What determines the minute volume of blood? Minute blood volume: formula. Cardiac index
Minute blood volume, the formula by which this indicator is calculated, as well as others important points must certainly be in the knowledge base of any medical student, and even more so of those already engaged in medical practice. What kind of indicator is this, how does it affect human health, why is it important for doctors, and also what depends on it - every young man or girl who wants to enter medical school is looking for answers to these questions. educational institution. These are the issues that are covered in this article.
Heart function
Performing the main function of the heart is the delivery to organs and tissues of a certain volume of blood per unit of time (blood volume in one minute), determined by the condition of the heart itself and the operating conditions in the circulatory system. This most important mission of the heart is studied during school years. Most anatomy textbooks, unfortunately, do not talk much about this function. Cardiac output is a derivative of stroke volume and heart rate.
MO(SV) = HR x SV
Cardiac index
Stroke volume is an indicator that determines the size and amount of blood expelled by the ventricles in one contraction; its value is approximately 70 ml. Cardiac index - the size of the 60-second volume converted to surface area human body. At rest, its normal value is about 3 l/min/m2.
Normally, a person’s minute blood volume depends on body size. For example, the cardiac output of a female weighing 53 kg will undoubtedly be significantly lower than that of a male weighing 93 kg.
Normally, in a man weighing 72 kg, the cardiac output pumped per minute is 5 l/min; with load, this figure can increase to 25 l/min.
What affects cardiac output?
These are several indicators:
- systolic volume of blood entering the right atrium and the ventricle (“right heart”), and the pressure it creates is preload.
- The resistance experienced by the heart muscle at the moment of ejection of the next volume of blood from the left ventricle is afterload.
- the period and speed of heart contractions and myocardial contractility, which change under the influence of the sensitive and parasympathetic nervous system.
Contractility - the ability to generate force by the heart muscle at any length muscle fiber. The combination of all these characteristics, of course, affects minute blood volume, speed and rhythm, as well as other cardiac parameters.
How is this process regulated in the myocardium?
Contraction of the heart muscle occurs if the calcium concentration inside the cell becomes more than 100 mmol; the susceptibility of the contractile apparatus to calcium is less important.
During the resting period of the cell, calcium ions make their way into the cardiomyocyte through the L-channels of the membrane, and are also released inside the cell itself into its cytoplasm from the sarcoplasmic reticulum. Due to the dual route of entry of this microelement, its concentration quickly increases, and this serves as the beginning of contraction of the cardiac myocyte. This double path of “ignition” is characteristic only of the heart. If there is no supply of extracellular calcium, then there will be no contraction of the heart muscle.
The hormone norepinephrine, which is released from sympathetic nerve endings, increases the rate of contraction and contractility of the heart, thereby increasing cardiac output. This substance belongs to physiological inotropic agents. Digoxin is an inotropic drug that is used in certain cases to treat heart weakness.
Stroke volume and filling pressure
The minute volume of blood that is formed at the end of diastole and the base of systole depends on elasticity muscle tissue and end-diastolic pressure. in the right parts of the heart is associated with the pressure of the venous system.
As end-diastolic pressure increases, the force of subsequent contractions and stroke volume increase. That is, the force of contraction is related to the degree of muscle stretch.
The stroke blood from both ventricles is assumed to be equal. If the output from the right ventricle exceeds the output from the left for some time, pulmonary edema may develop. However, there are protective mechanisms, during which, reflexively, due to increased stretching of muscle fibers in the left ventricle, the amount of blood expelled from it increases. This increase in cardiac output prevents pressure build-up in the pulmonary circulation and restores balance.
By the same mechanism, there is an increase in the release of blood volume during physical activity.
This mechanism - increased cardiac contraction when muscle fibers are stretched - is called the Frank-Starling law. It is an important compensatory mechanism in heart failure.
Effect of afterload
As blood pressure rises or afterload increases, the volume of blood ejected may also increase. This property was documented and experimentally confirmed many years ago, which made it possible to make appropriate amendments to calculations and formulas.
If blood is ejected from the left ventricle under conditions of increased resistance, then for some time the volume of residual blood in the left ventricle will increase, the extensibility of myofibrils increases, this increases the stroke volume, and as a result, the minute volume of blood increases in accordance with the Frank-Starling rule. After several such cycles, the blood volume returns to its original value.
Autonomous nervous system- external cardiac output regulator.
Ventricular filling pressure changes and contractility can alter stroke volume. Central venous pressure and the autonomic nervous system are factors that control cardiac output.
So, we have examined the concepts and definitions mentioned in the preamble of this article. We hope that the information presented above will be useful to all people interested in the topic discussed.
Every minute a person's heart pumps a certain amount of blood. This indicator is different for everyone, it can change according to age, physical activity and health status. Minute blood volume is important in determining the efficiency of the heart.
The amount of blood that human heart pumps in 60 seconds, has a definition of “minute blood volume” (MBV). Stroke (systolic) blood volume is the amount of blood ejected into the arteries in one heartbeat(systole). Systolic volume (SV) can be calculated by dividing SV by heart rate. Accordingly, as the SOC increases, the IOC also increases. The values of systolic and minute blood volumes are used by doctors to assess the pumping ability of the heart muscle.
MOC value depends not only on stroke volume and heart rate, but also from venous return (the amount of blood returned to the heart through the veins). Not all the blood is ejected in one systole. Some fluid remains in the heart as a reserve (reserve volume). It is used for increased physical activity and emotional stress. But even after the reserves are released, a certain amount of liquid remains, which is not released under any circumstances.
Norm of indicators
Normal in the absence of voltage MOK equal to 4.5-5 liters. That is, healthy heart pumps all the blood in 60 seconds. The systolic volume at rest, for example, with a pulse of up to 75 beats, does not exceed 70 ml.
During physical activity, the heart rate increases, and therefore the indicators increase. This happens at the expense of reserves. The body includes a self-regulation system. In untrained people minute surge blood increases 4-5 times, that is, 20-25 liters. In professional athletes, the value changes by 600-700%; their myocardium pumps up to 40 liters per minute.
An untrained body cannot withstand maximum stress for a long time, so it responds with a decrease in CO2.Minute volume, stroke volume, pulse rate are interconnected, they depend on many factors:
- Human weight. With obesity, the heart has to work twice as hard to supply oxygen to all cells.
- The relationship between body weight and myocardial weight. In a person weighing 60 kg, the mass of the heart muscle is approximately 110 ml.
- Condition of the venous system. Venous return should be equal to the IOC. If the valves in the veins do not work well, then not all the fluid returns back to the myocardium.
- Age. In children, the IOC is almost twice as large as in adults. Happens with age natural aging myocardium, so MOC and MOC decrease.
- Physical activity. Athletes have higher values.
- Pregnancy. The mother's body works in increased mode, the heart pumps much more more blood in a minute.
- Bad habits. When smoking and drinking alcohol, the blood vessels narrow, so the IOC decreases, since the heart does not have time to pump the required volume of blood.
Deviation from the norm
Decline in IOC indicators occurs in various heart pathologies:
- Atherosclerosis.
- Heart attack.
- Mitral valve prolapse.
- Blood loss.
- Arrhythmia.
- Taking some medical supplies: barbiturates, antiarrhythmic drugs, blood pressure lowering drugs.
Developing low cardiac output syndrome. This is expressed in a decrease in blood pressure, a drop in pulse, tachycardia, and pale skin.
The main physiological function of the heart is to pump blood into the vascular system.
The amount of blood ejected by a ventricle of the heart per minute is one of the most important indicators functional state hearts is called minute volume of blood flow, or minute volume of the heart. It is the same for the right and left ventricles. When a person is at rest, the minute volume averages 4.5-5.0 liters. By dividing the minute volume by the number of heartbeats per minute, you can calculate systolic volume blood flow With a heart rate of 70-75 per minute, the systolic volume is 65-70 ml of blood. Determination of the minute volume of blood flow in humans is used in clinical practice.
The most accurate method for determining the minute volume of blood flow in humans was proposed by Fick (1870). It consists of indirectly calculating the cardiac output, which is done by knowing: 1) the difference between the oxygen content in arterial and venous blood; 2) the volume of oxygen consumed by a person per minute. Let's say
that in 1 minute 400 ml of oxygen entered the blood through the lungs, every
100 ml of blood absorbs 8 ml of oxygen in the lungs; therefore, to assimilate everything
the amount of oxygen that entered the blood through the lungs per minute (in our case
at least 400 ml), it is necessary that 100 * 400/8 = 5000 ml of blood pass through the lungs. This
the amount of blood is the minute volume of blood flow, which in this case is 5000 ml.
When using the Fick method, you must take venous blood from the right side of the heart. IN last years Venous blood from a person is taken from the right half of the heart using a probe inserted into the right atrium through the brachial vein. This method of drawing blood is not widely used.
A number of other methods have been developed to determine minute, and therefore systolic, volume. Currently, some paints and radioactive substances are widely used. The substance injected into the vein passes through the right heart, pulmonary circulation, left heart and enters the systemic arteries, where its concentration is determined. At first it increases in waves and then falls. After some time, when a portion of blood containing its maximum amount passes through the left heart for the second time, its concentration is arterial blood again increases slightly (the so-called recirculation wave). The time from the moment of administration of the substance to the beginning of recirculation is noted and a dilution curve is drawn, i.e., changes in the concentration (increase and decrease) of the test substance in the blood. Knowing the amount of a substance introduced into the blood and contained in the arterial blood, as well as the time required for the passage of the entire amount of the injected substance through the circulatory system, we can calculate the minute volume (MV) of blood flow in l/min using the formula:
where I is the amount of administered substance in milligrams; C is its average concentration in milligrams per 1 liter, calculated from the dilution curve; T- duration of the first circulation wave in seconds.
Currently, a method has been proposed integral rheography. Rheography (impendanceography) is a method of recording the electrical resistance of tissues of the human body electric current passed through the body. To avoid causing tissue damage, currents of ultra-high frequency and very low strength are used. Blood resistance is much less than tissue resistance, so increasing the blood supply to tissues significantly reduces their electrical resistance. If we record the total electrical resistance chest in several directions, then periodic sharp decreases in it occur at the moment the heart ejects systolic blood volume into the aorta and pulmonary artery. In this case, the magnitude of the decrease in resistance is proportional to the magnitude of the systolic ejection.
Keeping this in mind and using formulas that take into account body size, constitutional features, etc., it is possible to determine the value of systolic blood volume using rheographic curves, and multiplying it by the number of heartbeats to obtain the value of cardiac output.
With each contraction of the human heart, the left and right ventricles expel approximately 60-80 ml of blood into the aorta and pulmonary arteries, respectively; this volume is called systolic or stroke volume of blood (SV). During ventricular systole, not all the blood contained in them is ejected, but only about half. The blood remaining in the ventricles is called reserve volume. Due to the presence of a reserve blood volume, the systolic volume can increase sharply even with the first contractions of the heart after the start of work. In addition to the reserve volume, there is also a residual volume of blood in the ventricles of the heart, which is not ejected even with the strongest contraction. By multiplying the MVR by the heart rate, you can calculate the minute blood volume (MBV), which averages 4.5-5 liters. An important indicator is the cardiac index - the ratio of the IOC to the body surface area; this value in adults is on average 2.5-3.5 l/min/m2. During muscle activity, systolic volume can increase to 100-150 ml or more, and IOC - up to 30-35 liters.
With each contraction of the heart, a certain amount of blood is released into the arteries under high pressure. Its free movement is hampered by peripheral vascular resistance. This creates pressure in the blood vessels, called blood pressure. It is not the same in various departments vascular system. Being highest in the aorta and large arteries, blood pressure decreases in small arteries, arterioles, capillaries, veins and becomes below atmospheric in the vena cava.
The value of blood pressure depends on the amount of blood flowing per unit time from the heart to the aorta, the intensity of blood outflow from the central vessels to the periphery, the capacity of the vascular bed, the elastic resistance of the arterial walls and blood viscosity. The flow of blood into the arteries, i.e., the systolic volume of blood depends on the force of heart contraction.
The pressure in the arteries is greater during systole and less during diastole. The highest pressure in the arteries is called systolic or maximum, the lowest is called diastolic or minimum. The pressure in the arteries does not drop to 0 during ventricular diastole. It is maintained due to the elasticity of the arterial walls, stretched during systole. During ventricular systole, the arteries fill with blood. The blood that does not have time to pass further into the peripheral vessels stretches the walls large arteries. During diastole, the blood in the arteries does not experience pressure from the heart. At this time, only the arterial walls, stretched during heart systole and returning due to their elasticity to their original state, exert pressure on it. Fluctuations in blood pressure during systole and diastole of the heart occur only in the aorta and arteries. In arterioles, capillaries and veins, blood pressure is constant throughout the cardiac cycle.
In adults healthy people systolic pressure in the brachial artery is most often in the range from 110 to 125 mm Hg. Art. According to the World Health Organization, in people 20-60 years old, systolic pressure is up to 140 mm Hg. Art. is normotonic, above 140 mm Hg. Art. - hypertensive, below 100 mm Hg. Art. - hypotonic. The difference between systolic and diastolic pressure is called pulse pressure or pulse amplitude. Its value is on average 40 mm Hg. Art. In older people, blood pressure is higher than in people due to increased stiffness of the arterial walls young. Children have lower blood pressure than adults. Blood pressure varies in different arteries. It can be different even in arteries of the same caliber, for example, in the right and left brachial arteries. Even more often, the pressure difference is observed in the arteries of the upper and lower limbs. Blood pressure changes under the influence of various factors (emotional arousal, physical work). IN pulmonary artery In humans, systolic pressure is 25-30 mmHg. Art., diastolic - 5-10mm. Thus, the pressure in the arteries of the lungs is many times lower than in the systemic circle. In the pulmonary veins it is on average 6-12 mm Hg. Art.
The vessels of the lungs can deposit blood, i.e., accommodate its excess volume that is not used by the organ itself. The accumulation of blood in the depot does not cause a significant increase in pressure in its vessels. The capacity of the pulmonary vessels is variable. When you inhale it increases, when you exhale it decreases. Pulmonary vessels can hold from 10 to 25% of the total blood volume.
Questions for self-control:
1. Heart structure and functions.
2. The valve apparatus of the heart and its location.
3. Conducting system of the heart, its topography and function.
4. What is the pericardium?
5. Basic properties of the heart (automation, contractility, excitability)
6. bridge, conductivity).
7. Tell us about cardiac cycle, its beginning, phases and continuation
8. body.
9. What are systole and diastole? What processes occur in the heart?
10. What is it during systole and diastole?
11. How is the neurohumoral regulation of the heart carried out?
12. List blood vessels, forming a small (pulmonary)
13. blood circulation.
14. What is it? big circle blood circulation? What blood vessels does it contain?
