the blockade of the conduction of excitation in the AV node. Improvement in the conduction of excitation under the influence of the sympathetic nerve concerns only the AV node. The interval between atrial and ventricular contraction is shortened. An increase in myocardial excitability is observed only if it was previously reduced. With simultaneous stimulation of the sympathetic and vagus nerves, the action of the vagus predominates. Despite the opposite influences of the sympathetic and vagus nerves, they are functional synergists. Depending on the degree of filling of the heart and coronary vessels blood, the vagus nerve can also have the opposite effect, i.e. not only slow down, but also increase the activity of the heart.

The transmission of excitation from the endings of the sympathetic nerve to the heart is carried out with the help of a mediator norepinephrine. It breaks down more slowly and lasts longer. At the endings of the vagus nerve, acetylcholine. It is rapidly degraded by ACh-esterase, so it only has local action. When transection of both nerves (both sympathetic and vagus), a higher rhythm of the AV node is observed. Consequently, his own rhythm is much higher than under the influence of the nervous system.

Nerve centers medulla oblongata, from which the vagus nerves depart to the heart, are in a state of constant central tone. Constant inhibitory influences come from them to the heart. When both vagus nerves are cut, the heart beats faster. The following factors affect the tone of the nuclei of the vagus nerve: an increase in the content of adrenaline, Ca 2+ ions, CO 2 in the blood. Breathing affects: when inhaling, the tone of the nucleus of the vagus nerve decreases, when exhaling, the tone rises and the activity of the heart slows down (respiratory arrhythmia).

Regulation of cardiac activity is carried out by the hypothalamus, limbic system, cortex hemispheres brain.

Important role receptors of the vascular system play in the regulation of the heart, forming vascular reflex zones.

The most significant: aortic, carotid sinus zone, zone of the pulmonary artery, the heart itself. The mechano- and chemoreceptors included in these zones are involved in stimulating or slowing down the activity of the heart, which leads to an increase or decrease in blood pressure.

Excitation from the receptors of the mouths of the caval veins leads to increased and increased heart rate, which is associated with a decrease in the tone of the vagus nerve, an increase in the tone of the sympathetic - Bainbridge reflex. The classic vagal reflex is the reflex loach. With a mechanical effect on the stomach or intestines of the frog, cardiac arrest is observed (influence of the vagus nerve). In humans, this is observed when hitting the anterior abdominal wall.

Oculo-cardiac reflex Danini-Ashner. When pressed on eyeballs there is a decrease in heart contractions by 10–20 per minute (the influence of the vagus nerve).

Increased and intensified contractions of the heart are observed with pain, muscle work, and emotions. The participation of the cortex in the regulation of the heart proves the method conditioned reflexes. If you repeatedly combine a conditioned stimulus (sound) with pressure on the eyeballs, which leads to a slowdown in heart contractions, then after a while only the conditioned stimulus (sound) will cause the same reaction - conditioned eye-heart reflex Danini-Ashner.

With neuroses, disturbances can also appear in the cardiovascular system, which are fixed according to the type of pathological conditioned reflexes. Of great importance in the regulation of the activity of the heart are signals from muscle proprioceptors. During muscle loads, the impulses from them have an inhibitory effect on the vagus centers, which leads to an increase in heart contractions. The rhythm of heart contractions can change under the influence of excitation from thermoreceptors. An increase in body temperature or environment causes an increase in contractions. Cooling of the body upon entering cold water, when bathing leads to a decrease in contractions.

humoral regulation. Carried out by hormones and ions interstitial fluid. Stimulate: catecholamines (adrenaline and norepinephrine), increase the strength and rhythm of contractions. Adrenaline interacts with beta receptors, adrenylyl cyclase is activated, cyclic AMP is formed, inactive phosphorylase turns into active, glycogen is broken down, glucose is formed, and as a result of these processes, energy is released. Adrenaline increases the permeability of membranes for Ca 2+ , which is involved in the processes of contraction of cardiomyocytes. Glucagon, corticosteroids - (aldosterone), angiotensin, serotonin, thyroxine also act on the force of contraction. Ca 2+ increases the excitability and conductivity of the myocardium.

Acetylcholine, hypoxemia, hypercapnia, acidosis, ions K +, HCO -, H + inhibit cardiac activity.

For the normal functioning of the heart great importance have electrolytes. The concentration of K + and Ca 2+ ions affect the automaticity and contractile properties of the heart. An excess of K + causes a slowdown in the rhythm, force of contraction, a decrease in excitability and conductivity. Washing the isolated heart of animals with a concentrated solution of K + leads to relaxation of the myocardium and cardiac arrest in diastole.

Ca 2+ ions speed up the rhythm, increase the strength of heart contractions, excitability, and conductivity. An excess of Ca 2+ leads to cardiac arrest in systole. Disadvantage - weakens the contraction of the heart.

Role higher departments CNS in the regulation of heart activity

The cardiovascular system through the suprasegmental parts of the autonomic nervous system - the thalamus, hypothalamus, cerebral cortex is integrated into the behavioral, somatic, vegetative reactions of the body. The influence of the cerebral cortex (motor and premotor zones) on the circulatory center of the medulla oblongata underlies conditioned reflex cardiovascular reactions. Irritation of the structures of the central nervous system, as a rule, is accompanied by an increase in heart rate and an increase in blood pressure.