Cardiac cycle and its phase structure. Systole

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The work of the heart is a continuous alternation of periods reductions(systole) and relaxation(diastole). The alternating periods of systole and diastole make up cardiac cycle.

Since at rest the heart rate is 60-80 cycles per minute, each of them lasts about 0.8 s. In this case, 0.1 s is occupied by atrial systole, 0.3 s by ventricular systole, and the rest of the time by total diastole of the heart.

By the beginning of systole, the myocardium is relaxed, and the cardiac chambers are filled with blood coming from the veins. At this time, the atrioventricular valves are open and the pressure in the atria and ventricles is almost the same. The generation of excitation in the sinoatrial node leads to atrial systole, during which, due to the pressure difference, the end-diastolic volume of the ventricles increases by approximately 15%. With the end of atrial systole, the pressure in them decreases.

Fig.7.11. Changes in left ventricular volume and pressure fluctuations in the left atrium, left ventricle and aorta during the cardiac cycle.

Since there are no valves between the great veins and the atria, during atrial systole the circular muscles surrounding the openings of the vena cava and pulmonary veins contract, which prevents the flow of blood from the atria back into the veins. At the same time, atrial systole is accompanied by a slight increase in pressure in the vena cava. Important in atrial systole, it ensures the turbulent nature of the blood flow entering the ventricles, which contributes to the closure of the atrioventricular valves. The maximum and average pressure in the left atrium during systole are 8-15 and 5-7 mm Hg, respectively, in the right atrium - 3-8 and 2-4 mm Hg. (Fig. 7.11).

I - beginning of atrial systole;
II - the beginning of ventricular systole and the moment of closure of the atrioventricular valves;
III - moment of opening of the semilunar valves;
IV - the end of ventricular systole and the moment of closure of the semilunar valves;
V - opening of the atrioventricular valves. The descent of the pinium, indicating the volume of the ventricles, corresponds to the dynamics of their emptying.

Phases of heart contraction

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With the transition of excitation to the atrioventricular node and the conduction system of the ventricles, the systole of the latter begins. Its initial stage (voltage period) lasts 0.08 s and consists of two phases:

1. Asynchronous contraction phase. Lasts (0.05 s) and represents the process of propagation of excitation and contraction throughout the myocardium. The pressure in the ventricles remains virtually unchanged.

2. Phase of isovolumic or isometric contraction. Occurs during further contraction, when the pressure in the ventricles increases to a value sufficient to close the atrioventricular valves, but not enough to open the semilunar valves.

A further increase in pressure leads to the opening of the semilunar valves and the beginning of the period of expulsion of blood from the heart, the total duration of which is 0.25 s.

This period consists of

• rapid expulsion phases (0.13 s), during which the pressure continues to rise and reaches maximum values ​​(200 mm Hg in the left ventricle and 60 mm Hg in the right), and
• slow expulsion phases (0.13 s), during which the pressure in the ventricles begins to decrease (to 130-140 and 20-30 mmHg, respectively), and after the end of the contraction it drops sharply.

IN main arteries the pressure decreases much more slowly, which ensures the closure of the semilunar valves and prevents the reverse flow of blood. The period of time from the beginning of ventricular relaxation to the closure of the semilunar valves is called the protodiastolic period.

After the end of ventricular systole, First stage diastole - phase of isovolumic (isometric) relaxation , which appears when the valves are still closed and lasts approximately 80 ms, i.e. until the moment when the pressure in the atria is higher than the pressure in the ventricles (2-6 mm Hg), which leads to the opening of the atrioventricular valves, after which the blood passes into the ventricle within 0.2-0.13 s. This period is called rapid filling phase. The movement of blood during this period is determined solely by the difference in pressure in the atria and ventricles, while its absolute value in all cardiac chambers continues to decrease. Diastole ends slow filling phase (diastasis), which lasts about 0.2 s. During this time, there is a continuous flow of blood from the main veins into both the atria and ventricles.

The frequency of generation of excitation by the cells of the conduction system and, accordingly, myocardial contractions is determined by the duration refractory phase, occurring after each systole. As in other excitable tissues, in the myocardium refractoriness is due to inactivation of sodium ion channels resulting from depolarization (Fig. 7.8).

To restore the incoming sodium current, a repolarization level of about -40 mV is required.

Up to this point there is absolute refractory period, which lasts about 0.27 s.

Followed by relative refractory period, during which the excitability of the cell is gradually restored, but remains reduced (duration 0.03 s). During this period, the heart muscle can respond additional reduction, if stimulated with a very strong stimulus.

The period of relative refractoriness is followed by a short period of supernormal excitability. During this period, myocardial excitability is high and it is possible to obtain an additional response in the form of muscle contraction by applying a subthreshold stimulus to it.

A long refractory period has important biological significance for the heart, because it protects the myocardium from rapid or repeated excitation and contraction. This eliminates the possibility of tetanic contraction of the myocardium and prevents the possibility of disruption of the pumping function of the heart.

Heart rate is determined by the duration of action potentials and refractory phases, as well as the speed of propagation of excitation along the conduction system and the temporal characteristics of the contractile apparatus of cardiomyocytes. The myocardium is not capable of tetanic contraction and fatigue, in the physiological understanding of this term. During contraction, cardiac tissue behaves like a functional syncytium, and the strength of each contraction is determined by the “all or nothing” law, according to which, when excitation exceeds a threshold value, contracting myocardial fibers develop a maximum force that does not depend on the magnitude of the suprathreshold stimulus.

Work of the heart represents a continuous alternation of periods of contraction ( systole) and relaxation ( diastole). Replacing each other systole And diastole make up cardiac cycle. Since at rest the heart rate is 60-80 cycles per minute, each of them lasts about 0.8 s. In this case, 0.1 s is occupied by atrial systole, 0.3 s by ventricular systole, and the rest of the time by total diastole of the heart.

TO beginning of systole myocardium relaxed, and the heart chambers are filled with blood coming from the veins. At this time, the atrioventricular valves are open and the pressure in the atria and ventricles is almost the same. The generation of excitation in the sinoatrial node leads to atrial systole, during which, due to the pressure difference, the end-diastolic volume of the ventricles increases by approximately 15%. With the end of atrial systole, the pressure in them decreases.

Since the valves between the main veins and the atria are absent; during atrial systole, the circular muscles surrounding the mouths of the vena cava and pulmonary veins contract, which prevents the outflow of blood from the atria back into the veins. At the same time, atrial systole is accompanied by a slight increase in pressure in the vena cava. Of great importance is ensuring the turbulent nature of the blood flow coming from the atria into the ventricles, which contributes to the closure of the atrioventricular valves. The maximum and average pressure in the left atrium during systole are 8-15 and 5-7 mmHg, respectively. Art., in the right atrium - 3-8 and 2-4 mm Hg. Art. (Fig. 9.11).

With transition stimulation to the atrioventricular node and the conduction system of the ventricles begins the last systole. Its initial stage ( tension period) lasts 0.08 s and consists of two phases. Asynchronous contraction phase(0.05 s) is the process of propagation of excitation and contraction throughout the myocardium. The pressure in the ventricles remains virtually unchanged. In the process of the beginning synchronous contraction of the ventricular myocardium, when the pressure in them increases to a value sufficient to close the atrioventricular valves, but insufficient to open the semilunar valves, the phase of isovolumic, or isometric, contraction begins.

Further increase in pressure leads to the opening of the semilunar valves and the onset of period of exile blood from the heart, the total duration of which is 0.25 s. This period consists of rapid expulsion phases(0.13 s), during which the pressure in the ventricles continues to increase and reaches maximum values, and slow expulsion phases(0.13 s), during which the pressure in the ventricles begins to decrease, and after the end of the contraction it drops sharply. In the main arteries, the pressure decreases much more slowly, which ensures the closure of the semilunar valves and prevents the reverse flow of blood. The period of time from the beginning of ventricular relaxation to the closure of the semilunar valves is called the protodiastolic period.

Cardiac cycle briefly

The heart contracts 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 release of blood from the cardiac cavities. First, the ears of the heart contract, then the atria and then the ventricles. Contraction runs through the heart in a wave from the ears to the ventricles. Contraction of the heart muscle is triggered by its excitation, and excitation begins from the sinoatrial node in the upper part of the atria.

1. Diastole – relaxation of the heart muscle (myocardium). In this case, the myocardium’s own blood supply increases and metabolic processes in him. During diastole, the cavities of the heart are filled with blood: simultaneously both atria and ventricles. It is important to note that blood fills simultaneously both atria and ventricles, because The valves between the atria and ventricles (atrioventricular) are open in diastole.

   Complete cardiac cycle

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

Pacemaker

Driver heart rate - This is a special part of the heart muscle that independently generates electrochemical impulses that excite the heart muscle and lead to its contraction.

In humans, the leading pacemaker is sinoatrial (sinoatrial) node. This is a section of cardiac tissue containing pacemaker cells , i.e. cells capable of spontaneous excitation. It is located on the fornix of the right atrium at the junction of the superior vena cava. 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 cardiac impulses, but only regulates (changes) the rhythm that is set by the pacemaker cardiac cells themselves. Each wave of cardiac excitation originates in the sinoatrial node, which leads to contraction of the heart muscle and serves as a stimulus for the occurrence of the next wave.

Phases of the cardiac cycle

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

1. Atrial systole (contraction) (together with ears) – 0.1 s . The atria contract and push the blood already in them into the ventricles. The ventricles also already have blood, which 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 the relaxation of the atria after contraction, it lasts 0,7 seconds. Thus, the rest time of the atria is much greater than the time they work, and this is important to know. From the ventricles, blood cannot return back to the atria thanks to special atrioventricular valves between the atria and ventricles (tricuspid on the right and bicuspid, or mitral, on the left). Thus, in diastole the walls of the atria are relaxed, but blood does not flow into them from the ventricles. 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 their relaxation, it opens the valves with its pressure and enters the ventricles. Atrial diastole has not yet ended.

And finally, a new wave of excitation is born in the sinoatrial 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 excitation wave comes from the atria, as well as along the interventricular septum, and reaches the ventricular myocardium. The ventricles contract. Blood is pumped under pressure from the ventricles into the arteries. From the left - into the aorta to run along big circle blood circulation, and from the right - into the pulmonary trunk to run through the pulmonary circulation. Maximum effort and maximum blood pressure are 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 lasts longer than work (0.5 s vs. 0.3). The ventricles have relaxed, the semilunar valves at their border with the arteries are closed, they do not allow blood to return to the ventricles. The atrioventricular (atrioventricular) valves are open at this time. The ventricles begin to fill with blood, which enters them from the atria, but so far without contraction of the atria. All 4 chambers of the heart, i.e. the ventricles and atria are relaxed.

5. Total diastole of the heart - 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, 2/3, and the atria - completely.

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

Video:Pumping blood to the heart

To reinforce this information, look at the animated diagram of the cardiac cycle:

Animated diagram of the cardiac cycle - I highly recommend clicking and viewing the details!

Details of the work of the ventricles of the heart

1. Systole.

2. Expulsion.

3. Diastole

Ventricular systole

1. Systole period , i.e. contraction consists of two phases:

1) Asynchronous contraction phase – 0.04 s . There is an uneven contraction of the ventricular wall. At the same time, the interventricular septum contracts. Due to this, pressure increases 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 also does not change. All valves are closed, the walls of the ventricles contract and tend to shrink. As a result, the walls of the ventricles become tense, 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 blood expulsion – 0.25 s.

1) Rapid expulsion phase – 0.12 s.

2) Slow expulsion phase – 0.13 s.

Expulsion (ejection) of blood from the heart

Blood is forced under pressure from the left ventricle into the aorta. The pressure in the aorta increases 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 transverse wave, which propagates at a certain speed through the vessels. This is a wave of expansion and contraction of the vascular wall - a pulse wave. Its speed does not match the speed of blood.

Pulse - This transverse wave expansion and contraction of the artery wall, generated by the expansion and contraction of the aorta during the ejection of blood into it from the left ventricle of the heart.

Ventricular diastole

Protodiastolic period – 0.04 s. From the end of 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 blood pressure in the circulation.

Isometric relaxation phase – 0.25 s. All valves are closed muscle fibers contracted, they have not yet stretched. But their tension decreases. The pressure in the atria becomes higher than that 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, first quickly and then slowly. Blood transits through the atria and fills the ventricles. The ventricles are filled with blood to 2/3 of their volume. At this moment, the heart is functionally 2-chambered, because only its left and right halves are separated. Anatomically, all 4 chambers are preserved.

Presystole . The ventricles are finally filled with blood as a result of atrial systole. The ventricles are still relaxed, while the atria are already contracting.

The heart acts as a pump and ensures constant blood flow through the vascular system body.

The activity of the heart consists of single cardiac cycles. Each cycle includes systole (contraction) and diastole (relaxation).

The duration of the cardiac cycle at heart rate = 75 beats/min is 0.8 s.

The cardiac cycle begins with atrial systole (lasts 0.1 s).

Atrial systole is followed by atrial diastole (0.7 s).

Simultaneously with the onset of atrial diastole, ventricular systole occurs (0.33 s), which is replaced by ventricular diastole (0.47 s).

Thus, 0.1 s before the end of ventricular diastole, a new atrial systole begins.

During atrial systole, the blood pressure in them increases from 2-4 to 5-9 mm Hg.

At this time, the ventricles are relaxed and the pressure in them is lower than in the atria, the leaflets of the atrioventricular valves hang down and blood flows from the atria to the ventricles along a pressure gradient, i.e. additional filling of the ventricles with blood occurs.

Reverse flow of blood from the atria into the hollow and pulmonary veins prevents the contraction of ring-shaped muscles (sphincters) covering the openings of the veins.

During this time, excitation from the sinus node reaches the ventricles and ventricular systole begins.

Ventricular systole consists of two phases: the tension phase and the ejection phase.

In the tension phase (0.08 s), the excitation wave does not immediately cover the ventricular muscles, but gradually spreads throughout the myocardium.

Therefore, part of the muscle fibers (which are closer to the atria) contracts, while the other part remains relaxed.

This period of systole is called the phase of asynchronous contraction (0.05 s).

The onset of excitation in this phase is accompanied by contraction of the papillary muscles and tension of the tendon threads, which prevents the eversion of the leaflet valves into the atria.

The pressure in the ventricles remains virtually unchanged.

As the process of excitation covers the entire contractile apparatus of the heart, the pressure in the ventricle increases, becomes greater than in the atria and the atrioventricular valves slam shut with the reverse flow of blood.

At the same time, the pressure in the arteries still exceeds the pressure in the ventricles, so the semilunar valves are also closed.

Thus, a period of contraction develops with the valves closed.

Since blood, like any liquid, is practically incompressible, for a short time (0.03 s) the muscles of the ventricles tense, but their volume does not change.

This period is called the isometric contraction phase.

The pressure increases greatly and reaches 115-125 in the left ventricle, and 25-30 mm Hg in the right. The pressure in the arterial vessels at this time, on the contrary, drops (due to the continued outflow of blood to the periphery).

When the pressure in the ventricles becomes higher than in the arteries, the semilunar valves open and blood is released under high pressure into the aorta and pulmonary artery.

The expulsion phase begins, which lasts 0.25 s.

In humans, expulsion of blood (systolic ejection) can occur when the pressure in the left ventricle reaches 65-75 mm Hg, and in the right - 5-12 mm Hg.

At the very beginning, when the pressure gradient is large, blood is expelled quickly from the ventricles into the vessels.

This is the rapid expulsion phase. It lasts 0.10-0.12 s. As the amount of blood in the ventricles decreases, the pressure in them drops.

At the same time, the flow of blood into the aorta and pulmonary artery is accompanied by an increase in pressure in the outgoing vessels.

The pressure difference decreases and the rate of expulsion decreases.

The slow expulsion phase begins (0.10-0.15 s).

Following the ejection phase, ventricular diastole occurs.

The ventricles begin to relax and the pressure in them drops further.

The pressure in the outgoing vessels becomes higher than in the ventricles, the blood changes its direction and the semilunar valves slam shut due to the reverse flow of blood.

The time from the beginning of ventricular relaxation to the moment of closure of the semilunar valves is called the protodiastolic period (0.04 s).

Then (about 0.08 s) the ventricles relax with the atrioventricular and semilunar valves closed.

This period of diastole is referred to as the isometric relaxation phase.

It continues until the pressure in the ventricles drops below that in the atria.

By that time the atria are already filled with blood, because Ventricular diastole partially coincides with atrial diastole, during which blood flows freely from the vena cava into the right atrium, and from the pulmonary veins into the left atrium.

As a result of a drop in pressure in the ventricles (where the pressure drops to 0) and an increase in pressure in the atria, a pressure difference occurs, the leaflet valves open, and blood from the atria begins to fill the ventricles. This is the ventricular filling phase (0.25 s).

At first, filling occurs quickly, because... the pressure gradient is large.

This period is called the rapid filling phase (0.08 s).

As the ventricles fill, the pressure in them increases, and in the atria it decreases. The pressure gradient decreases and the filling rate slows down.

This period is called the slow filling phase (0.17 s).

At the end of diastole, 0.1 s before its end, a new atrial systole begins, i.e. a new cardiac cycle begins.

At this time, additional filling of the ventricles with blood occurs.

This final period of ventricular diastole is called the presystolic period.

Details

The heart performs the function of a pump. Atria- containers that receive blood that continuously flows to the heart; they contain important reflexogenic zones, where volume receptors are located (for assessing the volume of incoming blood), osmoreceptors (for assessing osmotic pressure blood) etc.; in addition, they perform endocrine function(secretion of atrial natriuretic hormone and other atrial peptides into the blood); pumping function is also characteristic.
Ventricles perform mainly a pumping function.
Valves heart and large vessels: atrioventricular leaflet valves (left and right) between the atria and ventricles; semilunar valves of the aorta and pulmonary artery.
The valves prevent blood from flowing back. For the same purpose, there are muscle sphincters at the place where the vena cava and pulmonary veins flow into the atria.

CARDIAC CYCLE.

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

General diastole of the heart: the atria are relaxed, the ventricles are relaxed. Pressure = 0. Valves: atrioventricular are open, semilunar are closed. The ventricles are filled with blood, the volume of blood in the ventricles increases by 70%.
Atrial systole: blood pressure 5-7 mm Hg. Valves: atrioventricular are open, semilunar valves are 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) the tension period and (2) the ejection period.

Ventricular systole:

Direct ventricular systole

1)tension period

• asynchronous contraction phase
• isometric contraction phase

2)period of exile

• rapid expulsion phase
• slow expulsion phase

Asynchronous contraction phase: excitation spreads throughout 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 forearms). The semilunar valves are still closed (since 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 inflow from the atria, nor blood outflow 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 into the aorta, and from the right ventricle into the pulmonary artery. Isotonic contraction mode (muscle fibers are shortened, their tension does not change). The pressure rises to 120 mmHg in the aorta and to 30 mmHg in the pulmonary artery.

DIASTOLIC PHASES OF THE VENTRICLES.

VENTRICULAR DIASTOLE

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

Electrical activity in different phases of the cardiac cycle.

Left atrium: P wave => atrial systole (wave a) => additional filling of the ventricles (plays a significant role only with increased physical activity) => atrial diastole => influx of venous blood from the pulmonary veins to the left. atrium => atrial pressure (wave v) => wave c (P due to the closure of the mitral valve - towards the atrium).
Left ventricle: QRS => gastric systole => gastric pressure > atrial P => mitral valve closure. Aortic valve is still closed => isovolumetric contraction => gastric P > aortic P (80 mm Hg) => opening of aortic valve => ejection of blood, decrease in V ventricle => inertial blood flow through the valve =>↓ P in the aorta
and ventricle.

Ventricular diastole. R into the stomach.<Р в предсерд. =>opening of the mitral valve => passive filling of the ventricles even before atrial systole.
EDV = 135 ml (when the aortic valve opens)
ESV = 65 ml (when the mitral valve opens)
SV = KDO – KSO = 70 ml
EF = SV/ECD = normal 40-50%