The minute volume of blood, the formula by which this indicator is calculated, as well as other important points must certainly be in the baggage of knowledge of any medical student, and even more so of persons already engaged in medical activities. What is this indicator, how it affects human health, why it is important for doctors, and also what depends on it - every young person or girl who wants to go to a medical school is looking for answers to these questions. These questions are highlighted in this article.
Heart function
The main function of the heart is to deliver to the organs and tissues a certain amount of blood per unit time (blood volume per minute), due to the condition of the heart itself and the working conditions in the circulatory system. This most important mission of the heart is studied back in school. Most of the anatomy textbooks, unfortunately, talk a little about this feature. Cardiac output is a derivative of stroke volume and heart rate.
MO (NE) = heart rate x UO
Heart index
Impact volume - 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 is the size of a 60-second volume, calculated on the surface area of the human body. At rest, its normal value is about 3 l / min / m 2 .
Normally, the minute volume of human blood depends on the size of the body. For example, a cardiac output in a 53 kg female will undoubtedly be significantly lower than in a stronger sex weighing 93 kg.
Normally, for a man weighing 72 kg, the minute volume of the heart pumped per minute is 5 l / min., With a load, this figure can grow up to 25 l / min.
What affects cardiac output?
These are a few indicators:
- the systolic volume of blood flowing into the right atrium and ventricle (the "right heart"), and the pressure it creates is preload.
- the resistance that the heart muscle experiences when the next volume of blood is ejected from the left ventricle is 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 an effort by the heart muscle at any length of muscle fiber. The combination of all these characteristics, of course, affects the minute volume of blood, speed and rhythm, as well as other heart indicators.
How is this process regulated in the myocardium?
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 penetrate 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 double pathway of this microelement, its concentration increases rapidly, and this serves as the beginning of the contraction of cardiac myocyte. Such a double path of “ignition” is characteristic only of the heart. If there is no extracellular calcium, then there will be no contraction of the heart muscle.
The hormone norepinephrine, which is secreted from the ends of the sympathetic nerves, increases the speed of contractions 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 failure.
Impact volume and filling pressure
The minute volume of blood in the left ventricle, which is formed at the end of the diastole and the base of the systole, depends on the elasticity of muscle tissue and the final diastolic pressure. Blood pressure in the right heart is associated with venous pressure.
When the final diastolic pressure rises, the strength of subsequent contractions and stroke volume increase. That is, the force of contraction is related to the degree of muscle strain.
The shock systolic blood volume from both ventricles is supposedly equal. If the ejection from the right ventricle will exceed the ejection from the left ventricle for some time, pulmonary edema may develop. However, there are protective mechanisms in the course of 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, an increase in the release of blood volume during physical exertion occurs.
This mechanism - increased cardiac contraction during stretching of muscle fibers - is called the Frank-Starling law. It is an important compensatory mechanism in heart failure.
Afterload action
With an increase in blood pressure or an increase in afterload, the volume of blood ejected can also increase. This property has been documented and experimentally confirmed many years ago, which made it possible to make appropriate amendments to the calculations and formulas.
If 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, the minute volume of blood increases according to the Frank-Starling rule. After several such cycles, the blood volume returns to the original.
The autonomic nervous system is an 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 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 voiced.