The minute volume of blood, the formula by which this indicator is calculated, as well as other important points should certainly be in the knowledge base of any medical student, and even more so of those already involved in medical practice. What is this indicator, how does it affect human he alth, why is it important for doctors, and what depends on it - every young man or girl who wants to enter a medical school is looking for answers to these questions. These are the issues covered in this article.
Heart function
Fulfillment of the main function of the heart - delivery to organs and tissues of a certain volume of blood per unit time (blood volume per minute), due to the state of the heart itself and the conditions of work in the circulatory system. This most important mission of the heart is studied in school years. Most anatomy textbooks, unfortunately, don't talk much about this function. Cardiac output - derivative of shockheart rate and volume.
MO(SV)=HR x SV
Heart Index
Stroke volume - an indicator that determines the size and amount of blood expelled by the ventricles in one contraction, its value is approximately equal to 70 ml. Cardiac index - the size of a 60-second volume, converted to the surface area of the human body. At rest, its normal value is about 3 l/min/m2.
Normally, the minute volume of a person's blood depends on the size of the body. For example, the cardiac output of a 53kg female would certainly be significantly lower than that of a 93kg male.
Normally, in a man weighing 72 kg, the minute volume of the heart pumped per minute is 5 l / min., Under load, this figure can grow up to 25 l / min.
What influences cardiac output?
These are several indicators:
- systolic volume of blood entering the right atrium and ventricle ("right heart") and the pressure it creates - preload.
- resistance experienced by the heart muscle at the moment of ejection of the next volume of blood from the left ventricle - afterload.
- period and heart rate 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 of the muscle fiber. The totality of all the abovecharacteristics, of course, affects the minute volume of blood, speed and rhythm, as well as other cardiac indicators.
How is this process regulated in the myocardium?
The contraction of the heart muscle occurs if the concentration of calcium inside the cell becomes more than 100 mmol, the susceptibility of the contractile apparatus to calcium is less important.
In 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 into its cytoplasm from the sarcoplasmic reticulum. Due to the double route of intake of this trace element, its concentration rapidly increases, and this is the beginning of the contraction of the cardiac myocyte. Such a double path of "ignition" is characteristic only for 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 and contractility of the heart, thus increasing cardiac output. This substance belongs to physiological inotropic agents. Digoxin is an inotropic drug used in certain cases to treat heart failure.
Stroke volume and inflation pressure
The minute volume of blood in the left ventricle, which is formed at the end of diastole and the base of systole, depends on the elasticity of muscle tissue and end-diastolic pressure. Blood pressure in the right side of the heart is related to the pressure of the venous system.
When the finite growsdiastolic pressure, the strength of subsequent contractions and stroke volume increase. That is, the strength of the contraction is related to the degree of stretching of the muscle.
Stroke systolic blood volume from both ventricles is presumably 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 an increase in the stretching of muscle fibers in the left ventricle, the amount of blood expelled from it increases. This increase in cardiac output prevents pressure buildup in the pulmonary circulation and restores balance.
By the same mechanism, there is an increase in the release of blood volume during exercise.
This mechanism - an increase in cardiac contraction when a muscle fiber is stretched - is called the Frank-Starling law. It is an important compensatory mechanism for heart failure.
Afterload action
When blood pressure increases or afterload increases, the volume of blood ejected can also increase. This property was documented and experimentally confirmed many years ago, which made it possible to make appropriate corrections to the calculations and formulas.
If the blood from the left ventricle is ejected 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 - increasesminute volume of blood according to the Frank-Starling rule. After several such cycles, blood volume returns to its original value. The autonomic nervous system is the external regulator of cardiac output.
Ventricular filling pressure, changes in heart rate and contractility can change stroke volume. Central venous pressure and the autonomic nervous system are factors that control cardiac output.
So, we have considered 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 voiced.
