Regulation of heart function.
Regulation of the heart is a change in its activity in accordance with the needs of the body. The result of changes in heart function is the IOC.
IOC = HR HR. Regulatory mechanisms can ensure a change in the IOC through each of these quantities.
IOC under various conditions in a healthy person.
Changes in IOC are observed during orthostatics, physical inactivity, physical work, emotional stress, and in extreme situations.
Classification of mechanisms regulating the activity of the heart.
There are cellular, intraorgan and extracardiac levels of regulation.
Regulatory influences apply to all physiological properties: excitability, conductivity, contractility and automaticity.
1) A change in automaticity is reflected in a change in frequency - the chronotropic effect.
2) Change in contractility in contraction force - inotropic effect.
3) Change in excitability – bathmotropic.
4) Change in conductivity – dromotropic.
Cellular mechanisms of regulation.
We are talking about pacemaker cells. The cellular level of regulation provides chronotropic effect– change in heart rate.
Reasons causing changes in the chronotropic effect.
1) Change of pacemaker.
2) Change in the slope of slow diastolic depolarization.
3) Change in software.
4) Change in the value of the ICMP.
Mechanism. It is based on a change in the rate of diastolic depolarization.
Mechanism of cellular level regulation of contraction and relaxation(inotropic effect).
The regulated indicators are the strength and speed of contraction; degree and speed of relaxation.
The strength and speed of contraction depend on:
1) on the amount of actin and myosin;
2) the rate of formation of the acto-myosin complex;
3) the amount of Ca 2+ entering the fiber during AP generation.
The degree and speed of relaxation depend on the activity of the Ca 2+ pump in the cardiocyte.
Intraorgan mechanisms. At the intraorgan level, the strength of contractions is regulated depending → on venous return (Starling’s law).
↓ → from heart rate (Bowditch's law).
from resistance to blood flow (Anrep-Hill law).
1) In response to an increase in venous return.
This is a heterometric regulation of the force of contraction - Starling's law or the law of the heart.
An increase in myocardial length with an increase in venous return causes an increase in the force of contraction: venous return → myocardial length → contraction force → CO.
Mechanism Stretching the muscle promotes the formation of a large number of acto-myosin bridges and increases the force of contraction.
2) In response to an increase in resistance to blood flow.
In this case, venous return does not change, but the resistance to blood flow changes - for example, blood pressure in the systemic circulation increases.
This is a homeometric regulation of the force of contraction(Anrep-Hill law).
Manifestation. An increase in blood pressure is accompanied by an increase in the force of contraction.
The consequence of this law is the development of myocardial hypertrophy with a persistent increase in blood pressure.
3) In response to an increase in the frequency of SS(Bowditch's law).
An increase in the frequency of the SS to 170 beats per minute is accompanied by an increase in the force of contraction. This is due to the fact that Ca accumulates in the cardiomyocyte, which increases the force of contraction.
Extracardiac level. Subdues the cellular and organ (nervous, humoral).
Nervous influences through the sympathetic and parasympathetic nervous system.
Nervus vagus- nucleus in the medulla oblongata. The preganglionic fiber is interrupted in the intramural ganglion of the heart. The postganglionic fiber releases ACh, the receptor for it on the heart is M – ChR.
Irritation of the vagus nerve causes negative ino-chrono-dromo and bathmotropic effects, i.e. an inhibitory effect.
Mechanism– decrease in the rate of slow diastolic depolarization in the sinoatrial node, hyperpolarization.
Escape effect.
The nuclei of the vagus nerve have tone. In newborns this tone is not expressed. Fluctuations in the tone of the vagus nerve manifest themselves in the form of tachycardia and bradycardia, respiratory and cardiac arrhythmia.
The tone of the vagus nerve is maintained by afferent impulses from the aortic arch and carotid sinuses. When it is cut, the heart rate increases.
Sympathetic innervation carried out from 5 upper thoracic segments. Innervates the ventricles. Preganglionic fibers are interrupted in the stellate ganglion, postganglionic fibers secrete norepinephrine, β - adrenergic receptors.
Influence– exciting, positive foreign – chrono – dromo effects. The influence is carried out when signals arrive to the sympathetic centers from the suprasegmental region and from the periphery.
The sympathetic system has an adaptive-trophic effect, i.e., it ensures the adaptation of the cardiovascular system to increased stress (physical, mental, emotional).
Reflex influences on the heart.
There are vagal and sympathetic reflexes.
Vagal reflexes are associated with an increase in the tone of the vagus nerve nucleus and an increase in its inhibitory effect on the heart when irritating various reflexogenic zones.
Localization of zones.
1) In the cardiovascular system.
Example: blood pressure → baroreceptors of the aortic arch, pulmonary artery, vessels of internal organs, endo-myo- and pericardium → increase in the tone of the X pair → slowdown of heart contractions.
2) Outside the SSS.
a) From receptors of the ventricles and intestines - Goltz reflex. When a blow is struck to the abdomen, the heartbeat decreases until the heart stops (celiac nerve → vagus nerve → heart rate).
b) When pressing on the eyeball - Aschner reflex (eye-heart).
c) An increase in the tone of the vagus nerve is observed during exhalation, manifested in the form of respiratory-cardiac arrhythmia.
Sympathetic reflexes associated with a decrease in the inhibitory effect of the vagus nerve and increased tone of the sympathetic centers.
1) From the reflexogenic zones of the cardiovascular system. For example: an increase in pressure at the mouth of the vena cava with a large venous return excites the receptors at the mouth of the vena cava and the right heart → increased sympathetic influences on the heart → increased heart rate (Bainbridge unloading reflex).
2) ↓ Blood pressure → baroreceptors of the vascular system → ↓ vagal tone → Heart rate.
3) From chemoreceptors of the cardiovascular system.
From other receptors.
1) From painful ones.
2) From thermal thermoreceptors.
3) For emotional states.
Conditioned reflex regulation of heart function.
Example: pre-start increase in heart rate (22-35 beats).
The role of various parts of the central nervous system:
2) hypothalamus;
3) limbicoreticular complex;
3) medulla oblongata;
4) spinal cord.
Humoral regulation of heart function.
Carried out by substances carried in the blood. There are 1) direct and 2) indirect action.
Direct action.
1) Hormones: catecholamines increase the frequency of contractions, activate β-adrenergic receptors → AI → ATP → cAMP → phosphorylase → breakdown of glycogen → contraction. ↓
Ca 2+ - increases the coupling of excitation and contraction.
Catecholamines, in addition, they increase the permeability to Ca 2+ - cell membranes.
Chronotropic action reduces the time of slow depolarization.
2) Glucagon acts directly through the sympathoadrenal system.
3) Glucocorticoids– increase the strength of heart contractions.
4) Thyroxine– increases frequency.
Electrolytes.
Ca 2+ increases the strength of contractions. Overdose – stopping in systole.
K + - for excitability, overdose stops in diastole.
Indirect influence carried out through nerve centers.
H + - increases the influence of the sympathetic system. AH - vagus nerve tone.
Resting heart rate. Heart rate is one of the most informative indicators of the condition of not only the cardiovascular system, but also the entire body as a whole. From birth until the age of 20-30, heart rate at rest decreases from 100-110 to 70 beats/min in young untrained men and to 75 beats/min in women. Subsequently, with increasing age, heart rate increases slightly: in 60-76 year olds at rest, compared to young people, by 5-8 beats/min.
Heart rate during muscle work. The only way to increase the delivery of oxygen to working muscles is to increase the volume of blood supplied to them per unit time. For this, the IOC must increase. Since heart rate directly affects the value of the IOC, an increase in heart rate during muscle work is an obligatory mechanism aimed at satisfying the significantly increasing metabolic needs. Changes in heart rate during work are shown in Fig. 7.6.
If the power of cyclic work is expressed through the amount of oxygen consumed (as a percentage of the maximum oxygen consumption - MOC), then heart rate increases in a linear dependence on the work power (O2 consumption, Fig. 7.7). In women, subject to the same consumption of oxygen as men, the heart rate is usually 10-12 beats/min higher.
The presence of a directly proportional relationship between work power and heart rate makes heart rate an important informative indicator in the practical activities of a trainer and teacher. For many types of muscular activity, heart rate is an accurate and easily determined indicator of the intensity of physical activity performed, the physiological cost of work, and the characteristics of recovery periods.
For practical needs, it is necessary to know the maximum heart rate in people of different genders and ages. With age, the maximum heart rate values in both men and women decrease (Fig. 7.8.). The exact value of the heart rate for each individual person can only be determined experimentally by recording the heart rate while working with increasing power on a bicycle ergometer. In practice, for an approximate judgment about the maximum heart rate of a person (regardless of gender), the formula is used: HRmax = 220 - age (in years).
35. Nervous and humoral regulation of heart function at rest...
Nervous and humoral influences play the main role in regulating the activity of the heart. The heart contracts thanks to impulses coming from the main pacemaker, whose activity is controlled by the central nervous system.
Nervous regulation of the activity of the heart is carried out by the efferent branches of the vagus and sympathetic nerves. It was only thanks to the experiments of I.P. Pavlov (1883) that it was shown that different fibers of these nerves have different effects on the functioning of the heart. Thus, irritation of some fibers of the vagus nerve causes a decrease in heartbeat, and irritation of others causes their weakening. Some sympathetic nerve fibers increase the heart rate, while others increase it. Strengthening nerve fibers are trophic, that is, they act on the heart by increasing metabolism in the myocardium.
Based on an analysis of all the influences of the vagus and sympathetic nerves on the heart, a modern classification of their effects has been created. The chronotropic effect characterizes a change in heart rate, the bathmotropic effect characterizes a change in excitability, the drotropic effect characterizes a change in conductivity, and the inotropic effect characterizes a change in contractility. The vagus nerves slow down and weaken all these processes, while the sympathetic nerves speed up and strengthen them.
The centers of the vagus nerves are located in the medulla oblongata. Their second neurons are located directly in the nerve ganglia of the heart. The processes of these neurons innervate the sinoatrial and atrioventricular nodes and the muscles of the atria; the ventricular myocardium is not innervated by the vagus nerves. The neurons of the sympathetic nerves are located in the upper segments of the thoracic spinal cord, from here the excitation is transmitted to the cervical and upper thoracic sympathetic nodes and further to the heart. Impulses from nerve endings are transmitted to the heart through mediators. For the vagus nerves, the mediator is acetylcholine, for sympathies - norepinephrine.
The centers of the vagus nerves are constantly in a state of some excitation (tone), the degree of which varies under the influence of centripetal impulses from different receptors of the body. With a persistent increase in the tone of these nerves, the heartbeat becomes less frequent and sinus bradycardia occurs. The tone of the centers of the sympathetic nerves is less pronounced. Excitation in these centers increases with emotions and muscle activity, which leads to increased heart rate and increased heart rate.
The reflex regulation of the heart involves the centers of the medulla oblongata and spinal cord, hypothalamus, cerebellum and cerebral cortex, as well as receptors of some sensory systems (visual, auditory, motor, vestibular). Of great importance in the regulation of the heart and blood vessels are impulses from vascular receptors located in reflexogenic zones (aortic arch, bifurcation of the carotid arteries, etc.). The same receptors are present in the heart itself. Some of these receptors perceive changes in pressure in blood vessels (baroreceptors).
Humoral regulation of the activity of the heart is carried out through the influence of chemical substances in the blood on it; it was found that the above-mentioned substances are acetylcholine and norepinephrine.
Humoral influences on the heart can be exerted by hormones, breakdown products of carbohydrates and proteins, changes in pH, potassium and calcium ions. Adrenaline, norepinephrine and thyroxine increase the work of the heart, acetylcholine weakens it. A decrease in pH and an increase in urea and lactic acid levels increase cardiac activity. With an excess of potassium ions, the rhythm slows down and the strength of heart contractions, its excitability and conductivity decreases. High potassium concentrations lead to myocardial dissection and cardiac arrest in diastole. Calcium ions speed up the rhythm and strengthen heart contractions, increase the excitability and conductivity of the myocardium; When there is excess calcium, the heart stops in systole.
The functional state of the vascular system, like the heart, is regulated by nervous and humoral influences. The nerves that regulate vascular tone are called vasomotor and consist of two parts - vasoconstrictor and vasodilator. Sympathetic nerve fibers emerging as part of the anterior roots of the spinal cord have a constricting effect on the vessels of the skin, abdominal organs, kidneys, lungs and meninges, but dilate the vessels of the heart. . Vasodilator effects are exerted by parasympathetic fibers that exit the spinal cord as part of the dorsal roots.
Certain relationships between the vasoconstrictor and vasodilator nerves are maintained by the vasomotor center located in the medulla oblongata and discovered in 1871 by V.F. Ovsyannikov. The vasomotor center consists of pressor (vasoconstrictor) and depressor (vasodilator) sections. The main role in the regulation of vascular tone belongs to the pressor region. In addition, there are higher vasomotor centers located in the cerebral cortex and hypothalamus, and lower ones in the spinal cord. Nervous regulation of vascular tone is also carried out by reflex. On the basis of unconditioned reflexes (defensive, food, sexual) vascular conditioned reactions to words, types of objects, emotions, etc. are developed.
The main natural receptive fields where reflexes to blood vessels occur are the skin and mucous membranes (exteroceptive zones) and the cardiovascular system (interoceptive zones). The main interoreceptive zones are the sinocarotid and aortic; Subsequently, similar zones were discovered at the mouth of the vena cava, in the vessels of the lungs and gastrointestinal tract.
Humoral regulation of vascular tone is carried out by both vasoconstrictor and vasodilator substances. The first group includes hormones of the adrenal medulla - adrenaline and norepinephrine, as well as the posterior lobe of the pituitary gland - vasopressin. Humoral vasoconstrictor factors include serotonin, which is formed in the intestinal mucosa, in some parts of the brain and during the breakdown of platelets. A similar effect is exerted by the substance renin, which is formed in the kidneys, which activates the globulin found in the plasma - hypertensinogen, turning it into active hypertensin (angiotonin).
Currently, significant amounts of vasodilator substances have been found in many tissues of the body. This effect is exerted by medullin, produced by the medulla of the kidneys, and prostaglandins, found in the secretion of the prostate gland. In the submandibular and pancreas, in the lungs and skin, the presence of a very active polypeptide, bradykinin, has been established, which causes relaxation of the smooth muscles of arterioles and lowers blood pressure. Vasodilators also include acetylcholine, which is formed in the endings of the parasympathetic nerves, and histamine, which is found in the walls of the stomach, intestines, as well as in the skin and skeletal muscles (during their work).
All vasodilators, as a rule, act locally, causing dilatation of capillaries and arterioles. Vasoconstrictors primarily have a general effect on large blood vessels.
What is the normal heart rate? How to calculate and what is the maximum threshold at rest? How does your heart rate vary during exercise? How and when to control your own heart rate, which changes are considered normal and which are pathological.
What is heart rate
Heart rate is vital sign and represents number of heart beats per unit of time, usually per minute.
The heart rate is determined by a group of cells that are located in the heart itself at the level of the sinus node, and which have the ability to depolarize and spontaneously contract. Such cells control heart contractions and heart rate.
However, the work of the heart is controlled not only by these cells, but also depends on certain hormones (which speed up or slow down its work) and on the autonomic nervous system.
Normal heart rate - under load and at rest
Resting heart rate or physiological when the body is not subjected to stress or physical activity, it should be within:
- minimum – 60 beats per minute
- maximum – 80/90 beats per minute
- the average value during the rest period is 70-75 beats per minute
In fact, heart rate depends on many parameters, the most important of which is age.
Depending on age we have:
- Embryo: embryo in the uterine cavity, i.e. a child at the stage of early development has a pulse of 70-80 beats per minute. The frequency increases as the fetus develops in the womb and reaches values between 140 and 160 beats per minute.
- Newborns: In newborns, the heart rate ranges from 80 to 180 beats per minute.
- Children: In children, the frequency is 70-110 beats per minute.
- Teenagers: In teenagers, the heart rate varies from 70 to 120 beats per minute.
- Adults: For an adult, the normal average is 70 beats per minute for men and 75 beats per minute for women.
- Aged people: In older adults, heart rates range from 70 to 90 beats per minute, or slightly higher, but irregularities in heart rhythm often occur with age.
How to measure your heart rate
Measuring heart rate can be done using simple instruments such as the fingers of your own hand, or complex ones such as an electrocardiogram. There are also special tools for measuring heart rate during sports training.
Let's see what the main evaluation methods are:
- Manually: Manual heart rate measurement can be performed at the wrist (radial artery) or neck (carotid artery). To take the measurement, place two fingers over the artery and apply gentle pressure to feel the heartbeat. Then it is enough to count the number of blows per unit of time.
- Stethoscope: Another way to measure heart rate involves using a stethoscope. In this case, the heartbeat is listened to using a stethoscope.
- Heart rate monitor: This tool measures your heart rate through a headband with electrodes. Used primarily in sports to measure heart rate under load.
- ECG: Allows you to record the electrical activity of your heart and easily count the number of heartbeats per minute.
- Cardiotocography: A specific fetal heart rate assessment tool used during pregnancy.
Causes of changes in heart rate
The human heart rate is subjected to several changes during the day, which are determined by physiological processes. However, changes in heart rate can also be associated with pathological conditions.
Changes in pulse due to physiological reasons
Physiological changes in heart rate occur at various times during the day or as a response to certain physical conditions.
First of all:
- After meal: Eating leads to an increase in heart rate, which is associated with an increase in the volume of the stomach, which is located just below the heart. An enlarged stomach puts pressure on the muscles of the diaphragm, which causes the heart rate to increase. This problem can be solved by avoiding large meals and snacks before bed.
- Body temperature: An increase or decrease in body temperature affects the heart rate. An increase in body temperature, such as a general fever, determines an increase in heart rate of approximately 10 beats per minute for each degree of temperature above 37°C. For this reason, children with fever often have a significantly elevated heart rate. Otherwise, a significant decrease in body temperature, i.e. in cases of hypothermia, leads to a noticeable decrease in heart rate.
- During sleep: At night, the heart rate decreases by approximately 8%, since the body is at complete rest and does not require excessive work from the heart muscle.
- Pregnancy: During pregnancy, the heart rate increases as there is a need for more blood flow to the placenta for proper growth of the fetus.
- During sports training or when you're catching up to a bus, your heart rate increases to increase blood flow to your muscles, which need more oxygen under stress.
Pathological causes of increased heart rate
Pathological changes in heart rate are called arrhythmias. They are presented mainly tachycardia, in case of very high heart rate, And bradycardia if the heart rate is very low.
Let's take a closer look:
- Tachycardia: This is an increase in heart rate above 100 beats per minute. It manifests itself with symptoms such as rapid heartbeat, increased blood pressure, chest pain, a feeling of “heart in the throat,” nausea and cold sweat. It can occur due to reasons such as stress, anxiety, poor habits (smoking, alcohol or excessive caffeine consumption), as well as due to thyroid disease such as hyperthyroidism.
| If the heart rate is very high, e.g. a value in the range between 300 and 600 beats per minute, this indicates atrial fibrillation, that is, excessive contraction of the atria, which determines heart failure. This disease is typical for older people, since myocardial dysfunction accumulates with age and blood pressure increases, but it can also be associated with atrial hypertrophy. |
- Bradycardia: Decrease in heart rate below 60 beats per minute. Characterized by shortness of breath, fatigue, weakness, dizziness and fainting, loss of consciousness, and in severe cases, convulsions.
How is heart function regulated?
The conduction system of your heart regulates all its functioning, controlling:
- Heart rate is the number of heart beats per minute.
- The rhythm of heart contractions, coordinating the contraction of the four chambers of the heart.
The conduction system of the heart must provide:
- A steady heart rate of 60 to 100 beats per minute at rest. The conduction system of the heart can increase the heart rate during exercise and decrease it during sleep and rest.
- Coordinated contractions of the atria and ventricles (this is called sinus rhythm).
How does the conduction system of the heart work?
Like any part of the body, your heart muscle is made up of tiny cells. The conduction system of the heart controls the contractions of the heart by sending electrical signals to these cells.
In the heart, two different types of cells are involved in the work of contracting it:
- The cells of the conduction system conduct electrical signals.
- Muscle cells contract the chambers of the heart; this contraction is caused by electrical impulses.
The electrical signal travels through a network of cells that form pathways, stimulating the contraction of the atria and ventricles. The passage of a signal along the pathways is possible due to a complex reaction, each cell activates the next one, so the impulse flows through the cells in a certain order. Each cell transmits electrical charge sequentially, which leads to coordinated contractions and proper heartbeat.
How does the conduction system regulate the rhythm of the heart?
The electrical impulse is generated at the top of the heart, in a group of cells called sinus node(SU). The signal then descends, causing contraction of first the two atria and then the two ventricles. In a healthy heart, the electrical impulse travels very quickly, which creates clear and regular contractions.
In the following sections we will go into detail about how a signal propagates through a conduction system.
How does an electrical impulse cause the atria to contract?
When the control system “turns on”, the following process starts:
- The electrical impulse travels from the sinus through the muscle cells to the right and left atria.
- This causes the muscle cells in the atria to contract.
- The atria contract, pumping blood into the right and left ventricles.
After the electrical signal causes the atria to contract and pump blood into the ventricles, it travels to a group of cells at the base of the right atrium called the atrioventricular node (AV node). The AV node delays the electrical signal, allowing the ventricles to receive blood from the atria, and then the ventricles contract.
How does an electrical impulse cause the ventricles to contract?
When the electrical signal leaves the AV node, the following process begins:
- The signal travels down to a group of cells called His bundle, which is divided into two legs, right and left. They conduct electrical impulses to the right and left ventricles, respectively.
- The signal divided into these two branches causes contraction of both ventricles.
- During contraction of the ventricles, the right one pumps blood to the lungs, and the left one pumps blood throughout the body.
After contraction of the atria and ventricles occurs, all parts of the conduction system are independently restored.
How does the conduction system regulate heart rate?
SU cells located in the upper part of the heart are called natural pacemaker, since the frequency of the impulses they create sets the frequency of contractions of the entire heart (heart rate - heart rate). A normal resting heart rate ranges between 60 and 100 beats per minute, but can rise and fall depending on the body's needs. The heart rate value actually reflects the rate at which the ventricles contract. When various types of arrhythmias the atria and ventricles begin to contract at different frequencies; this should be remembered when using the term heart rate.
What makes the heart beat faster or slower?
Our brain and other organs send signals that cause our heart rate to slow down or shorten.
Although this process is based on a complex set of chemical reactions, in the end they all lead to a change in the frequency of “switching on” of the control system. It is the control system that begins to work more often or less often, which causes a change in heart rate.
For example, during physical activity, when the body requires more oxygen to function, the body signals the need to increase the heart rate to increase the rate of blood circulation, and therefore the flow of oxygen to the tissues. Heart rate can increase by more than 100 beats per minute to meet the body's growing needs for oxygenated blood.
Likewise, during rest or sleep, when the body needs less oxygen, the heart rate decreases.
For some athletes, a normal resting heart rate may be less than 60 beats per minute; their hearts are highly trained and should not beat frequently. Changes in heart rate are part of your body's normal functioning due to changes in its needs. Your heart rate can only be considered abnormal if your heart is beating too fast or too slow.
Structure of the heart
In humans and other mammals, as well as in birds, the heart is four-chambered and cone-shaped. The heart is located in the left half of the thoracic cavity, in the lower part of the anterior mediastinum on the tendon center of the diaphragm, between the right and left pleural cavity, fixed on large blood vessels and enclosed in a pericardial sac made of connective tissue, where there is always a fluid that moisturizes the surface of the heart and provides it with free contraction. A solid septum divides the heart into right and left halves and consists of the right and left atria and the right and left ventricles. In this way, a right heart and a left heart are distinguished.
Each atrium communicates with the corresponding ventricle through the atrioventricular orifice. At each orifice there is a valve that regulates the direction of blood flow from the atrium to the ventricle. The leaflet valve is a connective tissue petal, which with one edge is attached to the walls of the opening connecting the ventricle and the atrium, and with the other hangs freely into the cavity of the ventricle. Tendon filaments are attached to the free edge of the valves, and the other end grows into the walls of the ventricle.
When the atria contract, blood flows freely into the ventricles. And when the ventricles contract, the blood, with its pressure, lifts the free edges of the valves, they come into contact with each other and close the hole. Tendon threads prevent the valves from turning away from the atria. When the ventricles contract, blood does not enter the atria, but is sent to the arterial vessels.
In the atrioventricular ostium of the right heart there is a tricuspid (tricuspid) valve, in the left - a bicuspid (mitral) valve.
In addition, at the places where the aorta and pulmonary artery exit from the ventricles of the heart, semilunar, or pocket (in the form of pockets), valves are located on the inner surface of these vessels. Each flap consists of three pockets. Blood moving from the ventricle presses the pockets against the walls of the vessels and passes freely through the valve. During the relaxation of the ventricles, blood from the aorta and pulmonary artery begins to flow into the ventricles and, with its reverse movement, closes the pocket valves. Thanks to the valves, blood in the heart moves only in one direction: from the atria to the ventricles, from the ventricles to the arteries.
Blood enters the right atrium from the superior and inferior vena cava and the coronary veins of the heart itself (coronary sinus); four pulmonary veins flow into the left atrium. The ventricles give rise to vessels: the right one - the pulmonary artery, which is divided into two branches and carries venous blood to the right and left lungs, i.e. into the pulmonary circulation; The left ventricle gives rise to the aortic arch, through which arterial blood enters the systemic circulation.
The heart wall consists of three layers:
- internal - endocardium, covered with endothelial cells
- middle - myocardium - muscular
- outer - epicardium, consisting of connective tissue and covered with serous epithelium
On the outside, the heart is covered with a connective tissue membrane - the pericardial sac, or pericardium, also lined on the inside with serous epithelium. Between the epicardium and the heart sac there is a cavity filled with fluid.
The thickness of the muscle wall is greatest in the left ventricle (10-15 mm) and smallest in the atria (2-3 mm). The thickness of the wall of the right ventricle is 5-8 mm. This is due to the unequal intensity of work of different parts of the heart to push out blood. The left ventricle pumps blood into the systemic ventricle under high pressure and therefore has thick, muscular walls.
Properties of the heart muscle
The cardiac muscle, the myocardium, differs both in structure and properties from other muscles of the body. It consists of striated fibers, but unlike the fibers of skeletal muscles, which are also striated, the fibers of the cardiac muscle are interconnected by processes, so excitation from any part of the heart can spread to all muscle fibers. This structure is called a syncytium.
Contractions of the heart muscle are involuntary. A person cannot stop the heart or change its rate at will.
A heart removed from an animal's body and placed in certain conditions can contract rhythmically for a long time. This property of it is called automaticity. The automaticity of the heart is caused by the periodic occurrence of excitation in special cells of the heart, a cluster of which is located in the wall of the right atrium and is called the center of cardiac automaticity. The excitation that occurs in the cells of the center is transmitted to all the muscle cells of the heart and causes them to contract. Sometimes the center of automation fails, then the heart stops. Currently, in such cases, a miniature electronic stimulator is implanted on the heart, which periodically sends electrical impulses to the heart, and it contracts each time.
Work of the heart
The heart muscle, the size of a fist and weighing about 300 g, works continuously throughout life, contracts about 100 thousand times a day and pumps more than 10 thousand liters of blood. Such high performance is due to increased blood supply to the heart, the high level of metabolic processes occurring in it and the rhythmic nature of its contractions.
The human heart beats rhythmically at a frequency of 60-70 times per minute. After each contraction (systole), relaxation occurs (diastole), and then a pause during which the heart rests, and contraction again. The cardiac cycle lasts 0.8 s and consists of three phases:
- atrial contraction (0.1 s)
- ventricular contraction (0.3 s)
- relaxation of the heart with a pause (0.4 s).
If the heart rate increases, the time of each cycle decreases. This occurs mainly due to a shortening of the overall cardiac pause.
In addition, through the coronary vessels, the heart muscle receives about 200 ml of blood per minute during normal heart function, and at maximum load, coronary blood flow can reach 1.5-2 l/min. In terms of 100 g of tissue mass, this is much more than for any other organ except the brain. It also enhances the efficiency and fatigue of the heart.
During contraction of the atria, blood is ejected from them into the ventricles, and then, under the influence of ventricular contraction, is pushed into the aorta and pulmonary artery. At this time, the atria are relaxed and filled with blood flowing to them through the veins. After the ventricles relax during the pause, they fill with blood.
Each half of an adult's heart pumps approximately 70 ml of blood into the arteries in one contraction, which is called stroke volume. In 1 minute, the heart pumps out about 5 liters of blood. The work performed by the heart can be calculated by multiplying the volume of blood ejected by the heart by the pressure under which the blood is ejected into the arterial vessels (this is 15,000 - 20,000 kgm/day). And if a person performs very strenuous physical work, then the minute volume of blood increases to 30 liters, and the work of the heart increases accordingly.
The work of the heart is accompanied by various manifestations. So, if you put your ear or phonendoscope to a person’s chest, you can hear rhythmic sounds - heart sounds. There are three of them:
- the first sound occurs during ventricular systole and is caused by vibrations of the tendon threads and the closure of the leaflet valves;
- the second sound occurs at the beginning of diastole as a result of valve closure;
- the third tone - very weak, it can only be detected with the help of a sensitive microphone - occurs during the filling of the ventricles with blood.
Heart contractions are also accompanied by electrical processes, which can be detected as an alternating potential difference between symmetrical points on the surface of the body (for example, on the hands) and recorded with special devices. Recording of heart sounds - phonocardiogram and electrical potentials - electrocardiogram is shown in Fig. These indicators are used clinically to diagnose heart diseases.
Regulation of the heart
The work of the heart is regulated by the nervous system depending on the influence of the internal and external environment: the concentration of potassium and calcium ions, thyroid hormone, state of rest or physical work, emotional stress.
The nervous and humoral regulation of the activity of the heart coordinates its work with the needs of the body at any given moment, regardless of our will.
- The autonomic nervous system innervates the heart, like all internal organs. The nerves of the sympathetic department increase the frequency and strength of contractions of the heart muscle (for example, during physical work). Under resting conditions (during sleep), heart contractions become weaker under the influence of parasympathetic (vagus) nerves.
- Humoral regulation of the activity of the heart is carried out with the help of special chemoreceptors present in large vessels, which are excited under the influence of changes in blood composition. An increase in the concentration of carbon dioxide in the blood irritates these receptors and reflexively increases the work of the heart.
Particularly important in this sense is adrenaline, which enters the blood from the adrenal glands and causes effects similar to those observed when the sympathetic nervous system is irritated. Adrenaline causes an increase in heart rate and amplitude of heart contractions.
Electrolytes play an important role in the normal functioning of the heart. Changes in the concentration of potassium and calcium salts in the blood have a very significant effect on the automation and processes of excitation and contraction of the heart.
An excess of potassium ions inhibits all aspects of cardiac activity, acting negatively chronotropically (reduces the heart rate), inotropically (reduces the amplitude of heart contractions), dromotropically (impairs the conduction of excitation in the heart), bathmotropically (reduces the excitability of the heart muscle). With an excess of K+ ions, the heart stops in diastole. Sharp disturbances in cardiac activity also occur with a decrease in the content of K + ions in the blood (with hypokalemia).
Excess calcium ions acts in the opposite direction: positively chronotropic, inotropic, dromotropic and bathmotropic. With an excess of Ca 2+ ions, the heart stops in systole. With a decrease in the content of Ca 2+ ions in the blood, heart contractions are weakened.
Table. Neurohumoral regulation of the cardiovascular system
| Factor | Heart | Vessels | Blood pressure level |
| Sympathetic nervous system | narrows | increases | |
| Parasympathetic nervous system | expands | lowers | |
| Adrenalin | increases rhythm and strengthens contractions | narrows (except for heart vessels) | increases |
| Acetylcholine | slows down the rhythm and weakens contractions | expands | lowers |
| Thyroxine | quickens the rhythm | narrows | increases |
| Calcium ions | increase the rhythm and weaken contractions | narrow | raise |
| Potassium ions | slow down the rhythm and weaken contractions | expand | lower |
The work of the heart is also connected with the activities of other organs. If excitation is transmitted to the central nervous system from working organs, then from the central nervous system it is transmitted to the nerves that enhance the function of the heart. Thus, through a reflexive process, a correspondence is established between the activities of various organs and the work of the heart.
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