30.06.2019

Cardiac cycle. Contraction of the heart is what happens to the blood when the ventricles contract

The human heart works like a pump. Due to the properties of the myocardium (excitability, ability to contract, conductivity, automaticity), it is able to pump blood into the arteries, which enters it from the veins.

It moves without stopping due to the fact that a pressure difference is formed at the ends of the vascular system (arterial and venous) (0 mmHg in the main veins and 140 mmHg in the aorta).

The work of the heart consists of cardiac cycles - continuously alternating periods of contraction and relaxation, which are called systole and diastole, respectively.

Duration

As the table shows, the cardiac cycle lasts approximately 0.8 seconds, assuming that the average contraction frequency is from 60 to 80 beats per minute. Atrial systole takes 0.1 s, ventricular systole - 0.3 s, total diastole of the heart - the remaining time, equal to 0.4 s.

Phase structure

The cycle begins with atrial systole, which lasts 0.1 seconds. Their diastole lasts 0.7 seconds. Ventricular contraction lasts 0.3 seconds, their relaxation lasts 0.5 seconds. The general relaxation of the chambers of the heart is called a general pause, and in this case it takes 0.4 seconds. Thus, there are three phases of the cardiac cycle:

atrial systole – 0.1 sec.;
ventricular systole – 0.3 sec.;
cardiac diastole (general pause) – 0.4 sec.

The general pause preceding the start of a new cycle is very important for filling the heart with blood.

Before the onset of systole, the myocardium is in a relaxed state, and the chambers of the heart are filled with blood that comes from the veins.

The pressure in all chambers is approximately the same, since the atrioventricular valves are open. Excitation occurs in the sinoatrial node, which leads to contraction of the atria; due to the difference in pressure at the time of systole, the volume of the ventricles increases by 15%. When atrial systole ends, the pressure in them decreases.

Atrial systole (contraction)

Before the onset of systole, blood moves to the atria and they are successively filled with it. Part of it remains in these chambers, the rest is sent to the ventricles and enters them through the atrioventricular openings, which are not closed by valves.

At this moment, atrial systole begins. The walls of the chambers tense, their tone increases, the pressure in them increases by 5-8 mm Hg. pillar The lumen of the veins that carry blood is blocked by annular bundles of myocardium. The walls of the ventricles at this time are relaxed, their cavities are expanded, and blood from the atria quickly rushes there through the atrioventricular openings without difficulty. The duration of the phase is 0.1 seconds. Systole overlaps the end of the ventricular diastole phase. The muscle layer of the atria is quite thin, since they do not require much force to fill the neighboring chambers with blood.

Ventricular systole (contraction)

This is the next, second phase of the cardiac cycle and it begins with tension of the heart muscles. The voltage phase lasts 0.08 seconds and in turn is divided into two more phases:

Asynchronous voltage – duration 0.05 sec. Excitation of the walls of the ventricles begins, their tone increases.
Isometric contraction – duration 0.03 sec. The pressure in the chambers increases and reaches significant values.

The free leaflets of the atrioventricular valves floating in the ventricles begin to be pushed into the atria, but they cannot get there due to the tension of the papillary muscles, which stretch the tendon threads that hold the valves and prevent them from entering the atria. At the moment when the valves close and communication between the heart chambers stops, the tension phase ends.

As soon as the voltage reaches its maximum, a period of ventricular contraction begins, lasting 0.25 seconds. The systole of these chambers occurs precisely at this time. About 0.13 sec. The rapid expulsion phase lasts - the release of blood into the lumen of the aorta and pulmonary trunk, during which the valves adhere to the walls. This is possible due to an increase in pressure (up to 200 mmHg in the left and up to 60 in the right). The rest of the time falls on the slow ejection phase: blood is ejected under less pressure and at a lower speed, the atria are relaxed, and blood begins to flow into them from the veins. Ventricular systole is superimposed on atrial diastole.

General pause time

Ventricular diastole begins, and their walls begin to relax. This lasts for 0.45 seconds. The period of relaxation of these chambers is superimposed on the still ongoing atrial diastole, therefore these phases are combined and called a general pause. What happens during this time? The ventricle contracted, expelled blood from its cavity and relaxed. A rarefied space with pressure close to zero formed in it. The blood strives to get back, but the semilunar valves of the pulmonary artery and aorta, closing, prevent it from doing so. Then it is sent through the vessels. The phase that begins with relaxation of the ventricles and ends with the closure of the lumen of the vessels by the semilunar valves is called protodiastolic and lasts 0.04 seconds.

After this, an isometric relaxation phase begins, lasting 0.08 seconds. The cusps of the tricuspid and mitral valves are closed and do not allow blood to flow into the ventricles. But when the pressure in them becomes lower than in the atria, the atrioventricular valves open. During this time, blood fills the atria and now freely flows into other chambers. This is a fast filling phase lasting 0.08 seconds. Within 0.17 sec. the slow filling phase continues, during which blood continues to flow into the atria, and a small part of it flows through the atrioventricular openings into the ventricles. During the diastole of the latter, blood enters them from the atria during their systole. This is the presystolic phase of diastole, which lasts 0.1 seconds. Thus the cycle ends and begins again.

Heart sounds

The heart makes characteristic sounds similar to a knock. Each beat consists of two main tones. The first is the result of contraction of the ventricles, or, more precisely, the slamming of valves, which, when the myocardium is tense, block the atrioventricular openings so that blood cannot return to the atria. A characteristic sound is produced when their free edges close. In addition to the valves, the myocardium, the walls of the pulmonary trunk and aorta, and tendon threads take part in creating the shock.

The second sound is formed during ventricular diastole. This is the result of the semilunar valves, which prevent blood from flowing back, blocking its path. A knock is heard when they connect in the lumen of the vessels with their edges.

In addition to the main tones, there are two more - the third and fourth. The first two can be heard using a phonendoscope, while the other two can only be recorded by a special device.

Conclusion

Summarizing the phase analysis of cardiac activity, we can say that systolic work takes approximately the same amount of time (0.43 s) as diastolic work (0.47 s), that is, the heart works for half its life, rests for half, and the total cycle time is 0.9 seconds.

When calculating the overall timing of the cycle, you need to remember that its phases overlap each other, so this time is not taken into account, and as a result it turns out that the cardiac cycle lasts not 0.9 seconds, but 0.8.

Great Encyclopedia of Oil and Gas

Contraction - atrium

Contraction of the atria begins in the area of the mouths of the vena cava, as a result of which the mouths are compressed. Therefore, blood can only move in one direction into the ventricles through the atrioventricular orifices. Valves are located in these holes. At the moment of diastole and subsequent systole of the atria, the valve leaflets diverge, the valves open and allow blood to pass from the atria into the ventricles. The left ventricle contains a bicuspid mitral valve, and the right ventricle contains a tricuspid valve. When the ventricles contract, blood rushes towards the atria and slams the valve flaps. The opening of the valves towards the atria is prevented by tendon threads, with the help of which the edges of the valves are attached to the papillary muscles. The latter are finger-shaped outgrowths of the inner muscular layer of the ventricular wall. Being part of the ventricular myocardium, the papillary muscles contract along with them, pulling on the tendon threads, which, like the shrouds of a sail, hold the valve leaflets.

When the atria contract, blood is pushed into the ventricles; at the same time, the annular muscles located at the confluence of the vena cava and pulmonary veins into the atria contract, as a result of which blood cannot flow back into the veins. They are also known as atrioventricular (atrioventricular) valves.

The atrium-ventricular valves open when the atria contract, and when the ventricles contract, their valves close tightly, preventing blood from returning back to the atria. At the same time, the papillary muscles contract, stretching the chordae tendineae and preventing the valve flaps from turning toward the atria. At the base of the aorta and pulmonary artery there are semilunar valves, which look like pockets (Fig. 14.14, B) and do not allow blood from these vessels to flow back into the heart.

FKG; 1 - phase of atrial contractions; 2 - phase of asynchronous contraction of the ventricles; 3 - phase of isometric contraction of the ventricles; 4 - expulsion phase; 5 - protodiastolic period; 6 - phase of isometric ventricular relaxation; 7-phase of rapid ventricular filling; 8 - phase of slow ventricular filling.

Vibration of the walls of the heart, caused by contraction of the atria and additional flow of blood into the ventricles, leads to the appearance of the IV heart sound. During normal listening of the heart, the I and II sounds are clearly audible, they are loud, and the III and IV tones are quiet, detected only with a graphic recording of heart sounds.

A normal electrocardiogram (ECG) is shown in Fig. 1.4. P - wave corresponds to contraction of the atria caused by an electrical impulse that arises in the sinoatrial node and reaches the atria through the conduction system of the heart; P - - interval corresponds to excitation of the atrioventricular node, and Q S-complex - contraction of the ventricles; G - wave corresponds to the recovery phase of the ventricles. If excitation primarily occurs in the sinoatrial node, then this rhythm is called sinus. Pathological rhythms, the detection of which is very important for the diagnosis of the disease and its treatment, are called arrhythmias; a pathologically slow rhythm is sinus bradycardia, a pathologically accelerated rhythm is tachycardia.

Excitatory circulation is highly likely to be the cause of important cardiac arrhythmias such as flutter and fibrillation. Atrial flutter is an autonomous contraction of the atria, independent of the action of the cardiac pacemaker, caused by the circulation of an excitation wave around some inexcitable obstacle, usually around the superior or inferior vena cava.

The cardiogram identifies individual areas corresponding to different phases of the heart. Thus, the P wave occurs when the atria contract (which ensures that the relaxed ventricles are filled with blood), the QRS peak occurs when the ventricles of the heart contract, due to which blood is pushed into the aorta, the T wave is the period when the contraction of the ventricles ends and they go into a relaxed state.

The drug that stands out especially in its action is benzene - (3-gshperidinopropin - 1 -yl) benzene, which, in addition to a pronounced general inhibitory effect on the heart, causes dissociation of the rhythm of the ventricle and atrium. This dissociation is characterized by the occurrence of only one ventricular contraction per two atrium contractions. The saturated analogue does not cause such changes.

Undoubtedly, the atrial inflow phase is also active. During this phase, the atria are filled under the influence of reverse deformation of the elastic structures that have accumulated energy during the contraction of the atria. Previously, this phase of blood flow was actually not taken into account.

Phases of the cardiac cycle

The cardiac cycle is a complex and very important process. It includes periodic contractions and relaxations, which in medical language are called “systole” and “diastole”. The most important human organ (the heart), which comes second after the brain, resembles a pump in its operation.

Due to excitation, contraction, conduction, and also automaticity, it supplies blood to the arteries, from where it goes through the veins. Due to the different pressures in the vascular system, this pump works without interruption, so the blood moves without stopping.

What it is

Modern medicine explains in sufficient detail what the cardiac cycle is. It all starts with the systolic work of the atria, which takes 0.1 s. Blood flows to the ventricles while they are in the relaxation stage. As for leaflet valves, they open, and semilunar valves, on the contrary, close.

The situation changes when the atria relax. The ventricles begin to contract, this takes 0.3 s.

When this process just begins, all the valves of the heart remain in a closed position. The physiology of the heart is such that while the muscles of the ventricles contract, pressure is created, which gradually increases. This indicator also increases where the atria are located.

If we remember the laws of physics, it will become clear why blood tends to move from a cavity in which there is high pressure to a place where it is lower.

Along the way there are valves that do not allow blood to enter the atria, so it fills the cavities of the aorta and arteries. The ventricles stop contracting, and a moment of relaxation occurs at 0.4 s. In the meantime, blood flows into the ventricles without problems.

The purpose of the cardiac cycle is to maintain the functioning of a person's main organ throughout his life.

The strict sequence of phases of the cardiac cycle fits into 0.8 s. The cardiac pause takes 0.4 s. To fully restore heart function, such an interval is quite enough.

Duration of cardiac work

According to medical data, the heart rate ranges from 60 to 80 per minute if a person is in a calm state - both physically and emotionally. After human activity, heart beats become faster depending on the intensity of the load. By the level of arterial pulse you can determine how many heart contractions occur in 1 minute.

The walls of the artery vibrate, as they are affected by high blood pressure in the vessels against the background of the systolic work of the heart. As mentioned above, the duration of the cardiac cycle is no more than 0.8 s. The contraction process in the atrium lasts 0.1 s, where the ventricles last 0.3 s, the remaining time (0.4 s) is spent relaxing the heart.

The table shows the exact data of the heart beat cycle.

Where does blood come from and where does it move?

Phase duration in time

Systolic work of the atrium

Diastolic work of the atria and ventricles

Vein - atria and ventricles

Medicine describes 3 main phases that make up the cycle:

At first, the atria contract.
Ventricular systole.
Relaxation (pause) of the atria and ventricles.

An appropriate time is allocated for each phase. The first takes 0.1 s, the second 0.3 s, and the last phase takes 0.4 s.

At each stage, certain actions occur that are necessary for the proper functioning of the heart:

The first phase involves complete relaxation of the ventricles. As for the leaf valves, they open. The semilunar valves close.
The second phase begins with the atria relaxing. The semilunar valves open and the leaflet valves close.
When there is a pause, the semilunar valves, on the contrary, open, and the leaflet valves are in the open position. Some of the venous blood fills the area of the atria, and the rest collects in the ventricle.

The general pause before a new cycle of cardiac activity begins is of great importance, especially when the heart is filled with blood from the veins. At this moment, the pressure in all chambers is almost the same due to the fact that the atrioventricular valves are in an open state.

Excitation is observed in the area of the sinoatrial node, as a result of which the atria contract. When contraction occurs, the volume of the ventricles is increased by 15%. After systole ends, the pressure drops.

Heartbeat

For an adult, the heart rate does not go beyond 90 beats per minute. Children's heart rates increase. The heart of an infant produces 120 beats per minute, in children under 13 years of age this figure is 100. These are general parameters. Everyone’s values are slightly different - less or more, they are influenced by external factors.

The heart is entwined with nerve threads that control the cardiac cycle and its phases. The impulse coming from the brain to the muscle increases as a result of a serious stressful condition or after physical exertion. These can be any other changes in the normal state of a person under the influence of external factors.

The most important role in the work of the heart is played by its physiology, or rather, the changes associated with it. If, for example, the composition of the blood changes, the amount of carbon dioxide changes, or the level of oxygen decreases, this leads to a strong shock to the heart. The process of its stimulation intensifies. If changes in physiology affect the blood vessels, then the heart rate, on the contrary, decreases.

The activity of the heart muscle is determined by various factors. The same applies to the phases of cardiac activity. Among these factors is the central nervous system.

For example, elevated body temperatures contribute to an accelerated heart rate, while low ones, on the contrary, slow down the system. Hormones also affect heart rate. Together with the blood, they flow to the heart, thereby increasing the frequency of beats.

In medicine, the cardiac cycle is considered a rather complex process. It is influenced by numerous factors, some directly, others indirectly. But together, all these factors help the heart function properly.

The structure of heartbeats is no less important for the human body. She keeps him alive. An organ such as the heart is complex. It has a generator of electrical impulses, a certain physiology, and controls the frequency of strokes. That is why it works throughout the life of the body.

Only 3 main factors can influence it:

human life activity;
hereditary predisposition;
ecological state of the environment.

Numerous body processes are under the control of the heart, especially metabolic ones. In a matter of seconds, it can show violations and non-compliance with the established norm. That is why people should know what the cardiac cycle is, what phases it consists of, what their duration is, as well as physiology.

You can determine possible problems by assessing your heart function. And at the first sign of failure, contact a specialist.

Heartbeat phases

As already mentioned, the duration of the cardiac cycle is 0.8 s. The period of tension involves 2 main phases of the cardiac cycle:

When asynchronous contractions occur. The period of heart beats, which is accompanied by systolic and diastolic work of the ventricles. As for the pressure in the ventricles, it remains almost the same.
Isometric (isovolumic) contractions are the second phase, which begins some time after asynchronous contractions. At this stage, the pressure in the ventricles reaches the level at which the atrioventricular valves close. But this is not enough for the semilunar valves to open.

Pressure levels increase, thus the semilunar valves open. This causes blood to begin leaving the heart. The whole process takes 0.25 s. And it has a phase structure consisting of cycles.

Quick expulsion. At this stage, the pressure increases and reaches its maximum values.
Slow expulsion. The period when pressure parameters decrease. Once the contractions are over, the pressure will quickly subside.

After the systolic activity of the ventricles ends, a period of diastolic activity begins. Isometric relaxation. It lasts until the pressure rises to optimal parameters in the atrium.

At the same time, the atrioventricular valves open. The ventricles fill with blood. There is a transition to the rapid filling phase. Blood circulation is carried out due to the fact that different pressure parameters are observed in the atria and ventricles.

In other chambers of the heart, pressure continues to fall. After diastole, a slow filling phase begins, the duration of which is 0.2 s. During this process, the atria and ventricles are continuously filled with blood. By analyzing cardiac activity, you can determine how long the cycle lasts.

Diastolic and systolic work take almost the same time. Therefore, the human heart works for half of its life, and rests for the second half. The total duration time is 0.9 s, but due to the fact that the processes overlap each other, this time is 0.8 s.

Diseases
Body parts

A subject index to common diseases of the cardiovascular system will help you quickly find the material you need.

Select the body part you are interested in, the system will show materials related to it.

Use of site materials is possible only if there is an active link to the source.

Cardiac cycle

The heart is the main organ that performs an important function - maintaining life. The processes that occur in the organ cause the heart muscle to excite, contract and relax, thereby setting the rhythm of blood circulation. The cardiac cycle is the time period between which muscle contraction and relaxation occurs.

In this article we will take a detailed look at the phases of the cardiac cycle, find out what indicators of activity there are, and also try to understand how the human heart works.

If you have any questions while reading the article, you can ask them to the portal specialists. Consultations are provided free of charge 24 hours a day.

Work of the heart

The activity of the heart consists of a continuous alternation of contraction (systolic function) and relaxation (diastolic function). The change between systole and diastole is called the cardiac cycle.

In a person at rest, the contraction frequency averages 70 cycles per minute and has a duration of 0.8 seconds. Before contraction, the myocardium is in a relaxed state, and the chambers are filled with blood that comes from the veins. At the same time, all valves are open and the pressure in the ventricles and atria is equal. Myocardial excitation begins in the atrium. The pressure rises and due to the difference, blood is pushed out.

Thus, the heart performs a pumping function, where the atria are a container for receiving blood, and the ventricles “indicate” the direction.

It should be noted that the cycle of cardiac activity is provided by the impulse for muscle work. Therefore, the organ has a unique physiology and independently accumulates electrical stimulation. Now you know how the heart works.

Many of our readers actively use a well-known method based on natural ingredients, discovered by Elena Malysheva, to treat HEART DISEASES. We recommend that you check it out.

Cycle of cardiac work

Processes occurring during the cardiac cycle include electrical, mechanical and biochemical. The cardiac cycle can be influenced by both external factors (sports, stress, emotions, etc.) and the physiological characteristics of the body, which are subject to change.

The cardiac cycle consists of three phases:

Atrial systole has a duration of 0.1 second. During this period, the pressure in the atria increases, in contrast to the state of the ventricles, which are relaxed at this moment. Due to the difference in pressure, blood is pushed out of the ventricles.
The second phase consists of atrial relaxation and lasts 0.7 seconds. The ventricles are excited, and this lasts 0.3 seconds. And at this moment the pressure increases, and blood flows into the aorta and artery. Then the ventricle relaxes again for 0.5 seconds.
Phase number three is a time period of 0.4 seconds when the atria and ventricles are at rest. This time is called a general pause.

The figure clearly shows the three phases of the cardiac cycle:

At the moment, there is an opinion in the world of medicine that the systolic state of the ventricles contributes not only to the ejection of blood. At the moment of excitation, the ventricles undergo a slight displacement towards the upper region of the heart. This leads to the fact that blood is sucked from the main veins into the atria. At this moment the atria are in a diastolic state, and due to the incoming blood they are stretched. This effect is clearly pronounced in the right stomach.

The portal contains a table “Indicators of cardiac activity”. Viewing and downloading - free

Heartbeat

The frequency of contractions in an adult is in the range of beats per minute. The heart rate of children is slightly higher. For example, in infants the heart beats almost three times faster - 120 times per minute, and babies have a heartbeat of 100 beats per minute. Of course, these are approximate figures, because... Due to various external factors, the rhythm can last longer or shorter.

The main organ is enveloped in nerve threads that regulate all three phases of the cycle. Strong emotional experiences, physical activity and much more increase impulses in the muscles that come from the brain. Undoubtedly, physiology, or rather, its changes, plays an important role in the activity of the heart. For example, an increase in carbon dioxide in the blood and a decrease in oxygen gives a powerful boost to the heart and improves its stimulation. If changes in physiology affect the blood vessels, this leads to the opposite effect and the heart rate decreases.

As mentioned above, the work of the heart muscle, and therefore the three phases of the cycle, is influenced by many factors in which the central nervous system is not involved.

For example, high body temperature speeds up the rhythm, and low body temperature slows it down. Hormones, for example, also have a direct effect, because They enter the organ along with the blood and increase the rhythm of contractions.

The cardiac cycle is one of the most complex processes occurring in the human body, because... there are many factors involved. Some of them have a direct impact, others affect indirectly. But the totality of all processes allows the heart to carry out its work.

Having carefully studied Elena Malysheva’s methods in the treatment of tachycardia, arrhythmia, heart failure, stenacordia and general improvement of the body, we decided to offer it to your attention.

The structure of the cardiac cycle is the most important process that supports the functioning of the body. A complex organ with its own generator of electrical impulses, physiology and control of the frequency of contractions – it works all its life. The occurrence of diseases of the organ and its fatigue are influenced by three main factors - lifestyle, genetic characteristics and environmental conditions.

The main organ (after the brain) is the main link in blood circulation, therefore, it affects all metabolic processes in the body. The heart displays any failure or deviation from the normal state in a split second. Therefore, it is so important for every person to know the basic principles of work (three phases of activity) and physiology. This makes it possible to identify violations in the work of this body.

Do you often experience discomfort in the heart area (stabbing or squeezing pain, burning sensation)?
You may suddenly feel weak and tired.
The pressure is constantly fluctuating.
There is nothing to say about shortness of breath after the slightest physical exertion...
And you have been taking a bunch of medications for a long time, going on a diet and watching your weight.

Better read what Elena Malysheva says about this. For several years I suffered from arrhythmia, ischemic heart disease, angina pectoris - squeezing, stabbing pain in the heart, irregular heart rhythms, pressure surges, swelling, shortness of breath even with the slightest physical exertion. Endless tests, visits to doctors, and pills did not solve my problems. BUT thanks to a simple recipe, heart pain, blood pressure problems, shortness of breath - all this is a thing of the past. I feel great. Now my attending physician is surprised how this is so. Here is a link to the article.

Cardiac cycle. Atrial systole and diastole

Cardiac cycle and its analysis

The cardiac cycle is the systole and diastole of the heart, periodically repeating in a strict sequence, i.e. a period of time involving one contraction and one relaxation of the atria and ventricles.

In the cyclical functioning of the heart, two phases are distinguished: systole (contraction) and diastole (relaxation). During systole, the cavities of the heart are empty of blood, and during diastole they are filled with blood. The period that includes one systole and one diastole of the atria and ventricles and the following general pause is called the cardiac cycle.

Atrial systole in animals lasts 0.1-0.16 s, and ventricular systole lasts 0.5-0.56 s. The total pause of the heart (simultaneous diastole of the atria and ventricles) lasts 0.4 s. During this period the heart rests. The entire cardiac cycle lasts for 0.8-0.86 s.

The work of the atria is less complex than the work of the ventricles. Atrial systole ensures the flow of blood into the ventricles and lasts 0.1 s. Then the atria enter the diastole phase, which lasts for 0.7 s. During diastole, the atria fill with blood.

The duration of the various phases of the cardiac cycle depends on the heart rate. With more frequent heart contractions, the duration of each phase, especially diastole, decreases.

Phases of the cardiac cycle

The cardiac cycle is understood as a period covering one contraction - systole and one relaxation - diastole of the atria and ventricles - a general pause. The total duration of the cardiac cycle at a heart rate of 75 beats/min is 0.8 s.

Heart contraction begins with atrial systole, lasting 0.1 s. The pressure in the atria rises to 5-8 mm Hg. Art. Atrial systole is replaced by ventricular systole lasting 0.33 s. Ventricular systole is divided into several periods and phases (Fig. 1).

Rice. 1. Phases of the cardiac cycle

The voltage period lasts 0.08 s and consists of two phases:

phase of asynchronous contraction of the ventricular myocardium - lasts 0.05 s. During this phase, the excitation process and the subsequent contraction process spread throughout the ventricular myocardium. The pressure in the ventricles is still close to zero. By the end of the phase, the contraction covers all myocardial fibers, and the pressure in the ventricles begins to increase rapidly.
isometric contraction phase (0.03 s) - begins with the slamming of the atrioventricular valves. In this case, I, or systolic, heart sound occurs. The displacement of the valves and blood towards the atria causes an increase in pressure in the atria. The pressure in the ventricles increases rapidly: domm Hg. Art. in the left and domm rt. Art. in the right.

The leaflet and semilunar valves are still closed, the volume of blood in the ventricles remains constant. Due to the fact that the fluid is practically incompressible, the length of the myocardial fibers does not change, only their tension increases. Blood pressure in the ventricles increases rapidly. The left ventricle quickly becomes round and hits the inner surface of the chest wall with force. In the fifth intercostal space, 1 cm to the left of the midclavicular line, the apical impulse is detected at this moment.

Towards the end of the period of tension, the rapidly increasing pressure in the left and right ventricles becomes higher than the pressure in the aorta and pulmonary artery. Blood from the ventricles rushes into these vessels.

The period of blood expulsion from the ventricles lasts 0.25 s and consists of a fast phase (0.12 s) and a slow ejection phase (0.13 s). At the same time, the pressure in the ventricles increases: in the left domm. Art., and in the right up to 25 mm Hg. Art. At the end of the slow ejection phase, the ventricular myocardium begins to relax and diastole begins (0.47 s). The pressure in the ventricles drops, blood from the aorta and pulmonary artery rushes back into the ventricular cavities and “slams” the semilunar valves, and a second, or diastolic, heart sound occurs.

The time from the beginning of ventricular relaxation to the “closing” of the semilunar valves is called the protodiastolic period (0.04 s). After the semilunar valves close, the pressure in the ventricles drops. The leaflet valves are still closed at this time, the volume of blood remaining in the ventricles, and therefore the length of the myocardial fibers, does not change, therefore this period is called the period of isometric relaxation (0.08 s). Towards the end, the pressure in the ventricles becomes lower than in the atria, the atrioventricular valves open and blood from the atria enters the ventricles. The period of filling the ventricles with blood begins, which lasts 0.25 s and is divided into phases of fast (0.08 s) and slow (0.17 s) filling.

Vibration of the walls of the ventricles due to the rapid flow of blood to them causes the appearance of the third heart sound. Towards the end of the slow filling phase, atrial systole occurs. The atria pump additional blood into the ventricles (presystolic period equal to 0.1 s), after which a new cycle of ventricular activity begins.

Vibration of the walls of the heart, caused by contraction of the atria and additional flow of blood into the ventricles, leads to the appearance of the IV heart sound.

During normal listening of the heart, loud I and II tones are clearly audible, and quiet III and IV tones are detected only with graphical recording of heart sounds.

In humans, the number of heartbeats per minute can fluctuate significantly and depends on various external influences. When performing physical work or sports activity, the heart can contract up to 200 times per minute. In this case, the duration of one cardiac cycle will be 0.3 s. An increase in the number of heart contractions is called tachycardia, and the cardiac cycle decreases. During sleep, the number of heart contractions decreases to beats per minute. In this case, the duration of one cycle is 1.5 s. A decrease in the number of heart contractions is called bradycardia, while the cardiac cycle increases.

Structure of the cardiac cycle

Cardiac cycles follow at a frequency set by the pacemaker. The duration of a single cardiac cycle depends on the frequency of heart contractions and, for example, at a frequency of 75 beats/min it is 0.8 s. The general structure of the cardiac cycle can be represented in the form of a diagram (Fig. 2).

As can be seen from Fig. 1, with a cardiac cycle duration of 0.8 s (beat frequency 75 beats/min), the atria are in a systole state of 0.1 s and in a diastole state of 0.7 s.

Systole is a phase of the cardiac cycle that includes myocardial contraction and expulsion of blood from the heart into the vascular system.

Diastole is a phase of the cardiac cycle that includes relaxation of the myocardium and filling of the cavities of the heart with blood.

Rice. 2. Scheme of the general structure of the cardiac cycle. Dark squares show the systole of the atria and ventricles, light squares show their diastole.

The ventricles are in systole for about 0.3 s and in diastole for about 0.5 s. At the same time, the atria and ventricles are in diastole for about 0.4 s (total diastole of the heart). Ventricular systole and diastole are divided into periods and phases of the cardiac cycle (Table 1).

Table 1. Periods and phases of the cardiac cycle

Ventricular systole 0.33 s

Voltage period - 0.08 s

Asynchronous contraction phase - 0.05 s

Isometric contraction phase - 0.03 s

Ejection period 0.25 s

Rapid ejection phase - 0.12 s

Slow ejection phase - 0.13 s

Ventricular diastole 0.47 s

Relaxation period - 0.12 s

Protodiastolic interval - 0.04 s

Isometric relaxation phase - 0.08 s

Filling period - 0.25 s

Fast filling phase - 0.08 s

Slow filling phase - 0.17 s

The phase of asynchronous contraction is the initial stage of systole, during which a wave of excitation propagates throughout the ventricular myocardium, but there is no simultaneous contraction of cardiomyocytes and the pressure in the ventricles is from 6-8 domm Hg. Art.

The phase of isometric contraction is the stage of systole, during which the atrioventricular valves close and the pressure in the ventricles rapidly increases to a maximum of Hg. Art. in the right and domm rt. Art. in the left.

The rapid ejection phase is the stage of systole, during which there is an increase in pressure in the ventricles to maximum values of -mmHg. Art. in the right imm hg. Art. in the left and blood (about 70% of systolic output) enters the vascular system.

The slow ejection phase is the stage of systole in which blood (the remaining 30% of systolic ejection) continues to enter the vascular system at a slower rate. The pressure gradually decreases in the left ventricle sodomm Hg. Art., in the right - sdomm rt. Art.

The protodiastolic period is the transition period from systole to diastole, during which the ventricles begin to relax. The pressure decreases in the left ventricle to about Hg. Art., in temperament - up to 5-10 mm Hg. Art. Due to greater pressure in the aorta and pulmonary artery, the semilunar valves close.

The period of isometric relaxation is the stage of diastole, during which the ventricular cavities are isolated by closed atrioventricular and semilunar valves, they relax isometrically, the pressure approaches 0 mmHg. Art.

The rapid filling phase is the stage of diastole, during which the atrioventricular valves open and blood rushes into the ventricles at high speed.

The slow filling phase is the stage of diastole, during which blood slowly flows through the vena cava into the atria and through the open atrioventricular valves into the ventricles. At the end of this phase, the ventricles are 75% filled with blood.

The presystolic period is the stage of diastole, coinciding with atrial systole.

Atrial systole is a contraction of the atrial muscles, during which the pressure in the right atrium increases to 3-8 mm Hg. Art., in the left - up to 8-15 mm Hg. Art. and each ventricle receives about 25% of the diastolic blood volume (ppm).

Table 2. Characteristics of the phases of the cardiac cycle

Contraction of the myocardium of the atria and ventricles begins following their excitation, and since the pacemaker is located in the right atrium, its action potential initially spreads to the myocardium of the right and then the left atrium. Consequently, the myocardium of the right atrium responds with excitation and contraction somewhat earlier than the myocardium of the left atrium. Under normal conditions, the cardiac cycle begins with atrial systole, which lasts 0.1 s. The non-simultaneous coverage of the myocardial excitation of the right and left atria is reflected by the formation of the P wave on the ECG (Fig. 3).

Even before atrial systole, the AV valves are open and the cavities of the atria and ventricles are already largely filled with blood. The degree of stretching of the thin walls of the atrial myocardium by blood is important for the irritation of mechanoreceptors and the production of atrial natriuretic peptide.

Rice. 3. Changes in cardiac performance in different periods and phases of the cardiac cycle

During atrial systole, pressure in the left atrium can reach mm Hg. Art., and in the right - up to 4-8 mm Hg. Art., the atria additionally fill the ventricles with a volume of blood that at rest is about 5-15% of the volume located in the ventricles by this time. The volume of blood entering the ventricles during atrial systole can increase during physical activity and amount to 25-40%. The volume of additional filling can increase to 40% or more in people over 50 years of age.

The flow of blood under pressure from the atria promotes stretching of the ventricular myocardium and creates conditions for their more efficient subsequent contraction. Therefore, the atria play the role of a kind of amplifier of the contractile capabilities of the ventricles. When this atrial function is disrupted (for example, with atrial fibrillation), the efficiency of the ventricles decreases, a decrease in their functional reserves develops, and the transition to insufficiency of myocardial contractile function is accelerated.

At the moment of atrial systole, an a-wave is recorded on the venous pulse curve; in some people, when recording a phonocardiogram, a 4th heart sound may be recorded.

The volume of blood located after atrial systole in the cavity of the ventricles (at the end of their diastole) is called end-diastolic. It consists of the volume of blood remaining in the ventricle after the previous systole (end-systolic volume), the volume of blood that filled the ventricular cavity during its diastole to atrial systole, and the additional volume of blood entering the ventricle during atrial systole. The amount of end-diastolic blood volume depends on the size of the heart, the volume of blood flowing from the veins and a number of other factors. In a healthy young person at rest, it can be about ml (depending on age, gender and body weight, it can range from 90 to 150 ml). This volume of blood slightly increases the pressure in the ventricular cavity, which during atrial systole becomes equal to the pressure in them and can fluctuate in the left ventricle within mmHg. Art., and in the right - 4-8 mm Hg. Art.

Over a period of time of 0.12-0.2 s, corresponding to the PQ interval on the ECG, the action potential from the SA node spreads to the apical region of the ventricles, in the myocardium of which the excitation process begins, quickly spreading in the directions from the apex to the base of the heart and from the endocardial surface to epicardial. Following the excitation, myocardial contraction or ventricular systole begins, the duration of which also depends on the heart rate. Under resting conditions it is about 0.3 s. Ventricular systole consists of periods of tension (0.08 s) and expulsion (0.25 s) of blood.

Systole and diastole of both ventricles occur almost simultaneously, but occur under different hemodynamic conditions. A further, more detailed description of the events occurring during systole will be considered using the example of the left ventricle. For comparison, some data for the right ventricle are provided.

The period of ventricular tension is divided into phases of asynchronous (0.05 s) and isometric (0.03 s) contraction. The short-term phase of asynchronous contraction at the beginning of systole of the ventricular myocardium is a consequence of the non-simultaneous coverage of excitation and contraction of various parts of the myocardium. Excitation (corresponds to the Q wave on the ECG) and contraction of the myocardium occurs initially in the area of the papillary muscles, the apical part of the interventricular septum and the apex of the ventricles and spreads to the remaining myocardium in about 0.03 s. This coincides in time with the registration on the ECG of the Q wave and the ascending part of the R wave to its apex (see Fig. 3).

The apex of the heart contracts before its base, so the apical part of the ventricles is pulled towards the base and pushes the blood in the same direction. At this time, areas of the ventricular myocardium that are not affected by excitation can stretch slightly, so the volume of the heart practically does not change, the blood pressure in the ventricles does not yet change significantly and remains lower than the blood pressure in large vessels above the tricuspid valves. Blood pressure in the aorta and other arterial vessels continues to fall, approaching the minimum diastolic pressure value. However, the tricuspid vascular valves remain closed.

At this time, the atria relax and the blood pressure in them decreases: for the left atrium, on average, from 10 mm Hg. Art. (presystolic) up to 4 mm Hg. Art. By the end of the phase of asynchronous contraction of the left ventricle, the blood pressure in it rises to 9-10 mm Hg. Art. Blood, under pressure from the contracting apical part of the myocardium, picks up the leaflets of the AV valves, they close, taking a position close to horizontal. In this position, the valves are held by tendon threads of the papillary muscles. The shortening of the size of the heart from its apex to the base, which, due to the unchanged size of the tendon filaments, could lead to eversion of the valve leaflets into the atria, is compensated by contraction of the papillary muscles of the heart.

At the moment of closure of the atrioventricular valves, the 1st systolic heart sound is heard, the asynchronous phase ends and the isometric contraction phase begins, which is also called the isovolumetric (isovolumic) contraction phase. The duration of this phase is about 0.03 s, its implementation coincides with the time interval during which the descending part of the R wave and the beginning of the S wave are recorded on the ECG (see Fig. 3).

From the moment the AV valves close, under normal conditions the cavity of both ventricles becomes sealed. Blood, like any other fluid, is incompressible, so contraction of myocardial fibers occurs at their constant length or in an isometric mode. The volume of the ventricular cavities remains constant and myocardial contraction occurs in an isovolumic mode. The increase in tension and force of myocardial contraction under such conditions is converted into rapidly increasing blood pressure in the cavities of the ventricles. Under the influence of blood pressure on the area of the AV septum, a short-term shift occurs towards the atria, is transmitted to the inflowing venous blood and is reflected by the appearance of a c-wave on the venous pulse curve. Within a short period of time - about 0.04 s, the blood pressure in the cavity of the left ventricle reaches a value comparable to its value at this moment in the aorta, which decreased to a minimum level of - mm Hg. Art. The blood pressure in the right ventricle reaches mmHg. Art.

The excess of blood pressure in the left ventricle over the diastolic blood pressure in the aorta is accompanied by the opening of the aortic valves and the change from the period of myocardial tension to the period of blood expulsion. The reason for the opening of semilunar valves of blood vessels is the blood pressure gradient and the pocket-like feature of their structure. The valve leaflets are pressed against the walls of the vessels by the flow of blood expelled into them by the ventricles.

The period of blood expulsion lasts about 0.25 s and is divided into phases of fast expulsion (0.12 s) and slow expulsion of blood (0.13 s). During this period, the AV valves remain closed, the semilunar valves remain open. The rapid expulsion of blood at the beginning of the period is due to a number of reasons. About 0.1 s has passed since the onset of cardiomyocyte excitation and the action potential is in the plateau phase. Calcium continues to flow into the cell through open slow calcium channels. Thus, the tension of the myocardial fibers, which was already high at the beginning of expulsion, continues to increase. The myocardium continues to compress the decreasing blood volume with greater force, which is accompanied by a further increase in its pressure in the ventricular cavity. The blood pressure gradient between the ventricular cavity and the aorta increases and blood begins to be expelled into the aorta at high speed. During the rapid ejection phase, more than half of the stroke volume of blood expelled from the ventricle during the entire ejection period (about 70 ml) is ejected into the aorta. By the end of the phase of rapid expulsion of blood, the pressure in the left ventricle and aorta reaches its maximum - about 120 mm Hg. Art. in young people at rest, and in the pulmonary trunk and right ventricle - about 30 mm Hg. Art. This pressure is called systolic. The phase of rapid expulsion of blood occurs during the period of time when the end of the S wave and the isoelectric part of the ST interval before the beginning of the T wave are recorded on the ECG (see Fig. 3).

Under the condition of rapid expulsion of even 50% of the stroke volume, the rate of blood flow into the aorta in a short time will be about 300 ml/s (35 ml/0.12 s). The average rate of blood outflow from the arterial part of the vascular system is about 90 ml/s (70 ml/0.8 s). Thus, more than 35 ml of blood enters the aorta in 0.12 s, and during the same time about 11 ml of blood flows out of it into the arteries. Obviously, in order to accommodate for a short time a larger volume of inflowing blood compared to outflow, it is necessary to increase the capacity of the vessels receiving this “excess” volume of blood. Part of the kinetic energy of the contracting myocardium will be spent not only on the expulsion of blood, but also on stretching the elastic fibers of the wall of the aorta and large arteries to increase their capacity.

At the beginning of the phase of rapid expulsion of blood, stretching of the vessel walls is relatively easy, but as more blood is expelled and the vessels are stretched more and more, the resistance to stretching increases. The stretching limit of the elastic fibers is exhausted and the hard collagen fibers of the vessel walls begin to undergo stretching. The flow of blood is prevented by the resistance of peripheral vessels and the blood itself. The myocardium needs to spend a large amount of energy to overcome these resistances. The potential energy of muscle tissue and elastic structures of the myocardium itself, accumulated during the phase of isometric tension, is exhausted and the force of its contraction decreases.

The rate of blood expulsion begins to decrease and the rapid expulsion phase is replaced by a slow expulsion phase, which is also called the reduced expulsion phase. Its duration is about 0.13 s. The rate of decrease in ventricular volume decreases. At the beginning of this phase, blood pressure in the ventricle and aorta decreases at almost the same rate. By this time, the slow calcium channels close, and the plateau phase of the action potential ends. Calcium entry into cardiomyocytes decreases and the myocyte membrane enters phase 3 - final repolarization. Systole, the period of blood expulsion, ends and ventricular diastole begins (corresponding in time to phase 4 of the action potential). The implementation of reduced expulsion occurs during the period of time when the T wave is recorded on the ECG, and the end of systole and the beginning of diastole occur at the end of the T wave.

During the systole of the ventricles of the heart, more than half of the end-diastolic volume of blood (about 70 ml) is expelled from them. This volume is called stroke volume of blood. Stroke volume of blood can increase with increasing myocardial contractility and, conversely, decrease with insufficient contractility (see below for indicators of the pumping function of the heart and myocardial contractility).

The blood pressure in the ventricles at the beginning of diastole becomes lower than the blood pressure in the arterial vessels leaving the heart. The blood in these vessels experiences the forces of stretched elastic fibers of the vessel walls. The lumen of the vessels is restored and a certain amount of blood is displaced from them. Part of the blood flows to the periphery. The other part of the blood is displaced in the direction of the ventricles of the heart, and during its reverse movement fills the pockets of the tricuspid vascular valves, the edges of which are closed and held in this state by the resulting difference in blood pressure.

The time interval (about 0.04 s) from the beginning of diastole to the closure of the vascular valves is called the protodiastolic interval. At the end of this interval, the 2nd diastolic race of the heart is recorded and audible. When recording an ECG and a phonocardiogram simultaneously, the onset of the 2nd sound is recorded at the end of the T wave on the ECG.

Diastole of the ventricular myocardium (about 0.47 s) is also divided into periods of relaxation and filling, which, in turn, are divided into phases. From the moment the semilunar vascular valves close, the ventricular cavities become 0.08 closed, since the AV valves still remain closed at this time. Relaxation of the myocardium, caused mainly by the properties of the elastic structures of its intra- and extracellular matrix, is carried out under isometric conditions. In the cavities of the ventricles of the heart, less than 50% of the end-diastolic volume of blood remains after systole. The volume of the ventricular cavities does not change during this time, the blood pressure in the ventricles begins to decrease rapidly and tends to 0 mmHg. Art. Let us remember that by this time blood continued to return to the atria for about 0.3 s and the pressure in the atria gradually increased. At the moment when the blood pressure in the atria exceeds the pressure in the ventricles, the AV valves open, the phase of isometric relaxation ends and the period of filling the ventricles with blood begins.

The filling period lasts about 0.25 s and is divided into fast and slow filling phases. Immediately after the opening of the AV valves, blood quickly flows along a pressure gradient from the atria into the ventricular cavity. This is facilitated by a certain suction effect of the relaxing ventricles, associated with their straightening under the action of elastic forces that arise during compression of the myocardium and its connective tissue framework. At the beginning of the rapid filling phase, sound vibrations in the form of the 3rd diastolic heart sound can be recorded on the phonocardiogram, which are caused by the opening of the AV valves and the rapid passage of blood into the ventricles.

As the ventricles fill, the difference in blood pressure between the atria and ventricles decreases, and after about 0.08 s, the rapid filling phase is replaced by a slow filling phase of the ventricles with blood, which lasts about 0.17 s. The filling of the ventricles with blood in this phase is carried out mainly due to the preservation in the blood moving through the vessels of the residual kinetic energy imparted to it by the previous contraction of the heart.

0.1 s before the end of the phase of slow filling of the ventricles with blood, the cardiac cycle ends, a new action potential arises in the pacemaker, the next atrial systole occurs and the ventricles are filled with end-diastolic volumes of blood. This period of time of 0.1 s, which completes the cardiac cycle, is sometimes also called the period of additional filling of the ventricles during atrial systole.

An integral indicator characterizing the mechanical pumping function of the heart is the volume of blood pumped by the heart per minute, or minute blood volume (MBV):

where heart rate is the heart rate per minute; SV - stroke volume of the heart. Normally, at rest, the IOC for a young man is about 5 liters. Regulation of the IOC is carried out by various mechanisms through changes in heart rate and (or) stroke volume.

The influence on heart rate can be exerted through changes in the properties of cardiac pacemaker cells. The influence on stroke volume is achieved through the effect on the contractility of myocardial cardiomyocytes and the synchronization of its contraction.

Cardiac cycle, its phases. Systole and diastole of the atria of the ventricles

The heart works in a periodic mode - the contraction phase (systole) is replaced by a relaxation phase (diastole). The sum of the systolic and diastolic time intervals forms the contraction period T = t s + t d. The reciprocal of the period is called the heart rate. Under normal conditions, the average frequency is f = 75 1/min. Therefore, the period of work of the heart is:

T = 1/f = 1 min/75 = 60 s/75 = 0.8 s

Systole is 0.3 s, diastole is 0.5 s.

Cardiac systole begins with contraction of the atria. As a result of a decrease in the volume of these chambers, pressure increases and blood flows through the atrioventricular (atrioventricular) valves into the ventricular cavity. When the ventricular myocardium contracts, when the pressure becomes greater than in the atria, these valves close and the pressure in the ventricles quickly increases. When it exceeds the pressure in the arterial system, the valves of the aorta and pulmonary artery open, through which blood enters the systemic and pulmonary circulation. The time during which ventricular tension develops with the valves closed is called the isometric cardiac tension phase. In this case, the volume of the ventricular chambers does not change.

In one contraction, each ventricle releases 70-100 ml (70-100 cm 3) of blood into the arteries. This portion of Vc is called the systolic volume of the heart. Since the contraction frequency is f = 75 1/min, the minute volume of the heart (blood flow intensity, volumetric velocity) is determined as the product of systolic volume and frequency:

Q = V with f = 7075 = 5250 ml/min = 5.25 l/min

When there is a need to increase the intensity of the blood supply to the body (for example, when performing heavy physical work), minute volume can increase 3-4 times in untrained individuals and 5-7 times in athletes. As follows from the above formula, this is possible due to an increase in the heart rate f and systolic volume Vc. The first mechanism plays a decisive role - the frequency of contractions can increase 3-3.5 times, the minute volume in extreme situations reaches 200 ml. The force that the myocardium develops depends on the size and shape of the heart. With some approximation, we can assume that the ventricles have a spherical shape. Undoubtedly, such an assumption introduces an error into the results of further calculations. In the cavities of the ventricles, the total force acts on the blood: F = =PS, where S is the surface area. Since it is assumed that this surface is spherical, then S = 4пr 2, and the volume of the cavity V = 4пr 3 /3 (r is the radius of the ventricular cavity). Under normal conditions, the volume of the ventricles varies from V 1 = 95 cm 3 at the beginning of systole to 25 cm 3 at its end. The radius of the ventricle before contraction will be equal to:

r 1 == 2.83 cm

At the end of systole:

r 2 = = 1.81 cm

The corresponding surface areas are:

S 1 = 4пr 1 2 = 43.148 = 100 cm 2 ; S 2 = 4пr 2 2 = 43.143.3 = 41 cm 2

The magnitude of the force at the beginning of systole (at a pressure of 70 mm Hg = 9.3 kPa) is equal to F 1 = 93.3 N, and at the end (at a pressure of 120 mm Hg = 16 kPa) F 2 = 66 N. The change in the geometric dimensions of the chambers of the heart is such that at the beginning of contraction a large force develops.

The heart performs mechanical work, which is spent on increasing the mechanical energy of the blood flowing through the left and right hearts (see Fig. 73).

After blood passes through the right heart (right atrium and ventricle), the mechanical energy increases by E 1 = E 1 " - E 1 ", and after the left - by E 2 = E 2 " - E 2 ". The work of the heart is spent on the total change in energy A =E 1 +E 2. Calculations show that the work of the right heart A P is approximately 6 times less than that of the left Al, and therefore all work: A = A P + A L = A L + A L = 7A L /6 = 7(E 2) /6.

A change in mechanical energy can be represented as an increase in potential and kinetic energy: E 2 =E P2 +E K2. The increase in potential energy is due to the effect of mechanical forces on the blood from the walls of the chambers of the heart: E P2 = P"V - P"V, where P" and P" are the blood pressures in the aorta and pulmonary veins, respectively, V is the volume of blood that pumps the left ventricle.

If we consider one contraction, then V = V C (V C is systolic volume). Since the blood pressure in the aorta (average 100 mm Hg) is significantly higher than in the pulmonary veins (2-4 mm Hg), the value P "V C can be neglected and then the change in potential energy E P2 = P "V C. Increase in kinetic energy:

E K2 =(mW") 2 / 2 - (mW") 2 / 2 = (m/2)[(W") 2 - (W") 2 ]

Here W", W" are the blood speed in the aorta and pulmonary veins, respectively. The resulting change in the mechanical energy of the blood passing through the left heart will be equal to:

E 2 = Р"V С + (m/2)[(W") 2 - (W") 2 ]

Expressing the mass through its density and systolic volume: m = V C, all the work done by the heart during one contraction can be represented:

Let us present the corresponding values of the quantities included in the formula for the work: average blood pressure P" = 13 kPa, V = 70 ml, blood density  = 10 kg/m 3, blood speed in the aorta W" = 0.5 m/s, in veins about 0.2 m/s. Substituting all the given values, we find that for one contraction the heart performs work A of the order of 1.1 J. During the day, the work of the heart will be equal to: A st = NA, where N is the number of heart contractions during the day equal to the ratio of the duration of the day to the period of contractions N= 243600: 0.8 = 1.110 5. Therefore, A st = 1.110 5 1.1 = 1.2110 5 J. A simple calculation shows that over the average human lifespan of 75 years, the heart performs work approximately equal to 3.310 9 J Since the duration of systole is t s = 00.3 s, the power developed by the heart will be equal to: N = A/ t s = = 1.1: 0.3 = 3.7 W.

Let us note one more important circumstance. The work of the heart is spent on increasing kinetic energy (increasing speed) and potential energy of the blood (its volumetric compression). The calculation shows that the energy costs for blood movement are about 1% of the total change in all energy, and 99% is spent on increasing potential energy. This means that the main work of the heart is spent not on movement, but on volumetric compression of the blood.

When the heart works, when blood from the ventricles enters the arteries, the heart valves and vessel walls vibrate. In this case, sounds called heart sounds occur. In fact, the spectrum of these sounds, according to the above classification, belongs to noise. If there is a narrowing of the openings through which blood enters the aorta and pulmonary artery, the speed of blood passage increases, exceeds the critical one, and turbulent noise appears. A similar phenomenon is also observed if during diastole the heart valves do not close tightly and when the ventricles relax, blood flows from the arteries back to the heart. This condition is called valve insufficiency. The reverse flow of blood through loosely closed valves is turbulent, which also leads to noise. Therefore, listening to sounds above the heart (auscultation) allows one to detect pathomorphological changes in the heart.

What it is

The situation changes when the atria relax. The ventricles begin to contract, this takes 0.3 s.

If we remember the laws of physics, it will become clear why blood tends to move from a cavity in which there is high pressure to a place where it is lower.

The purpose of the cardiac cycle is to maintain the functioning of a person's main organ throughout his life.

The strict sequence of phases of the cardiac cycle fits into 0.8 s. The cardiac pause takes 0.4 s. To fully restore heart function, such an interval is quite enough.

Duration of cardiac work

The table shows the exact data of the heart beat cycle.

Phases

Medicine describes 3 main phases that make up the cycle:

At first, the atria contract.
Ventricular systole.
Relaxation (pause) of the atria and ventricles.

An appropriate time is allocated for each phase. The first takes 0.1 s, the second 0.3 s, and the last phase takes 0.4 s.

At each stage, certain actions occur that are necessary for the proper functioning of the heart:

The first phase involves complete relaxation of the ventricles. As for the leaf valves, they open. The semilunar valves close.
The second phase begins with the atria relaxing. The semilunar valves open and the leaflet valves close.
When there is a pause, the semilunar valves, on the contrary, open, and the leaflet valves are in the open position. Some of the venous blood fills the area of the atria, and the rest collects in the ventricle.

Heartbeat

The activity of the heart muscle is determined by various factors. The same applies to the phases of cardiac activity. Among these factors is the central nervous system.

For example, elevated body temperatures contribute to an accelerated heart rate, while low ones, on the contrary, slow down the system. Hormones also affect heart rate. Together with the blood, they flow to the heart, thereby increasing the frequency of beats.

Only 3 main factors can influence it:

human life activity;
hereditary predisposition;
ecological state of the environment.

Numerous body processes are under the control of the heart, especially metabolic. In a matter of seconds, it can show violations and non-compliance with the established norm. That is why people should know what the cardiac cycle is, what phases it consists of, what their duration is, as well as physiology.

You can determine possible problems by assessing your heart function. And at the first sign of failure, contact a specialist.

Heartbeat phases

As already mentioned, the duration of the cardiac cycle is 0.8 s. The period of tension involves 2 main phases of the cardiac cycle:

When asynchronous contractions occur. The period of heart beats, which is accompanied by systolic and diastolic work of the ventricles. As for the pressure in the ventricles, it remains almost the same.
Isometric (isovolumic) contractions are the second phase, which begins some time after asynchronous contractions. At this stage, the pressure in the ventricles reaches the level at which the atrioventricular valves close. But this is not enough for the semilunar valves to open.

Pressure levels increase, thus the semilunar valves open. This causes blood to begin leaving the heart. The whole process takes 0.25 s. And it has a phase structure consisting of cycles.

Quick expulsion. At this stage, the pressure increases and reaches its maximum values.
Slow expulsion. The period when pressure parameters decrease. Once the contractions are over, the pressure will quickly subside.

After the systolic activity of the ventricles ends, a period of diastolic activity begins. Isometric relaxation. It lasts until the pressure rises to optimal parameters in the atrium.

CARDIAC CYCLE

The main components of the cardiac cycle are systole (contraction) and diastole (expansion) of the atria and ventricles. To date, there is no consensus on the phases of the cycle and the meaning of the term “diastole”. Some authors call only the process of myocardial relaxation diastole. Most authors include in diastole both a period of muscle relaxation and a period of rest (pause), for the stomach

daughters this is a period of filling. Obviously, one should distinguish between systole, diastole and rest (pause) of the atria and ventricles, since diastole, like systole, is a dynamic process.

The cardiac cycle is divided into three main phases, each of which has periods.

Atrial systole - 0.1 s (additional filling of the ventricles with blood).

Ventricular systole - 0.33 s. The tension period is 0.08 s (the asynchronous contraction phase is 0.05 s and the isometric contraction phase is 0.03 s).

The period of blood expulsion is 0.25 s (fast expulsion phase - 0.12 s and slow expulsion phase - 0.13 s).

General cardiac pause - 0,37 With (the period of relaxation is the diastole of the ventricles and their rest, coinciding with the end of the rest of the atria).

The period of ventricular relaxation is 0.12 s (protodiastole - 0.04 s and the isometric relaxation phase - 0.08 s).

The period of the main filling of the ventricles with blood is 0.25 s (fast filling phase - 0.08 s and slow filling phase - 0.17 s).

The entire cycle of cardiac activity lasts 0.8 s at a contraction frequency of 75 per minute. Ventricular diastole and their pause at this heart rate are 0.47 s (0.8 s - 0.33 s = 0.47 s), the last 0.1 s coincides with atrial systole. The cycle is presented graphically in Fig. 13.2.

Let's consider each phase of the cardiac cycle.

A. Atrial systole provides additional blood supply to the ventricles; it begins after a general pause of the heart. At this point, all the muscles of the atria and ventricles are relaxed. The atrioventricular valves are open, they sag into the ventricles, the sphincters, which are the ring muscles of the atria in the area where the veins flow into the atria and perform the function of valves, are relaxed.

Since the entire working myocardium is relaxed, the pressure in the cavities of the heart is zero. Due to the pressure gradient in the cavities of the heart and arterial system, the semilunar valves are closed.

Excitation and, consequently, the wave of contraction of the atria begins in the area of the confluence of the vena cava, therefore, simultaneously with the contraction of the working myocardium of the atria, the muscles of the sphincters that perform the function of valves also contract - they close, the pressure in the atria begins to increase, and an additional portion of blood (approximately VS from the course -diastolic volume) enters the ventricles.

During atrial systole, blood from them does not return to the vena cava and pulmonary veins, since the sphincters are closed. By the end of systole, the pressure in the left atrium increases to 10-12 mm Hg, in the right - to 4-8 mm Hg. The same pressure is created in the ventricles towards the end of atrial systole. Thus, during atrial systole, the atrial sphincters are closed and the atrioventricular valves are open. Since the blood pressure in the aorta and pulmonary artery is higher during this period, the semilunar valves are naturally still closed. After the end of atrial systole, after 0.007 s (intersystolic interval), ventricular systole, atrial diastole and atrial rest begin. The latter last 0.7 s, while the atria are filled with blood (reservoir function of the atria). The significance of atrial systole also lies in the fact that the resulting pressure provides additional stretching of the ventricular myocardium and subsequent intensification of their contractions during ventricular systole.

B. Ventricular systole consists of two periods - tension and expulsion, each of which is divided into two phases. In the phase of asynchronous (non-simultaneous) contraction excitation of muscle fibers spreads throughout both ventricles. Contraction begins from the areas of the working myocardium closest to the conduction system of the heart (papillary muscles, septum, apex of the ventricles). By the end of this phase, all muscle fibers are involved in contraction, so the pressure in the ventricles begins to increase rapidly, as a result of which the atrioventricular valves close and isometric contraction phase. The papillary muscles, which contract together with the ventricles, stretch the tendon threads and prevent the valves from everting into the atria. In addition, the elasticity and extensibility of the

walking threads soften the impact of blood on the atrioventricular valves, which ensures the durability of their operation. The total surface of the atrioventricular valves is larger than the area of the atrioventricular orifice, so their leaflets are pressed tightly against each other. Thanks to this, the valves close reliably even with changes in the volume of the ventricles and blood does not return back to the atria during ventricular systole. During the isometric contraction phase, ventricular pressure increases rapidly. In the left ventricle it increases to 70-80 mm Hg, in the right - to 15-20 mm Hg. As soon as the pressure in the left ventricle is greater than the diastolic pressure in the aorta (70-80 mm Hg), and in the right ventricle - greater than the diastolic pressure in the pulmonary artery (15-20 mm Hg), the semilunar valves open and the period of exile.

Both ventricles contract simultaneously, and the wave of their contraction begins at the apex of the heart and spreads upward, pushing blood out of the ventricles into the aorta and pulmonary trunk. During the expulsion period, the length of the muscle fibers and the volume of the ventricles decrease, the atrioventricular valves are closed, since the pressure in the ventricles is high, and in the atria it is zero. During the period of rapid ejection, the pressure in the left ventricle reaches 120-140 mm Hg. (systolic pressure in the aorta and large arteries of the systemic circle), and in the right ventricle - 30-40 mm Hg. During the period of slow ejection, the pressure in the ventricles begins to fall. The state of the heart valves has not yet changed - only the atrioventricular valves are closed, the semilunar valves are open, the atrial sphincters are also open, because the entire atrial myocardium is relaxed, blood fills the atria.

During the period of expulsion of blood from the ventricles, the process of absorption of blood from large veins into the atria takes place. This is due to the fact that the plane of the atrioventricular “septum”, which is formed by the corresponding valves, shifts towards the apex of the heart, while the atria, which are in a relaxed state, stretch, which helps them fill with blood.

Following the ejection phase, ventricular diastole and their pause (rest) begin, with which the atrial pause partially coincides, therefore this period of cardiac activity is proposed to be called a general cardiac pause.

B. General cardiac pause begin with pro-diastole - This is the period from the beginning of relaxation of the ventricular muscles to the closure of the semilunar valves. The pressure in the ventricles becomes slightly lower than in the aorta and pulmonary artery, so the semilunar valves close. During the isometric relaxation phase The semilunar valves are already closed, and the atrioventricular valves are not yet open. As ventricular relaxation continues, ventricular pressure drops, causing the atrioventricular valves to open with the mass of blood accumulated in the atria during diastole. Begins ventricular filling period the expansion of which is ensured by several factors.

1. Relaxation of the ventricles and expansion of their chambers occurs mainly due to part of the energy that is spent during systole to overcome the elastic forces of the heart (potential energy). During the systole of the heart, its elastic connective tissue frame and muscle fibers, which have different directions in different layers, are compressed. The ventricle in this regard can be compared to a rubber bulb, which takes its previous shape after being pressed; the expansion of the ventricles has some suction effect.

2. The left ventricle (right - to a lesser extent) during the isometric contraction phase instantly becomes round, therefore, as a result of the gravitational forces of both ventricles and the blood in them, the large vessels on which the heart “hangs” quickly stretch. In this case, the atrioventricular “septum” moves slightly downward. When the muscles of the ventricles relax, the atrioventricular “septum” rises again, which also contributes to the expansion of the ventricular chambers and accelerates their filling with blood.

3. In the rapid filling phase, the blood accumulated in the atria immediately falls into the relaxed ventricles and promotes their expansion.

4. Relaxation of the ventricular myocardium is facilitated by blood pressure in the coronary arteries, which at this time begins to flow intensively from the aorta into the thickness of the myocardium (“hydraulic frame of the heart”).

5. Additional stretching of the ventricular muscles is carried out due to the energy of atrial systole (increased pressure in the ventricles during atrial systole).

6. Residual energy of venous blood imparted to it by the heart during systole (this factor acts in the slow filling phase).

Thus, during the general pause of the atria and ventricles, the heart rests, its chambers are filled with blood, the myocardium is intensively supplied with blood, receives oxygen and nutrients. This is very important, since during systole the coronary vessels are compressed by contracting muscles, while there is practically no blood flow in the coronary vessels.

The heart is a muscular organ in humans and animals that pumps blood through blood vessels.

Functions of the heart - why do we need a heart?

Our blood provides the entire body with oxygen and nutrients. In addition, it also has a cleansing function, helping in the removal of metabolic waste.

The function of the heart is to pump blood through blood vessels.

How much blood does the human heart pump?

The human heart pumps from 7,000 to 10,000 liters of blood in one day. This amounts to approximately 3 million liters per year. That works out to 200 million liters over a lifetime!

The amount of blood pumped within a minute depends on the current physical and emotional load - the greater the load, the more blood the body requires. So the heart can conduct from 5 to 30 liters through itself in one minute.

The circulatory system consists of about 65 thousand vessels, their total length is about 100 thousand kilometers! Yes, we didn't make a mistake.

Circulatory system

The human cardiovascular system is formed by two circles of blood circulation. With each heartbeat, blood moves in both circles at once.

Pulmonary circulation

Deoxygenated blood from the superior and inferior vena cava enters the right atrium and then into the right ventricle.
From the right ventricle, blood is pushed into the pulmonary trunk. The pulmonary arteries carry blood directly to the lungs (to the pulmonary capillaries), where it receives oxygen and releases carbon dioxide.
Having received enough oxygen, the blood returns to the left atrium of the heart through the pulmonary veins.

Systemic circulation

From the left atrium, blood moves into the left ventricle, from where it is subsequently pumped through the aorta into the systemic circulation.
After going through a difficult path, the blood again arrives through the vena cava to the right atrium of the heart.

Normally, the amount of blood pushed out of the ventricles of the heart is the same with each contraction. Thus, an equal volume of blood simultaneously enters the greater and lesser circulation.

What is the difference between veins and arteries?

Veins are designed to transport blood to the heart, and the job of arteries is to supply blood in the opposite direction.
In veins, blood pressure is lower than in arteries. Accordingly, the walls of arteries are more elastic and dense.
Arteries saturate “fresh” tissue, and veins take away “waste” blood.
In the case of vascular damage, arterial or venous bleeding can be distinguished by its intensity and the color of the blood. Arterial - strong, pulsating, beating like a “fountain”, the color of the blood is bright. Venous - bleeding of constant intensity (continuous flow), the color of the blood is dark.

The weight of a human heart is only about 300 grams (on average 250 grams for women and 330 grams for men). Despite its relatively low weight, it is undoubtedly the main muscle in the human body and the basis of its life activity. The size of the heart is indeed approximately equal to a human fist. Athletes' hearts can be one and a half times larger than those of the average person.

Anatomical structure

The heart is located in the middle of the chest at the level of 5-8 vertebrae.

Normally, the lower part of the heart is located mostly in the left side of the chest. There is a variant of congenital pathology in which all organs are mirrored. It is called transposition of internal organs. The lung, next to which the heart is located (normally the left one), is smaller in size relative to the other half.

The back surface of the heart is located near the spinal column, and the front surface is reliably protected by the sternum and ribs.

The human heart consists of four independent cavities (chambers) divided by partitions:

two upper ones - the left and right atria;
and two lower ones - the left and right ventricles.

The right side of the heart includes the right atrium and ventricle. The left half of the heart is represented, respectively, by the left ventricle and atrium.

The inferior and superior vena cava enter the right atrium, and the pulmonary veins enter the left atrium. From right ventricle the pulmonary arteries (also called the pulmonary trunk) emerge. From left ventricle the ascending aorta rises.

The heart has protection from overstretching and other organs, which is called the pericardium or pericardial sac (a kind of membrane in which the organ is enclosed). It has two layers: an outer dense, durable connective tissue called fibrous membrane of the pericardium and internal ( serous pericardium).

Thus, the heart itself consists of three layers: epicardium, myocardium, endocardium. It is the contraction of the myocardium that pumps blood through the vessels of the body.

The walls of the left ventricle are approximately three times larger than the walls of the right! This fact is explained by the fact that the function of the left ventricle is to push blood into the systemic circulation, where the resistance and pressure are much higher than in the pulmonary circulation.

The device of heart valves

Special heart valves allow you to constantly maintain blood flow in the correct (unidirectional) direction. The valves alternately open and close, either letting blood through or blocking its path. Interestingly, all four valves are located along the same plane.

Between the right atrium and the right ventricle is located tricuspid (tricuspid) valve. It contains three special leaflet plates that, during contraction of the right ventricle, can provide protection from the reverse flow (regurgitation) of blood into the atrium.

Works in a similar way mitral valve, only it is located on the left side of the heart and is bicuspid in its structure.

Aortic valve prevents the reverse flow of blood from the aorta into the left ventricle. Interestingly, when the left ventricle contracts, the aortic valve opens as a result of blood pressure on it, so it moves into the aorta. After which, during diastole (the period of relaxation of the heart), the reverse flow of blood from the artery promotes the closure of the valves.

Normally, the aortic valve has three leaflets. The most common congenital heart abnormality is bicuspid aortic valve. This pathology occurs in 2% of the human population.

Pulmonary valve at the moment of contraction of the right ventricle, it allows blood to flow into the pulmonary trunk, and during diastole it does not allow it to flow in the opposite direction. It also consists of three doors.

Cardiac vessels and coronary circulation

The human heart requires nutrition and oxygen, just like any other organ. The vessels that supply (nourish) the heart with blood are called coronary or coronary. These vessels branch from the base of the aorta.

The coronary arteries supply the heart with blood, and the coronary veins remove deoxygenated blood. Those arteries that are located on the surface of the heart are called epicardial. Subendocardial arteries are called coronary arteries hidden deep in the myocardium.

Most of the blood outflow from the myocardium occurs through three cardiac veins: large, middle and small. Forming the coronary sinus, they flow into the right atrium. The anterior and small veins of the heart deliver blood directly to the right atrium.

Coronary arteries are divided into two types - right and left. The latter consists of the anterior interventricular and circumflex arteries. The great cardiac vein branches into the posterior, middle and small veins of the heart.

Even absolutely healthy people have their own unique characteristics of coronary circulation. In reality, the vessels may look and be located differently than shown in the picture.

How does the heart develop (form)?

Pulse path

This system ensures automatism of the heart - excitation of impulses generated in cardiomyocytes without an external stimulus. In a healthy heart, the main source of impulses is the sinoatrial (sinus) node. He is the leader and blocks the impulses from all other pacemakers. But if any disease occurs that leads to sick sinus syndrome, then other parts of the heart take over its function. Thus, the atrioventricular node (automatic center of the second order) and the His bundle (AC of the third order) are able to activate when the sinus node is weak. There are cases when secondary nodes enhance their own automaticity even during normal operation of the sinus node.

Sinus node located in the upper posterior wall of the right atrium in close proximity to the mouth of the superior vena cava. This node initiates pulses with a frequency of approximately 80-100 times per minute.

Atrioventricular node (AV) located in the lower part of the right atrium in the atrioventricular septum. This septum prevents the impulse from propagating directly into the ventricles, bypassing the AV node. If the sinus node is weakened, then the atrioventricular node will take over its function and begin to transmit impulses to the heart muscle at a frequency of 40-60 contractions per minute.

Next, the atrioventricular node passes into His bundle(atrioventricular bundle divided into two legs). The right leg rushes towards the right ventricle. The left leg is divided into two more halves.

The situation with the left bundle branch has not been fully studied. It is believed that the left leg with fibers from the anterior branch rushes to the anterior and lateral wall of the left ventricle, and the posterior branch supplies fibers to the posterior wall of the left ventricle and the lower parts of the lateral wall.

In case of weakness of the sinus node and atrioventricular block, the His bundle is capable of creating impulses at a speed of 30-40 per minute.

The conducting system deepens and further branches into smaller branches, eventually moving into Purkinje fibers, which penetrate the entire myocardium and serve as a transmission mechanism for contraction of the ventricular muscles. Purkinje fibers are capable of initiating impulses at a frequency of 15-20 per minute.

Exceptionally trained athletes can have normal resting heart rates down to the lowest recorded figure of just 28 beats per minute! However, for the average person, even one leading a very active lifestyle, a heart rate below 50 beats per minute may be a sign of bradycardia. If your heart rate is this low, you should be examined by a cardiologist.

Heartbeat

A newborn's heart rate may be around 120 beats per minute. As a person gets older, the pulse stabilizes between 60 and 100 beats per minute. Well-trained athletes (we are talking about people with well-trained cardiovascular and respiratory systems) have a heart rate of 40 to 100 beats per minute.

The rhythm of the heart is controlled by the nervous system - the sympathetic strengthens contractions, and the parasympathetic weakens.

Cardiac activity, to a certain extent, depends on the content of calcium and potassium ions in the blood. Other biologically active substances also contribute to the regulation of heart rhythm. Our heart may begin to beat faster under the influence of endorphins and hormones released when listening to our favorite music or kissing.

In addition, the endocrine system can have a significant impact on the heart rhythm - both the frequency of contractions and their strength. For example, the release of the well-known adrenaline by the adrenal glands causes an increase in heart rate. The hormone with the opposite effect is acetylcholine.

Heart sounds

One of the simplest methods for diagnosing heart disease is to listen to the chest using a stethoscope (auscultation).

In a healthy heart, during standard auscultation, only two heart sounds are heard - they are called S1 and S2:

S1 is the sound heard when the atrioventricular (mitral and tricuspid) valves close during ventricular systole (contraction).
S2 - the sound heard when the semilunar (aortic and pulmonary) valves close during diastole (relaxation) of the ventricles.

Each sound consists of two components, but to the human ear they merge into one due to the very short period of time between them. If, under normal conditions of auscultation, additional tones become audible, this may indicate some kind of disease of the cardiovascular system.

Sometimes additional abnormal sounds may be heard in the heart, called a heart murmur. As a rule, the presence of murmurs indicates some kind of heart pathology. For example, noise can cause blood to flow back in the opposite direction (regurgitation) due to malfunction or damage to a valve. However, noise is not always a symptom of a disease. To clarify the reasons for the appearance of additional sounds in the heart, it is worth doing echocardiography (ultrasound of the heart).

Heart diseases

It is not surprising that the number of cardiovascular diseases is increasing in the world. The heart is a complex organ that actually rests (if it can be called rest) only in the intervals between heartbeats. Any complex and constantly working mechanism itself requires the most careful treatment and constant prevention.

Just imagine what a monstrous burden falls on the heart given our lifestyle and low-quality, abundant nutrition. Interestingly, mortality from cardiovascular diseases is also quite high in high-income countries.

The huge amounts of food consumed by the population of wealthy countries and the endless pursuit of money, as well as the associated stress, destroy our hearts. Another reason for the spread of cardiovascular diseases is physical inactivity - catastrophically low physical activity that destroys the entire body. Or, on the contrary, an illiterate passion for heavy physical exercise, often occurring against the background of which people do not even suspect and manage to die right during “health” activities.

Lifestyle and heart health

The main factors that increase the risk of developing cardiovascular diseases are:

Obesity.
High blood pressure.
Increased blood cholesterol levels.
Physical inactivity or excessive physical activity.
Abundant, low-quality food.
Depressed emotional state and stress.

Make reading this great article a turning point in your life - give up bad habits and change your lifestyle.

Act on damage, damage, scrap of inventory items

Form TORG-15 is drawn up in the case when during transportation, movement between and within a warehouse, during storage...
Homemade cereal bars with persimmons

Nutritionists say that for good health and a slim figure, you must include snacks in your diet....
Carrot salad for the winter: recipes

Delicious pickled carrots for the winter can be prepared in a variety of containers, it can be a wooden container,...
Omelet with crab sticks Scrambled eggs with crab sticks and tomatoes

When I have a few minutes left to prepare breakfast, the simplest and fastest recipes are used. This option...
Dessert recipes for the New Year

An elegant table, a decorated Christmas tree, tangerine spirit spilled throughout all the rooms, soon the most magical holiday - New...
Strong conspiracies for money and wealth

Each of us has repeatedly faced financial difficulties and difficult periods in life when Fortune mocked...

2024-01-13 01:35:31

How to get rid of a love spell

If you are new to magic, then it will be useful for you to get acquainted with the signs by which you can accurately...
2024-01-13 01:35:31

Dream Interpretation all the secrets of dreams are revealed All the secrets of dreams and dreams are revealed

SECRETS OF DREAMS Why does day follow night? What is life? What is death and what is sleep? These questions...

2024-01-13 01:35:31

Fortune telling with Lenormand cards for the future Fortune telling with Lenormand cards 4 cards

Main meaning: Whatever version of Madame Lenormand’s deck we take, we can definitely say that this is one of...
2024-01-12 01:28:20

Procedure for filling out an income tax return

An example of a correct income tax return in 2017, download for free in excel the new current...

2024-01-12 01:28:20

Presentation Contribution to the study of the Taman Peninsula P

P. S. Pallas (1741 - 1811) - naturalist and traveler-encyclopedist, who glorified his name with major contributions...
2024-01-12 01:28:20

How to choose a purchasing method?

Today, all issues related to the placement of government orders are regulated by the Law on Contract...

2024-01-12 01:28:20

PBU for profit calculations Accounting for the organization's profit PBU 18 02

Accounting Regulations Accounting for income tax calculations of organizations PBU 18/02 (as amended by Order...
2024-01-11 03:01:07

Sales Manager Full name

A trainee salesperson is usually called those salespeople who are not yet ready to work completely independently....

2024-01-11 03:01:07
Download books, textbooks, guides on neurology and diseases of the nervous system

Educational institution "Gomel State Medical University" Department of Neurology and...
2024-01-11 03:01:07
Early development according to Tyulenev: a strict system on the path to genius

One of the most controversial and controversial methods for the early development of children was developed in the 80s...
2021-11-25 03:13:05
Spanish Front sight for two - how it affects libido in women and men

Contents Dietary supplement based on an extract obtained from the fly beetle (or...
2021-11-25 03:13:05
Diet of Ekaterina Mirimanova: menu for every day in detail

Ekaterina MirimanovaSystem minus 60. RevolutionSystem minus 60 with Ekaterina Mirimanova“System minus 60....
2021-11-25 03:13:05
What can unpleasant bloating and heaviness in the stomach indicate?

Heaviness and bloating are the causes of both ordinary overeating and more serious problems with...
2021-11-25 03:13:05
What are the features of the second positive blood group?

The second blood group, Rh-negative, appeared many years ago, when a person ceased to be...
2024-01-15 01:17:34
Act on damage, damage, scrap of inventory items

Form TORG-15 is drawn up when during transportation, movement between and within the warehouse,...

2024-01-14 01:13:32

Homemade cereal bars with persimmons

Nutritionists say that for good health and a slim figure, you must include snacks in your...
2024-01-14 01:13:32

Carrot salad for the winter: recipes

Delicious pickled carrots for the winter can be prepared in a variety of containers, it can be a wooden...

Duration

Phase structure

Atrial systole (contraction)

Ventricular systole (contraction)

General pause time

Heart sounds

Conclusion

Great Encyclopedia of Oil and Gas

Contraction - atrium

Phases of the cardiac cycle

What it is

Duration of cardiac work

Heartbeat

Heartbeat phases

Cardiac cycle

Work of the heart

Cycle of cardiac work

Heartbeat

Cardiac cycle. Atrial systole and diastole

Cardiac cycle and its analysis

Phases of the cardiac cycle

Structure of the cardiac cycle

What it is

Duration of cardiac work

Phases

Heartbeat

Heartbeat phases

Functions of the heart - why do we need a heart?

How much blood does the human heart pump?

Circulatory system

What is the difference between veins and arteries?

Cardiac vessels and coronary circulation

How does the heart develop (form)?

Heartbeat

Heart sounds

Heart diseases

Lifestyle and heart health

Random articles

Advertising

Low hemoglobin

ALT and AST

Lymphocytes

Leukocytes

New

Popular