Blood circulation of the fetus and newborn. Yolk period. Allanthic blood circulation. Placental circulation.

During intrauterine development, fetal blood circulation goes through three successive stages: vitelline, allantoid and placental.

Yolk period of development of the circulatory system in humans it is very short - from the moment of implantation to the 2nd week of the embryo’s life. Oxygen and nutrients enter the embryo directly through trophoblast cells, which do not yet have blood vessels during this period of embryogenesis. A significant portion of the nutrients accumulates in the yolk sac, which also has its own meager reserves of nutrients. From the yolk sac, oxygen and necessary nutrients travel through the primary blood vessels to the embryo. This is how yolk blood circulation occurs, which is inherent in the earliest stages of ontogenetic development.

Allanthoid circulation begins to function approximately from the end of the 8th week of pregnancy and continues for 8 weeks, i.e. until the 15-16th week of pregnancy. The allantois, which is a protrusion of the primary intestine, gradually grows to the avascular trophoblast, carrying with it fetal vessels. When the allantois comes into contact with the trophoblast, the fetal vessels grow into the avascular villi of the grophoblast, and the chorion becomes vascular. The establishment of allantoic blood circulation is a qualitatively new stage in the intrauterine development of the embryo, since it allows for wider transport of oxygen and necessary nutrients from mother to fetus. Allantoic circulation disorders(disorders of trophoblast vascularization) underlie the causes of embryo death.

Placental circulation replaces allantoid. It begins in the 3-4th month of pregnancy and reaches its peak at the end of pregnancy. The formation of placental blood circulation is accompanied by the development of the fetus and all functions of the placenta (respiratory, excretory, transport, metabolic, barrier, endocrine, etc.). It is with the hemochorial type of placement that the most complete and adequate exchange between the organisms of the mother and fetus is possible, as well as the implementation of adaptive reactions of the mother-fetus system.

Fetal circulatory system differs in many ways from that of a newborn. This is determined by both anatomical and functional characteristics of the fetal body, reflecting its adaptation processes during intrauterine life.

The anatomical features of the fetal cardiovascular system primarily consist in the existence of the foramen ovale between the right and left atria and the ductus arteriosus connecting the pulmonary artery to the aorta. This allows a significant amount of blood to bypass the non-functioning lungs. In addition, there is communication between the right and left ventricles of the heart. The blood circulation of the fetus begins in the vessels of the placenta, from where blood, enriched with oxygen and containing all the necessary nutrients, enters the umbilical cord vein.

Then arterial blood through ductus venosus (Arantius) enters the liver. The fetal liver is a kind of blood depot. The left lobe plays the largest role in blood deposition. From the liver, through the same venous duct, blood flows into the inferior vena cava, and from there into the right atrium. The right atrium also receives blood from the superior vena cava. Between the confluence of the inferior and superior vena cava there is a valve of the inferior vena cava, which separates both blood flows. This valve directs the blood flow of the inferior vena cava from the right atrium to the left through the functioning foramen ovale. From the left atrium, blood flows into the left ventricle, and from there into the aorta. From the ascending aortic arch, blood enters the vessels of the head and upper body.

Deoxygenated blood, entering the right atrium from the superior vena cava, flows into the right ventricle, and from it into the pulmonary arteries. From the pulmonary arteries, only a small part of the blood enters the non-functioning lungs. The bulk of blood from the pulmonary artery is directed through the arterial (botal) duct to the descending aortic arch. Blood from the descending aortic arch supplies the lower half of the body and lower extremities. After this, oxygen-poor blood flows through the branches of the iliac arteries into the paired arteries of the umbilical cord and through them into the placenta.

Volume distribution of blood in fetal circulation look like this: approximately half of the total blood volume from the right side of the heart enters through the foramen ovale into the left side of the heart, 30% is discharged through the ductus arteriosus into the aorta, 12% enters the lungs. This distribution of blood is of very great physiological importance from the point of view of the individual organs of the fetus receiving blood rich in oxygen, namely, purely arterial blood is contained only in the umbilical cord vein, in the venous duct and liver vessels; mixed venous blood containing sufficient oxygen is found in the inferior vena cava and the ascending aortic arch, so the liver and upper body of the fetus are better supplied with arterial blood than the lower half of the body. Subsequently, as pregnancy progresses, there is a slight narrowing of the oval opening and a decrease in the size of the inferior vena cava. As a result, in the second half of pregnancy, the imbalance in the distribution of arterial blood decreases somewhat.

Physiological features of fetal blood circulation are important not only from the point of view of supplying it with oxygen. Fetal blood circulation is no less important for the implementation of the most important process of removing CO2 and other metabolic products from the fetal body. The anatomical features of the fetal circulation described above create the prerequisites for the implementation of a very short route for the elimination of CO2 and metabolic products: aorta - umbilical cord arteries - placenta.

Fetal cardiovascular system has pronounced adaptive reactions to acute and chronic stressful situations, thereby ensuring an uninterrupted supply of oxygen and essential nutrients to the blood, as well as the removal of CO2 and metabolic end products from the body. This is ensured by the presence of various neurogenic and humoral mechanisms that regulate heart rate, stroke volume, peripheral constriction and dilatation of the ductus arteriosus and other arteries. In addition, the fetal circulatory system is in close relationship with the hemodynamics of the placenta and mother. This relationship is clearly visible, for example, when compression syndrome of the inferior vena cava occurs. The essence of this syndrome is that in some women at the end of pregnancy, compression of the inferior vena cava and, apparently, partly of the aorta, occurs by the uterus. As a result, when a woman lies on her back, a redistribution of blood occurs, with a large amount of blood retained in the inferior vena cava, and blood pressure in the upper body decreases. Clinically, this is expressed in the occurrence of dizziness and fainting. Compression of the inferior vena cava by the pregnant uterus leads to circulatory disorders in the uterus, which in turn immediately affects the condition of the fetus (tachycardia, increased motor activity). Thus, consideration of the pathogenesis of inferior vena cava compression syndrome clearly demonstrates the presence of a close relationship between the maternal vascular system, hemodynamics of the placenta and fetus.

Fetal circulation has certain peculiarities (Fig. 51).

Figure 51. Scheme of the fetal blood circulation: 1 - placenta; 2 -- umbilical arteries; 3 -- umbilical vein; 4 -- portal vein; 5 -- venous duct; 6 -- inferior vena cava; 7 -- oval hole; 8 -- superior vena cava; 9 -- ductus arteriosus; 10 -- aorta; 11 -- hypogastric arteries.

Oxygen from atmospheric air first penetrates into the mother's blood through the lungs, where gas exchange occurs for the first time. The second time gas exchange occurs in the placenta. During the prenatal period, fetal breathing occurs through the placenta - placental respiration .

Wherein Fetal and maternal blood do not mix . Through the placenta, the fetus receives nutrients and removes waste. From the placenta, blood flows to the fetus through the umbilical vein. As we know, veins are vessels that bring blood. In this case flows through the umbilical vein not venous, but arterial blood - this is the only exception to the rule. In the fetal body, vessels (venous capillaries of the liver) depart from the umbilical vein, feeding the liver, which receives the blood richest in oxygen and nutrients. The bulk of the blood from the umbilical vein through venous - Arantsiev - flow (G.C. Aranzi, 1530--1589, Italian anatomist and surgeon) enters the inferior vena cava. Here the arterial blood mixes with the venous blood of the inferior vena cava - first mixing . Then the mixed blood enters the right atrium and, practically without mixing with the blood coming from the superior vena cava, enters the left atrium through the open foramen ovale (window) between the atria. The valve of the inferior vena cava prevents the mixing of blood in the right atrium. Next, the mixed blood enters the left ventricle and aorta. The coronary arteries branch off from the aorta and supply the heart. The ascending aorta gives off the brachiocephalic trunk, subclavian and carotid arteries. The brain and upper extremities receive sufficiently oxygenated and nutrient-rich blood. In the descending part of the aorta there is a second connection (communication) between the large and small circles of blood circulation - arterial - Botallov - duct (L. Botallo, 1530-1600, Italian surgeon and anatomist), which connects the aorta and pulmonary artery. Here, blood is discharged from the pulmonary artery (blood from the superior vena cava - right atrium - right ventricle) into the aorta - second mixing blood. Internal organs (except the liver and heart) and lower extremities receive the least oxygenated blood with the lowest nutrient content. Therefore, the lower part of the body and legs are developed to a lesser extent in a newborn baby. They arise from the common iliac arteries umbilical arteries along which it flows deoxygenated blood to the placenta.

Between the greater and lesser circulation there are two anastomoses (connections) - the venous (Arantius) duct and the arterial (Botallov) duct. According to this anastomosis blood is drained by pressure gradient from the pulmonary circulation to the systemic . Since in the prenatal period the fetal lungs are not functioning , they are in a collapsed state, including the vessels of the pulmonary circulation. Therefore, the resistance to blood flow in these vessels is large and blood pressure in the pulmonary circulation is higher than in the large circulation .

After birth the child begins to breathe, with the first breaths the lungs expand, the resistance of the vessels of the pulmonary circulation decreases, the blood pressure in the circulatory circles equalizes. Therefore, blood discharge no longer occurs; the anastomoses between the circulation circles are closed first functionally and then anatomically. The round ligament of the liver is formed from the umbilical vein, the venous ligament is formed from the venous (Arantsian) duct, the ligament arteriosus is formed from the ductus arteriosus (Botallova), and the medial umbilical ligaments are formed from the umbilical arteries. The foramen ovale overgrows and turns into an oval fossa. Anatomically, the arterial (Botallov) duct closes by 2 months of life, the oval window - by 5-7 months of life. If closure of these anastomoses does not occur, a heart defect is formed.

The heart of a newborn occupies a fairly large volume of the chest and a higher position than in adults, which is associated with the high position of the diaphragm. The ventricles are not sufficiently developed in relation to the atria, the thickness of the walls of the left and right ventricles is the same - the ratio is 1:1 (at 5 years old - 1:2.5, at 14 years old - 1:2.75).

The myocardium in newborns has signs embryonic structure : muscle fibers are thin, poorly divided, have a large number of oval nuclei, and there is no striation. The connective tissue of the myocardium is poorly expressed, there are practically no elastic fibers. The myocardium has a very good blood supply with a well-developed vascular network. The nervous regulation of the heart is imperfect, which causes quite frequent dysfunctions in the form of embryocardia, extrasystole, and respiratory arrhythmia.

With age, myofibril striations appear, connective tissue develops intensively, muscle fibers thicken, and by the beginning of puberty, myocardial development, as a rule, ends.

The arteries in children are relatively wider than in adults. Their lumen is even larger than the lumen of the veins. But, since veins grow faster than arteries, by the age of 15 the lumen of the veins becomes twice as large as the arteries. Vascular development is largely completed by age 12.

Cardiovascular examination plan

I. Complaints.

Pain in the heart area (localization, nature, irradiation, time of occurrence, connection with physical and/or emotional stress);

Feeling of “interruptions” in the work of the heart, palpitations (intensity, duration, frequency, conditions of occurrence);

Shortness of breath (conditions of occurrence: at rest or during physical activity, difficulty inhaling and/or exhaling);

Paleness, cyanosis of the skin (localization, prevalence, conditions of appearance);

Presence of edema (localization, time of appearance during the day);

Presence of rashes (erythema annulare, rheumatic nodules, butterfly rash on the face);

Pain and swelling in the joint area (localization, symmetry, severity, duration);

Restriction or difficulty in movements in joints (localization, time of appearance during the day, duration);

Retarded physical development;

Frequent colds, pneumonia;

The presence of attacks with loss of consciousness, cyanosis, shortness of breath, convulsions;

II. Objective research.

1.Inspection:

Assessment of physical development;

Proportionality of development of the upper and lower halves of the body;

-skin examination:

Ø color (in the presence of pallor, cyanosis, marbled pattern - indicate localization, prevalence, conditions of occurrence);

Øpresence of rashes (erythema annulare, rheumatic nodules, “butterfly” symptom on the face);

Øseverity of the venous network on the head, chest, abdomen, limbs;

Examination of fingers (presence of “drumsticks”, “watch glasses”);

The presence of shortness of breath (difficulty in inhaling, exhaling, participation of auxiliary muscles, conditions of occurrence - at rest or during physical activity);

Pulsation of neck vessels (arterial, venous);

Symmetry of the chest, the presence of a “heart hump”;

The presence of cardiac pulsation, pulsation of the base of the heart;

The presence of epigastric pulsation (ventricular or aortic);

-apical impulse:

Localization (along intercostal spaces and lines);

Øarea (in square centimeters);

Presence of edema (localization, prevalence).

2.Palpation:

Cardiac impulse (presence, localization, prevalence);

Apex beat (localization, prevalence, resistance, height);

Systolic or diastolic tremor (presence, location, prevalence);

Pulsation of peripheral arteries (symmetry, frequency, rhythm, filling, tension, shape, size):

Øradial arteries;

Øcarotid arteries;

Øfemoral arteries;

Arteries of the dorsum of the foot;

Study of venous pulsation (on the jugular veins);

The presence of edema (on the lower extremities, face; in infants - in the sternum, abdomen, lower back, sacrum, scrotum in boys);

Palpation of the liver (size, pain, consistency);

Pulsation of blood vessels in the skin of the back (below the angles of the shoulder blades).

3.Percussion:

Borders of relative dullness of the heart (right, upper, left);

Borders of absolute dullness of the heart (right, upper, left);

Width of the vascular bundle (symptom of the Philosopher's cup);

Diameter of relative and absolute dullness of the heart (in cm).

4. Auscultation.

A. Auscultation of the heart - It is carried out in an upright position of the child, lying on his back. In the presence of auscultatory changes - lying on the left side, in school-age children - at the height of inspiration, at the height of exhalation, after moderate physical activity (Shalkov tests No. 1 - 6).

When listening to 5 standard points, the entire heart area, left axillary, subscapular, interscapular areas must be described:

Heart rate;

Rhythm of tones;

Number of tones;

Strength (loudness) of I and II tones at each point;

The presence of splitting, bifurcation of the first and/or second tone (at what points, in what position of the child);

-If there are pathological noises, characterize them:

Øsystolic and/or diastolic;

Østrength, duration, timbre, character (increasing or weakening);

Øprevalence and places of best listening;

ØSirradiation outside the heart - to the left axillary, subscapular, interscapular areas, to the area of ​​​​the vessels of the neck;

Ødependence on body position;

Ødynamics after physical activity;

Pericardial friction rub (presence, location, prevalence).

B. Auscultation of blood vessels(in the presence of pathological noises, indicate location, intensity, nature):

Arteries (aorta, carotid arteries, subclavian arteries, femoral arteries);

Jugular veins.

B. Blood pressure measurement(systolic and diastolic):

On the arms (left and right);

On your feet (left and right).

5. Conducting functional tests:

Clino-orthostatic (Martine);

Orthostatic (Shellonga);

Differentiated tests according to Shalkoff;

Tests with breath holding on inhalation (Shtange) and exhalation (Gencha).

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The blood circulation of the fetus has several features.

• One of them is that the placenta performs the function of the lung.
• Oxygenated blood enters the fetus from the placenta through the umbilical vein.
• Approximately 50% of the blood passes through the liver, and from there, through the characteristic ductus venosus of the fetus, it enters the inferior vena cava. The remaining blood from the umbilical vein (highly oxygenated) flows directly into the inferior vena cava
• From the latter, the part of the blood divided by the crista dividens is directed through the oval window inherent in the fetus to the left atrium.
• Blood from the superior vena cava enters the right atrium, right ventricle and pulmonary trunk.
• In the fetus, in the absence of breathing, the pulmonary arterioles create great resistance to blood flow. As a result, blood from the pulmonary trunk flows through the wide ductus arteriosus into the aorta, where during this period the blood pressure is lower than in the pulmonary trunk.
• The effective cardiac output of the fetus is the sum of the left ventricular output and the minute volume of blood flowing through the ductus arteriosus, and reaches 220 ml/(kg.min).
• About 65% of this blood returns to the placenta, and the remaining 35% of the blood perfuses the newborn's organs and tissues. (Fig. 18.4).

The upper end of the inferior vein communicates directly with the left atrium through the foramen ovale (see inset) and with the right atrium.

RA and RV - right atrium and ventricle;
LA and LV - left atrium and ventricle;
SVC - superior vena cava;
IVC - inferior vena cava;
AP - ductus arteriosus;
VP - ductus venosus;
OO - foramen ovale.

Features of the regulation of blood circulation in the fetus and newborns

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As for the peculiarities of the regulation of fetal blood circulation, the first half of pregnancy is characterized by the dominance of humoral rather than neuronal adrenergic mechanisms. As the fetus matures, both sympathetic and parasympathetic regulation increases. For example, atropine administered to a woman at different stages of pregnancy, due to its blockade of cholinergic fibers, promotes a progressive increase in the fetal heart rate. This means that during maturation the cholinergic regulation of the heart increases.

From the moment of the first breath, the resistance in the blood vessels of the lungs decreases by 7 times and blood flow to the left atrium improves. As a result, the pressure in the left atrium increases and the passage of blood through the foramen ovale is difficult. Functional closure of the oval window usually occurs by 3 months of age, but in 25% of adults during cardiac catheterization, the probe can be passed through the tissue covering it. In response to hypoxia in the newborn, the pulmonary vessels narrow, which leads to a decrease in blood flow to the left atrium and a drop in pressure in it. Blood again begins to pass through the oval window from the right atrium to the left, which leads to deepening hypoxia. In addition, it causes patent ductus arteriosus.

Normal in a newborn, due to the opening of the pulmonary vessels and the beginning of breathing, there is no need not only for the oval window, but also for the ductus arteriosus. The functional closure of the latter is usually completed by the 10th-15th hour of life.

The ductus arteriosus differs from the aorta of the pulmonary trunk in a large number of circularly located muscle fibers. In the fetus, maintaining the duct open is associated with the presence of prostaglandins in the blood. The main factor causing its closure in a newborn is oxygen. If the PO 2 of the blood passing through the duct reaches 50 mm Hg, it narrows. The age of the fetus at the time of birth also plays an important role: the walls of the ductus arteriosus of premature infants are less sensitive to the effects of oxygen, even with a developed muscle layer. Consequently, in premature children or those born in hypoxic conditions, the risk of patent ductus arteriosus and conus ovale increases.

Newborn heart weight relative to his body weight, almost twice that of an adult. The relative value of the IOC has the same pattern, which is explained by the need to compensate for the child’s high energy metabolism, future breathing and heart rate. The decrease in the relative value of IOC with age is due to a decrease in heart rate, an increase in the total peripheral vascular resistance in the systemic circulation and a decrease in central venous pressure.

The functional state of the circulatory system of a newborn is also affected by the characteristics of his physique. The relative size of the head (relative to the size of the body) is 4 times greater than that of an adult, and the relative length of the lower limbs is half that of adults. This leads to the fact that the proportion of IOC in the vessels of the descending aorta system in newborns is 40%, while in adults it is 75%. As a result, constriction of the vessels of the descending aorta in a newborn does not cause such a pronounced pressor reaction as in an adult.

Reaction of the cardiovascular system of a newborn to an orthostatic test(rapid change in body position from horizontal to vertical) differs from the reaction of an adult. If in an adult the transition to a vertical position is accompanied by accumulation of blood in the lower extremities and a slight decrease in venous return, then in a newborn the venous return may even increase, because short lower limbs do not allow centrifugal forces acting in the head-leg direction to significantly reduce central venous pressure, and the outflow of blood from a relatively large head even causes an increase in this pressure and venous return.

Capillary filtration coefficient in newborns it is twice as high as in adults. In premature newborns it may be even greater. There are several reasons for high capillary filtration in newborns: dilatation of arterioles, high capillary density, high venous pressure, relatively large plasma volume, low plasma protein content, and high levels of tissue metabolism. Central venous pressure in a newborn is higher than in an adult, which is due to the weak distensibility of the veins, their narrow lumen, large volume of plasma, high heart rate (the heart does not have time to fill with blood as much as with a lower heart rate and, accordingly, prolonged diastole) .

In the early stages of postnatal ontogenesis, the heart continues to remain under the dominant influence of sympathetic nerves. However, parasympathetic influences gradually increase during the development of the child. Thus, when atropine is administered to a newborn child, the heart rate increases by 15%, while in adults, at appropriate dosages, it increases by 80%. The weak influence of the vagus nerve on the heart of a newborn is associated not only with the immaturity of central regulation, but also with the instability of acetylcholine synthesis in presynaptic plaques.

The decrease in heart rate observed with age is based on increased influence of parasympathetic fibers, stimulation of vascular mechanoreceptors by increasing blood pressure levels, increasing activity of skeletal muscles, leading to increased influence of the vagus nerve. Thus, the heart rate of a 7-8 month old child is about 120 beats/min instead of 140-150 beats/min in a newborn, which is explained by the formation of a sitting posture during this period. The influence of the vagus nerve on the heart is even more pronounced due to the implementation of the standing pose at 9-12 months.

In the process of age-related development, the thickness of the wall of large elastic arteries increases, and the walls of muscular-type vessels thicken. As a result, the stiffness of blood vessels increases and the speed of propagation of the pulse wave increases.

In newborns, the reninangiotensive system is a more important mechanism for regulating blood pressure than the baroreceptor reflex. There are two points of view regarding the role of vascular chemoreceptors: the more common one is that in the neonatal period they have the same excitability as in an adult; the other is that chemoreceptors, sensitive to carbon dioxide tension in the blood, mature gradually.

Increasing constriction of arterioles underlies the characteristic tendency of ontogenetic development - a gradual increase in blood pressure from birth to adolescence. Determinants of blood pressure in the age aspect are also the characteristics of the genotype, the phenomenon of acceleration, and the level of puberty. The most significant determinants of blood pressure in children and adolescents are body length and weight. At the same calendar age, blood pressure will be higher in individuals with greater body length and weight. The norm of blood pressure during these periods of ontogenesis is purely individual and often does not coincide with generally accepted standards.

Low vascular resistance to blood flow in children, weakly expressed reactions of their tone to external stimuli do not contribute to maintaining homeostasis. In particular, even with slight cooling, heat transfer increases sharply due to the fact that the skin vessels remain dilated. Rapid improvement of vasomotor reactions to external stimuli begins at 6 years of age. Their development can be accelerated by hardening procedures. Vasomotor reactions from uneconomical generalized ones at this age become more local; at an early age, the activity of a certain muscle group begins to involve the blood vessels of many non-working muscles in the working hyperemia.

From 7-8 years of age, children experience a pre-start reaction of the circulatory system: Even before muscle work begins, the heart rate increases and blood pressure rises. This indicates the appearance in the circulatory system of conditioned reflex reactions, which become more pronounced in the process of further ontogenetic development. At the same time, the child’s body, even under conditions of systematic physical training, does not acquire the economization of the functions of the cardiovascular system that is characteristic of adults.

Changes in blood circulation during adolescence

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Pronounced changes in blood circulation occur during adolescence, which is one of the critical stages of development.

The mass of the heart and the size of its chambers increase faster than the diameter of blood vessels. The lumen of the vessels relative to the size of the heart at this age is also small because, as a result of an abrupt increase in body length, the vessels are stretched. Myocardial growth in adolescents outpaces valve growth, which leads to transient valve insufficiency. It is enhanced by the asynchrony of the work of the papillary muscles of the myocardium. These features of the development of the heart and blood vessels in adolescents affect the nature of blood flow and contribute to the appearance of functional heart murmurs. Due to the acceleration phenomenon, in many adolescents the rate of heart development lags behind the characteristics of physical development (body length and weight, chest circumference). At the same time, despite high rates of physical development, adaptive reactions of the cardiovascular system may be inadequate to the power of physical activity.

During puberty andrenergic regulation of the circulatory system is enhanced. The endocrine system also plays an important role in regulating the heart and blood vessels. For example, the proper development of the heart is facilitated by the gonadotropic function of the pituitary gland and the level of sex hormones in the blood (hypophysectomy in experimental animals leads to a decrease in heart weight in relation to body weight). In adolescence, gender differences in the cardiovascular system intensify - the myocardium of teenage boys is characterized by greater functionality than that of girls. In girls, due to the menstrual cycle, there is a premenstrual increase in systolic blood pressure and a decrease in heart rate. The value of blood pressure in girls reaches adult levels earlier than in boys (approximately 3.5 years after the appearance of the first menstruation).

During the teenage spurt of body length, a transient increase in heart rate may be observed. Its adult level is established at the end of adolescence; Girls' heart rates are 10% higher than boys'. The slower heart rate in the latter is associated with larger heart sizes and greater force of heart contractions, as well as more pronounced parasympathetic regulation of the heart.

Adaptive changes in the cardiovascular system associated with muscle load are improved in adolescents mainly due to an increase in heart rate, while the stroke volume of blood changes slightly.

Despite the fact that by adolescence the role of the muscle pump increases and the phases of the cardiac cycle lengthen, especially diastole, and thereby create favorable conditions for filling the heart with blood and implementing the Starling mechanism, the relative value of the IOC decreases. Its decrease is due to a decrease in heart rate, an increase in the total peripheral resistance of arterial vessels (due to the growth of the muscle layer in the arterioles and a delay in relation to the size of the heart in the increase in the diameter of the arterial vessels), a decrease in the relative amount of circulating blood and the relative mass of the heart. In general, the magnitude of the increase in IOC does not keep pace with the increase in body weight.

Heart development. The heart develops from two symmetrical rudiments, which then merge into one tube located in the neck. Due to the rapid growth of the tube in length, it forms an S-shaped loop). The first contractions of the heart begin at a very early stage of development, when muscle tissue is barely visible. In the S-shaped cardiac loop, there is an anterior arterial, or ventricular, part, which continues into the truncus arteriosus, which is divided into two primary aortas, and a posterior venous, or atrial, into which the vitelline-mesenteric veins flow, vv. omphalomesentericae. At this stage, the heart is single-cavity; its division into right and left halves begins with the formation of the atrial septum. By growing from top to bottom, the septum divides the primary atrium into two - left and right, and in such a way that subsequently the confluence of the vena cava is in the right, and the pulmonary veins are in the left. The atrial septum has a hole in the middle, foramen ovale, through which in the fetus part of the blood from the right atrium flows directly into the left. The ventricle is also divided into two halves by a septum, which grows from below towards the atrial septum, without, however, completing the complete separation of the ventricular cavities. Outside, corresponding to the boundaries of the ventricular septum, grooves appear, sulci interventriculares. Completion of the formation of the septum occurs after the truncus arteriosus, in turn, is divided by the frontal septum into two trunks: the aorta and the pulmonary trunk. The septum dividing the truncus arteriosus into two trunks, continuing into the ventricular cavity towards the ventricular septum described above and forming pars membranacea septi interventriculare, completes the separation of the ventricular cavities from each other.

Blood circulation of the fetus and newborn. During intrauterine development, the fetal blood circulation goes through three successive stages: vitelline, allantoic and placental.

The yolk period of development of the human circulatory system is very short - from the moment of implantation to the 2nd week of the embryo’s life. Oxygen and nutrients enter the embryo directly through trophoblast cells, which do not yet have blood vessels during this period of embryogenesis. A significant portion of the nutrients accumulates in the yolk sac, which also has its own meager reserves of nutrients. From the yolk sac, oxygen and necessary nutrients travel through the primary blood vessels to the embryo. This is how yolk blood circulation occurs, which is inherent in the earliest stages of ontogenetic development.

Allantoic circulation begins to function approximately from the end of the 8th week of pregnancy and continues for 8 weeks, i.e. until the 15-16th week of pregnancy. The allantois, which is a protrusion of the primary intestine, gradually grows to the avascular trophoblast, carrying with it the fetal vessels. When the allantois comes into contact with the trophoblast, the fetal vessels grow into the avascular villi of the grophoblast, and the chorion becomes vascular. The establishment of allantoic blood circulation is a qualitatively new stage in the intrauterine development of the embryo, since it allows for wider transport of oxygen and necessary nutrients from mother to fetus.

Placental circulation replaces allantoic circulation. It begins in the 3-4th month of pregnancy and reaches its peak at the end of pregnancy. The formation of placental blood circulation is accompanied by the development of the fetus and all functions of the placenta (respiratory, excretory, transport, metabolic, barrier, endocrine, etc.).

Venous blood entering the right atrium from the superior vena cava flows into the right ventricle, and from it into the pulmonary arteries. From the pulmonary arteries, only a small part of the blood enters the non-functioning lungs. The bulk of blood from the pulmonary artery is directed through the arterial (botal) duct to the descending aortic arch. Blood from the descending aortic arch supplies the lower half of the body and lower extremities. After this, oxygen-poor blood flows through the branches of the iliac arteries into the paired arteries of the umbilical cord and through them into the placenta.

The volume distribution of blood in the fetal circulation is as follows: approximately half of the total blood volume from the right side of the heart enters through the foramen ovale into the left side of the heart, 30% is discharged through the ductus arteriosus into the aorta, 12% enters the lungs. This distribution of blood is of very great physiological importance from the point of view of the individual organs of the fetus receiving blood rich in oxygen, namely, purely arterial blood is contained only in the umbilical cord vein, in the venous duct and liver vessels; mixed venous blood containing sufficient oxygen is found in the inferior vena cava and the ascending aortic arch, so the liver and upper body of the fetus are better supplied with arterial blood than the lower half of the body. Subsequently, as pregnancy progresses, there is a slight narrowing of the oval opening and a decrease in the size of the inferior vena cava. As a result, in the second half of pregnancy, the imbalance in the distribution of arterial blood decreases somewhat.