Duration of atrial diastole. Cardiac cycle: systole, diastole, contractions

CYCLE OF HEART ACTIVITY

The main components of the heart cycle are systole (contraction) and diastole (expansion) of the atria and ventricles. Until now, there is no consensus about the phases of the cycle and the meaning of the term "diastole". Some authors call diastole only the process of myocardial relaxation. Most authors include in diastole both a period of relaxation of the muscles and a period of rest (pause), for

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

The cycle of cardiac activity 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 voltage 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 expulsion of blood is 0.25 s (the phase of rapid expulsion is 0.12 s and the phase of slow expulsion is 0.13 s).

General pause of the heart - 0,37 from (the period of relaxation is the diastole of the ventricles and their rest, which coincides with the end of atrial rest).

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

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

The whole cycle of cardiac activity lasts 0.8 s at a frequency of contractions of 75 per 1 min. The diastole of the ventricles and their pause at such a heart rate is 0.47 s (0.8 s - 0.33 s \u003d 0.47 s), the last 0.1 s coincide with atrial systole. The cycle is graphically shown in Fig. 13.2.

Consider each phase of the heart cycle.

A. Atrial systoleprovides additional blood supply to the ventricles, it begins after a general pause of the heart. At this point, the entire musculature of the atria and ventricles is relaxed. The atrio-ventricular valves are open, they sag into the ventricles, the sphincters are relaxed, which are the annular muscles of the atria in the area where the veins fall into the atria and act as valves.

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 the arterial system, the semilunar valves are closed.

Excitation and, consequently, the wave of contraction of the atria begin in the area of \u200b\u200bthe confluence of the vena cava, therefore, simultaneously with the contraction of the working myocardium of the atria, the muscles of the sphincters, which function as valves, also contract - they close, the pressure in the atria begins to increase, and an additional portion of blood (approximately VS from of course -diastolic volume) enters the ventricles.

During atrial systole, blood from them does not return back to the vena cava and pulmonary veins, since the sphincters are closed. By the end of the systole, the pressure in the left atrium rises to 10-12 mm Hg, in the right - up to 4-8 mm Hg. The same pressure at the end of the atrial systole is created in the ventricles. Thus, during atrial systole, the atrial sphincters are closed, 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, in 0.007 s (intersystolic interval), ventricular systole, atrial diastole and their 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 pressure arising in this case provides additional stretching of the ventricular myocardium and the subsequent increase in their contractions during ventricular systole.

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

hoisting filaments soften the blow of blood against the atrioventricular valves, which ensures their durability. The total surface of the atrioventricular valves is larger than the area of \u200b\u200bthe atrioventricular opening, so their cusps are tightly pressed against each other. Due to this, the valves reliably close even with changes in ventricular volume and blood does not return during ventricular systole back into the atria. During the isometric contraction phase, ventricular pressure builds up rapidly. In the left ventricle, it increases to 70-80 mm Hg, in the right - up 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 - more than the diastolic pressure in the pulmonary artery (15-20 mm Hg), the semilunar valves open and begin 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 from 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 expulsion, the pressure in the left ventricle reaches 120-140 mm Hg. (systolic pressure in the aorta and large arteries of the great circle), and in the right ventricle - 30-40 mm Hg. During the period of slow expulsion, the pressure in the ventricles begins to drop. 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 sucking blood from large veins into the atria is realized. This is due to the fact that the plane of the atrioventricular "septum", which is formed by the corresponding valves, is displaced towards the apex of the heart, while the atria, which are in a relaxed state, stretch, which contributes to their filling with blood.

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

B. General pause of the heartbegin with protodiastoles -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. Isometric relaxation phasethe semilunar valves are already closed, and the atrioventricular valves are not yet open. As relaxation of the ventricles continues, the pressure in them drops, which leads to the opening of the atrioventricular valves by the mass of blood accumulated during diastole in the atria. Begins period of filling the ventricles,the expansion of which is provided by several factors.

1. The relaxation of the ventricles and the 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 systole of the heart, its connective tissue elastic frame and muscle fibers, which have a different direction in different layers, are compressed. In this respect, the ventricle can be compared to a rubber bulb, which takes its previous shape after being pressed on, the expansion of the ventricles has some suction effect.

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

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

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

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

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

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 the contracting muscles, while there is practically no blood flow in the coronary vessels.

CYCLE OF WORK OF THE HEART

Heart cycle - a concept that reflects the sequence of processes occurring in one reduction hearts and its subsequent relaxation. Each cycle includes three large stages: systole atria , systoleventricles and diastole ... Term systole means muscle contraction. Allocate electric systole - electrical activity that stimulates myocardium and calls mechanical systole - contraction of the heart muscle and reduction of the heart chambers in volume. Term diastole means muscle relaxation. During the cardiac cycle, an increase and decrease in blood pressure occurs, respectively, high pressure at the time of ventricular systole is called systolic, and low during their diastole - diastolic.

The repetition rate of the cardiac cycle is called heart rate, it is set pacemaker.

Periods and phases of the cardiac cycle

Schematic relationship of the phases of the cardiac cycle, ECG, FCG, sphygmograms... Denoted eCG teeth, PCG tone numbers and parts of the sphygmogram: a - anacrot, d - dikrota, k - catacrota. The phase numbers correspond to the table. The time scale is saved.

A summary table of periods and phases of the cardiac cycle with approximate pressures in the chambers of the heart and the position of the valves is shown at the bottom of the page.

Ventricular systole

Ventricular systole - the period of contraction of the ventricles, which allows the blood to be pushed into the arterial bed.

Several periods and phases can be distinguished in the contraction of the ventricles:

Voltage period - characterized by the onset of contraction of the muscle mass of the ventricles without changing the volume of blood inside them.

Asynchronous reduction - the onset of excitation of the ventricular myocardium, when only individual fibers are involved. The change in ventricular pressure is sufficient to close the atrioventricular valves at the end of this phase.
Isovolumetric reduction - almost the entire myocardium of the ventricles is involved, but there is no change in the volume of blood inside them, since the outflow (semilunar - aortic and pulmonary) valves are closed. Term isometric reduction not entirely accurate, since at this time there is a change in the shape (remodeling) of the ventricles, tension of the chords.

Exile period - characterized by the expulsion of blood from the ventricles.

Rapid banishment - the period from the moment the semilunar valves open until systolic pressure is reached in the cavity of the ventricles - during this period the maximum amount of blood is thrown out.
Slow banishment - the period when the pressure in the ventricular cavity begins to decrease, but still more than the diastolic pressure. At this time, the blood from the ventricles continues to move under the action of the kinetic energy imparted to it, until the pressure in the cavity of the ventricles and outflow vessels is equalized.

In a state of calm, the ventricle of the heart of an adult for each systole ejects from 60 ml of blood (stroke volume). The cardiac cycle lasts up to 1 s, respectively, the heart makes 60 beats per minute (heart rate, heart rate). It is easy to calculate that even at rest, the heart distills 4 liters of blood per minute (cardiac output, MOC). During the maximum load, the stroke volume of a trained person's heart can exceed 200 ml, the pulse can exceed 200 beats per minute, and the blood circulation can reach 40 liters per minute.

Diastole

Diastole - the period of time during which the heart relaxes to receive blood. In general, it is characterized by a decrease in pressure in the cavity of the ventricles, the closure of the semilunar valves and the opening of the atrioventricular valves with the advancement of blood into the ventricles.

Ventricular diastole

Protodiastole - the period of onset of myocardial relaxation with a drop in pressure lower than in the outflow vessels, which leads to the closure of the semilunar valves.
Isovolumetric relaxation - is similar to the phase of the isolumetric contraction, but exactly the opposite. Muscle fibers are lengthened, but without a change in the volume of the ventricular cavity. The phase ends with the opening of the atrioventricular (mitral and tricuspid) valves.

Filling period

Fast filling - the ventricles quickly regain their shape in a relaxed state, which significantly reduces the pressure in their cavity and sucks blood from the atria.
Slow filling - the ventricles have almost completely restored their shape, the blood is already flowing due to the pressure gradient in the vena cava, where it is 2-3 mm Hg higher. Art.

Atrial systole

It is the final phase of diastole. At a normal heart rate, the contribution of atrial contraction is small (about 8%), since during a relatively long diastole, the blood already has time to fill the ventricles. However, with an increase in the frequency of contractions, the duration of diastole generally decreases and the contribution of atrial systole to ventricular filling becomes very significant.

In mammals, the heart is located in chest between the lungs, behind the sternum. It is surrounded by a cone-shaped sac - the pericardial sac, or pericardium, the outer layer of which consists of inextensible white fibrous tissue, and the inner layer consists of two sheets, visceral and parietal.

Cardiac cycle

The visceral leaf is fused with the heart, and the parietal leaf is fused with fibrous tissue. Pericardial fluid is released into the gap between these sheets, which reduces friction between the walls of the heart and surrounding tissues. The generally inelastic nature of the pericardium prevents excessive stretching of the heart or overflowing with blood.

The heart consists of four chambers: two upper - thin-walled atria and two lower - thick-walled ventricles (Fig. 14.50). The right half of the heart is completely separated from the left. The function of the atria is to collect and retain blood for a short time until it passes into the ventricles. The distance from the atria to the ventricles is very small, so a large force of contraction is not required from the atria. IN right atrium deoxygenated blood comes from the systemic circle, and oxygenated blood from the lungs to the left. The muscle walls of the left ventricle are at least three times thicker than the walls of the right ventricle. This difference is due to the fact that the right ventricle supplies blood only to the pulmonary (small) circle of blood circulation, while the left one drives blood through the systemic (large) circle supplying blood to the entire body. Accordingly, the blood entering the aorta from the left ventricle is under significantly higher pressure (approximately 105 mm Hg) than the blood entering the pulmonary artery (16 mm Hg).

Figure: 14.50. Mammalian heart (cutaway)

14.34. What other advantages does the lower blood pressure in the pulmonary circle compared to the large circle?

With the contraction of the atria, blood is pushed into the ventricles, and at the same time the annular muscles located at the confluence of the vena cava and pulmonary veins into the atria contract and block the mouth of the veins, so that blood cannot flow back into the veins. The left atrium is separated from the left ventricle by a bicuspid valve, and the right atrium from the right ventricle by a tricuspid valve. Strong tendon threads are attached to the valve cusps from the side of the ventricles, which are attached at the other end to the cone-shaped papillary (papillary) muscles, which are outgrowths of the inner wall of the ventricles. When the atria contracts, the valves open, and when the ventricles contract, the valve flaps close tightly, preventing blood from returning to the atria. At the same time, the papillary muscles contract, pulling the tendon threads and preventing the valves from turning towards the atria. At the base of the pulmonary artery and aorta are connective tissue pockets - semilunar valves that allow blood to flow into these vessels and prevent it from returning to the heart.

The walls of the heart are made up of heart muscle fibers, connective tissue and the smallest blood vessels... Each muscle fiber contains one or two nuclei, myofilaments, and many large mitochondria. Muscle fibers branch out and are connected to each other, forming a complex network. This ensures that the contraction waves propagate quickly through the fibers so that each chamber contracts as one piece. The walls of the heart do not contain any neurons (Figures 14.51 and 14.52).

Figure: 14.51. The structure of the heart muscle

Figure: 14.52. Micrograph of a section of the heart muscle

Cardiac cycle. Phases of the cardiac cycle.

Details

The heart acts as a pump. Atria - containers that receive blood, which continuously flows to the heart; they contain important reflex zoneswhere volumoreceptors are located (for assessing the volume of inflowing blood), osmoreceptors (for assessing the osmotic pressure of blood), etc.; in addition, they perform an endocrine function (secretion of atrial natriuretic hormone and other atrial peptides into the blood); also characterized by a pumping function.
Ventricles perform mainly a pumping function.
Valves heart and large vessels: atrio-ventricular cusp valves (left and right) between the atria and ventricles; lunar valves of the aorta and pulmonary artery.
The valves prevent back blood flow. For the same purpose, there are muscle sphincters at the place where the hollow and pulmonary veins enter the atria.

CYCLE OF HEART ACTIVITY.

Electrical, mechanical, biochemical processes that occur during one complete contraction (systole) and relaxation (diastole) of the heart are called the heart cycle. The cycle consists of 3 main phases:
(1) atrial systole (0.1 sec),
(2) ventricular systole (0.3 sec),
(3) total pause or total heart diastole (0.4 sec).

General diastole of the heart: the atria are relaxed, the ventricles are relaxed. Pressure \u003d 0. Valves: atrioventricular open, semilunar closed. The ventricles are filled with blood, and the volume of blood in the ventricles increases by 70%.
Atrial systole: blood pressure 5-7 mm Hg.

The duration of the total cardiac pause is

Valves: atrioventricular open, semilunar closed. Additional filling of the ventricles with blood occurs, the volume of blood in the ventricles increases by 30%.
Ventricular systole consists of 2 periods: (1) a period of tension and (2) a period of expulsion.

Ventricular systole:

Directly ventricular systole

1) voltage period

asynchronous contraction phase
isometric contraction phase

2) exile period

rapid expulsion phase
slow ejection phase

Asynchronous pruning phase: excitement spreads through the ventricular myocardium. Individual muscle fibers begin to contract. The pressure in the ventricles is about 0.

Isometric contraction phase: all fibers of the ventricular myocardium contract. The pressure in the ventricles increases. The atrioventricular valves close (because the pressure in the ventricles becomes greater than in the precursor). The semilunar valves are still closed (because the pressure in the ventricles is still less than in the aorta and pulmonary artery). The volume of blood in the ventricles does not change (at this time, there is neither blood flow from the atria, nor outflow of blood into the vessels). Isometric contraction mode (the length of the muscle fibers does not change, the tension increases).

Exile period: all fibers of the ventricular myocardium continue to contract. The blood pressure in the ventricles becomes greater than the diastolic pressure in the aorta (70 mm Hg) and pulmonary artery (15 mm Hg). The semilunar valves open. Blood flows from the left ventricle to the aorta, from the right ventricle to the pulmonary artery. Isotonic contraction (muscle fibers are shortened, their tension does not change). The pressure rises to 120 mm Hg in the aorta and 30 mm Hg in the pulmonary artery.

DIASTOLIC PHASES OF THE VENTRICLES.

VENTRICULAR DIASTOL

isometric relaxation phase
fast passive filling phase
slow passive filling phase
phase of rapid active filling (due to atrial systole)

Electrical activity in different phases of the heart cic.

Left atrium: P wave \u003d\u003e atrial systole (wave a) \u003d\u003e additional filling of the ventricles (plays an essential role only with increasing physical load) \u003d\u003e atrial diastole \u003d\u003e inflow of venous blood from pulmonary veins to the left. atrium \u003d\u003e atrial pressure (wave v) \u003d\u003e wave c (P due to the closure of the mitral valve - towards the atrium).
Left ventricle: QRS \u003d\u003e stomach systole \u003d\u003e yellow pressure\u003e atrial P \u003d\u003e mitral valve closure. Aortic valve still closed \u003d\u003e isovolumetric contraction \u003d\u003e stomach P\u003e aortic P (80 mm Hg) \u003d\u003e aortic valve opening \u003d\u003e ejection of blood, V ventricular V decrease \u003d\u003e inertial blood flow through the valve \u003d\u003e ↓ P in the aorta
and the ventricle.

Ventricular diastole. P in the stomach.<Р в предсерд. =>opening of the mitral valve \u003d\u003e passive filling of the ventricles even before atrial systole.
EDV \u003d 135 ml (when the aortic valve opens)
KCO \u003d 65 ml (when the mitral valve opens)
UO \u003d KDO - KCO \u003d 70 ml
EF \u003d UO / EDV \u003d normal 40-50%

Home → Physiology → Circulatory system -\u003e Cardiac cycle

Cardiac cycle

In the vessels, the blood moves due to the pressure gradient from high to low. The ventricles are the organ that creates the indicated gradient.
The change in the states of contraction (systole) and relaxation (diastole) of the parts of the heart, which is repeated cyclically, is called the cardiac cycle. With a heart rate (HR) of 75 per minute, the duration of the entire cycle is 0.8 s.
It is convenient to view the cardiac cycle starting from the total diastole of the atria and ventricles (cardiac pause). In this case, the heart is in this state: the half-monthly valves are closed, and the atrioventricular valves are open. Blood from the veins flows freely and completely fills the cavities of the atria and ventricles. The blood pressure in them, as in the veins lying nearby, is about 0 mm Hg. Art. At the end of the total diastole, approximately 180-200 mji of blood are placed in the right and left halves of the heart of an adult.
Atrial systole. Excitation, originated in the sinus node, first enters the atrial myocardium - atrial systole occurs (0.1 s). In this case, due to the contraction of the muscle fibers located around the vein openings, their lumen is blocked. A kind of closed atrioventricular cavity is formed. With the contraction of the atrial myocardium, the pressure in them rises to 3-8 mm Hg. Art. (0.4-1.1 kPa). As a result, part of the blood from the atria through the open atrioventricular openings passes into the ventricles, bringing the volume of blood in them to 130-140 ml (end-diastolic volume of the ventricles - EDV). After this, atrial diastole begins (0.7 s).
Ventricular systole. Currently, the leading excitation system extends to the cardiomyocytes of the ventricles and ventricular systole begins, which lasts about 0.33 s. it is divided into two periods. Each of the periods, respectively, consists of phases.
The first period of stress continues until the semi-monthly valves open. To open them, the pressure in the ventricles must rise to top levelthan in the corresponding arterial trunks. Diastolic pressure in the aorta is about 70-80 mm Hg. Art. (9.3-10.6 kPa), and in the pulmonary artery - 10-15 mm Hg. Art. (1.3-2.0 kPa). The voltage period lasts about 0.08 s.
It begins with the phase of asynchronous contraction (0.05 s), as evidenced by the non-simultaneous contraction of all ventricular fibers. The first to contract are cardiomyocytes, which are located near the fibers of the conducting system.
The next phase of isometric contraction (0.03 s) is characterized by the involvement of all ventricular fibers in the contraction process. The beginning of the contraction of the ventricles leads to the fact that when the valves are still half-monthly closed, the blood rushes to the site of no pressure - towards the atria. The atrioventricular valves lying in its path are closed by the blood stream. The tendon filaments prevent them from turning into the atrium, and the papillary muscles, by contracting, make them even more stable. As a result, closed cavities of the ventricles are temporarily created. And until, due to contraction in the ventricles, the blood pressure rises above the level required to open the half-monthly valves, there is no significant fiber contraction. Only their inner tension rises. Thus, in the isometric contraction phase, all heart valves are closed.
The period of expulsion of blood begins with the opening of the valves of the aorta and pulmonary artery.

What are the phases of cardiac activity

It lasts 0.25 s and consists of phases of fast (0.12 s) and slow (0.13 s) blood expulsion. The aortic valves open when the blood pressure is about 80 mmHg. Art. (10.6 kPa), and pulmonary - 15 mm Hg. c (2.0 kPa). The relatively narrow openings of the arteries can immediately skip the entire volume of expulsion of blood (70 ml), therefore, the contraction of the myocardium leads to a further increase in blood pressure in the ventricles. In the left one, it rises to 120-130 mm Hg. Art. (16.0-17.3 kPa), and in the right-up to 20-25 mm Hg. Art. (2.6-3.3 kPa). The created high pressure gradient between the ventricle and the aorta (pulmonary artery) promotes the rapid ejection of some of the blood into the vessel.
However, due to the relatively small capacity of the vessel, in which there was still blood, overflow. Now the pressure is already growing in the vessels. The pressure gradient between the ventricles and vessels gradually decreases, and the blood flow rate slows down.
Due to the fact that the diastolic pressure in the pulmonary artery is lower, the opening of the valves for the expulsion of blood from the right ventricle begins somewhat earlier than from the left. And through a low gradient, the expulsion of blood ends later. Therefore, the diastolic rate of the right ventricle is 10-30 ms longer than the left one.
Diastole. At the end, when the pressure in the vessels rises to the level of the pressure in the cavities of the ventricles, the expulsion of blood stops. Their diastole begins, which lasts about 0.47 s. The time of the end of the systolic expulsion of blood coincides with the time of the termination of the contraction of the ventricles. Usually, 60-70 ml of blood remains in the ventricles (end-systolic volume - CSR). The cessation of expulsion leads to the fact that the blood contained in the vessels closes the half-monthly valves in reverse flow. This period is called protodiastolic (0.04 s). After this, the tension subsides, and an isometric period of relaxation begins (0.08 s), after which the ventricles, under the influence of the incoming blood, begin to expand.
At present, the atria after systole are already completely filled with blood. Atrial diastole lasts about 0.7 s. The atria are filled mainly with blood, passively follows from the veins. But it is possible to distinguish the "active" component, which manifests itself in connection with the overlap of its diastole from the systolic ventricles. With the contraction of the latter, the plane of the atrioventricular septum shifts towards the apex of the heart; as a result, a pre-wetting effect is formed.
When the voltage of the ventricular wall decreases, the atrioventricular valves open with blood flow. The blood filling the ventricles gradually straightens them.
The period of filling the ventricles with blood is divided into phases of fast (with atrial diastole) and slow (with systolic atria) filling. Before the start of a new cycle (atrial systole), the ventricles, like the atria, have time to completely fill with blood. Therefore, due to the flow of blood during atrial systole, the intragastric volume increases by about 20-30%. But this indicator significantly increases with the intensification of the heart, when the total diastole decreases and the blood does not have time to fill the ventricles.

The function of the heart is to pump blood from the veins into the arteries and ensure its constant movement. This is achieved by rhythmic contractions of the heart. The contraction of the heart (systole) alternates with its relaxation (diastole). The heart contracts automatically. An excitement (impulse) arises in its conducting system, which spreads through the myocardium and causes it to contract. Normally, these impulses occur in the sinus node at almost equal intervals of 60-80 times per minute, from where they propagate through the atrial myocardium and cause their contraction. Atrial systole lasts 0.1 sec. Further, the impulse passes through the Ashoff-Tavara node, reaches the bundle of His and along its legs spreads to the myocardium of the ventricles, causing their contraction.

When the impulse passes to the ventricles, the speed of its propagation decreases, so the atrial systole has time to end before the excitation of the ventricles ends and their contraction begins. Ventricular systole lasts about 0.3 seconds. and is replaced by their diastole, the duration of which depends on the heart rate (on average, about 0.5 seconds). The function of automatism is inherent in almost the entire conducting system, therefore, in some diseases (see), impulses to contraction can occur in the bundle of His and Purkinje fibers.

The sequence of the heart is as follows. During the systole of the ventricles, the blood pressure in their cavities rises, as a result, the atrioventricular valves slam shut and the valves of the aorta and pulmonary trunk open; the ventricles expel the blood they contain into the aorta and pulmonary trunk. With the onset of diastole of the ventricles, the blood pressure in their cavities begins to fall, and when it becomes less than in the aorta and pulmonary trunk, the semilunar valves of the aorta and pulmonary trunk close, preventing blood flow from the aorta and pulmonary trunk back to the heart ventricles. At the same time, the atrioventricular valves open, and blood from the atria, the diastole of which begins shortly before the diastole of the ventricles, and then from venous system through the relaxed atria it enters the ventricles. In about 0.15 sec. before the next systole of the ventricles, atrial systole begins, as a result of which the blood in the atria is pumped into the ventricles; then the cardiac cycle starts over.

Since the pressure in the aorta is 4-5 times higher than in the pulmonary trunk, the left ventricle during systole has to develop significantly more tension than the right one. The intracardiac pressure in the cavities of the heart is also not the same (for example, in the left ventricle during systole it reaches 110-130 mm Hg, in the right ventricle - 25-30 mm Hg). With heart disease, the pressure in its cavities may increase. In these cases, the myocardium of that part of the heart where the pressure is increased must be contracted with greater force, which leads to a compensatory increase in the muscle mass of this part of the heart (compensatory myocardial hypertrophy). During systole, about 70 ml of blood (stroke, or systolic, volume) is expelled from the ventricles of the heart. In 1 minute, each ventricle ejects from 3 to 5 liters of blood (minute volume). With intensive work, the minute volume can exceed 25 liters. Although it itself produces impulses for contraction, its activity is controlled by the nervous system. The increased influence on the heart of the vagus nerve slows down the production of impulses in the sinus node, inhibits their conduction, and reduces the number of heart contractions. Increased effect on the heart of the sympathetic nervous system increases the excitability of the myocardium, accelerates the production of impulses in the sinus node and their conduction, increases the number of heart contractions.

Heart cycle briefly

The heart beats rhythmically and cyclically. One cycle lasts 0.8-0.85 seconds, which is approximately 72-75 contractions (beats) per minute.

Main phases:

Systole - contraction of the muscle layer (myocardium) and the release of blood from the cardiac cavities. First, the ears of the heart contract, then the atria and after them the ventricles. The contraction runs through the heart in a wave from the ears to the ventricles. The contraction of the heart muscle is triggered by its excitement, and the excitation starts from the sinoatrial node in the upper part of the atria.

Diastole - relaxation of the heart muscle (myocardium). In this case, there is an increase in its own blood supply to the myocardium and metabolic processes in it. During diastole, the cavities of the heart are filled with blood: at the same timeboth atria and ventricles. It is important to note that blood fills at the same timeboth atria and ventricles, t. the valves between the atria and the ventricles (atrioventricular) are open in diastole.
Complete cardiac cycle

From the point of view of the movement of excitation along the heart muscle, the cycle should begin with excitation and contraction of the atria, because it is on them that the excitement from the main pacemaker of the heart goes - sinoatrial node.

Pacemaker

Heart rate driver - it is a special area of \u200b\u200bthe heart muscle that independently generates electrochemical impulses that excite the heart muscle and lead to its contraction.

In humans, the leading pacemaker is sinus-atrial (sino-atrial) node... This is a piece of heart tissue that contains "Pacemaker" cells , i.e. cells capable of spontaneous excitation. It is located on the fornix of the right atrium at the place where the superior vena cava flows into it. The node consists of a small number of cardiac muscle fibers innervated by the endings of neurons from the autonomic nervous system. It is important to understand that autonomic innervation does not create an independent rhythm of the heart's impulses, but only regulates (changes) the rhythm that is set by the pacemaker heart cells themselves. In the sino-atrial node, each wave of excitation of the heart is generated, which leads to a contraction of the heart muscle and serves as a stimulus for the emergence of the next wave.

Phases of the cardiac cycle

So, the wave of contraction of the heart provoked by the wave of excitation begins from the atria.

1. Systole (contraction) of the atria (with ears) - 0.1 s ... The atria contract and push the blood already in them into the ventricles. There is already blood in the ventricles, which has poured into them from the veins during diastole, passing through the atria and open atrioventricular valves. Due to their contraction, the atria add additional portions of blood to the ventricles.

2. Diastole (relaxation) of the atria - this is relaxation of the atria after contraction, it lasts 0,7 seconds. Thus, the rest time at the atria is much longer than their work time, and this is important to know. Blood from the ventricles cannot return back to the atria due to the special atrioventricular valves between the atria and the ventricles (tricuspid on the right and bicuspid, or mitral, on the left). Thus, in diastole, the walls of the atria are relaxed, but the blood in them from the ventricles does not flow. During this period, the heart has 2 empty and 2 filled chambers. Blood from the veins begins to flow into the atria. At first, blood slowly fills the relaxed atria. Then, after the contraction of the ventricles and the relaxation that has occurred in them, it opens the valves with its pressure and enters the ventricles. Atrial diastole has not yet ended.

And now, finally, a new wave of excitation is born in the sino-atrial node and under its influence the atria move to systole and push the blood accumulated in them into the ventricles.

3. Ventricular systole – 0.3 s ... The wave of excitation goes from the atria, as well as along the interventricular septum, and reaches the ventricular myocardium. The ventricles contract. Pressurized blood is expelled from the ventricles into the arteries. From the left to the aorta to run along a large circle circulation, and from the right - into the pulmonary trunk in order to run along the small circle of blood circulation. The maximum effort and maximum blood pressure is provided by the left ventricle. It has the most powerful myocardium of all the chambers of the heart.

4. Ventricular diastole - 0.5 s ... Note that again rest is longer than work (0.5s versus 0.3). The ventricles have relaxed, the semilunar valves at their border in the arteries are closed, they do not allow blood to return to the ventricles. The atrioventricular (atrioventricular) valves are open at this time. The filling of the ventricles with blood begins, which enters them from the atria, but so far without atrial contraction. All 4 chambers of the heart, i.e. ventricles and atria are relaxed.

5. Total heart diastole - 0.4 s ... The walls of the atria and ventricles are relaxed. The ventricles are filled with blood flowing into them through the atria from the vena cava, by 2/3, and the atria - completely.

6. New cycle ... The next cycle begins - atrial systole .

Video:Pumping blood to the heart

To consolidate this information, take a look at the animated diagram of the cardiac cycle:

Animated diagram of the cardiac cycle - I strongly advise you to click and view the details!

Details of the work of the ventricles of the heart

1. Systole.

2. Expulsion.

3. Diastole

Ventricular systole

1. Period of systole , i.e. contraction, consists of two phases:

1) Asynchronous pruning phase – 0.04 s ... An uneven contraction of the ventricular wall occurs. At the same time, there is a reduction in the interventricular septum. Due to this, pressure builds up in the ventricles, and as a result, the atrioventricular valve closes. As a result, the ventricles are isolated from the atria.

2) Isometric contraction phase ... This means that the length of the muscles does not change, although their tension increases. The volume of the ventricles does not change either. All valves are closed, the walls of the ventricles contract and tend to contract. As a result, the walls of the ventricles tighten, but the blood does not move. But at the same time, the blood pressure inside the ventricles increases, it opens the semilunar valves of the arteries and an outlet appears for the blood.

2. Period of expulsion of blood – 0.25 s

1) Rapid expulsion phase - 0.12 s.

2) Slow ejection phase - 0.13 s.

Expulsion (release) of blood from the heart

Pressurized blood is forced from the left ventricle into the aorta. The pressure in the aorta rises sharply, and it expands, accepting a large portion of blood. However, due to the elasticity of its wall, the aorta immediately contracts again and drives blood through the arteries. The expansion and contraction of the aorta generates a shear wave that propagates at a certain speed through the vessels. This is a wave of expansion and contraction of the vessel wall - pulse wave... Its speed does not match the speed of blood flow.

Pulse - this is a transverse wave of expansion and contraction of the artery wall, generated by the expansion and contraction of the aorta when blood is ejected into it from the left ventricle of the heart.

Ventricular diastole

Protodiastolic period - 0.04 s. From the end of the ventricular systole to the closure of the semilunar valves. During this period, part of the blood returns back to the ventricle from the arteries under the pressure of blood in the circulation.

Isometric relaxation phase - 0.25 p. All valves are closed, muscle fibers are contracted, they have not yet stretched. But their tension decreases. The pressure in the atria becomes higher than in the ventricles, and this blood pressure opens the atrioventricular valves to allow blood to pass from the atria to the ventricles.

Filling phase ... There is a general diastole of the heart, during which all its chambers are filled with blood, and at first quickly, and then slowly. Blood transits through the atria and fills the ventricles. The ventricles are filled with blood by 2/3 of the volume. At this moment, the heart is functionally 2-chambered, because only its left and right halves are divided. Anatomically, all 4 chambers are preserved.

Presystola ... The ventricles finally fill with blood as a result of atrial systole. The ventricles are still relaxed, while the atria are already contracted.