Circulation of the fetus and newborn. Yolk period. Allantoid circulation. placental circulation.

During fetal development, fetal circulation goes through three successive stages: yolk, allantoid and placental.

The yolk period of the 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 the cells of the trophoblast, which do not yet have vessels during this period of embryogenesis. Much of the nutrients are stored in the yolk sac, which also has its own meager supply of nutrients. From the yolk sac, oxygen and the necessary nutrients travel through the primary blood vessels to the embryo. This is how the yolk circulation is carried out, which is inherent in the earliest stages of ontogenetic development.

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

placental circulation comes to replace the allantoid. It begins at the 3-4th month of pregnancy and reaches its peak at the end of pregnancy. The formation of placental 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 implantation 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 very different from that of a newborn. This is determined by both the anatomical and functional features of the fetal body, reflecting its adaptive processes during intrauterine life.

The anatomical features of the fetal cardiovascular system primarily consist in the existence of an oval opening between the right and left atria and the arterial duct connecting the pulmonary artery to the aorta. This allows a significant amount of blood to bypass 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 the blood, enriched with oxygen and containing all the necessary nutrients, enters the umbilical cord vein.

Then arterial blood through venous (arantian) duct enters the liver. The fetal liver is a kind of blood depot. In the deposition of blood, its left lobe plays the greatest role. From the liver, through the same venous duct, blood enters 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 is the 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 a functioning foramen ovale. From the left atrium, blood enters the left ventricle, and from there to 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 the blood from the pulmonary artery through the arterial (botallian) duct is directed to the descending aortic arch. The blood of the descending aortic arch supplies the lower half of the trunk and lower limbs. After that, blood, poor in oxygen, through the branches of the iliac arteries enters the paired arteries of the umbilical cord and through them into the placenta.

Volumetric distributions of blood in fetal circulation look like this: approximately half of the total blood volume from the right heart enters through the foramen ovale to the left heart, 30% through the arterial (botall) duct is discharged into the aorta, 12% enters the lungs. Such a distribution of blood is of great physiological importance from the point of view of obtaining oxygen-rich blood by individual organs of the fetus, namely, purely arterial blood is found only in the vein of the umbilical cord, in the venous duct and vessels of the liver; mixed venous blood, containing a sufficient amount of oxygen, is located in the inferior vena cava and the ascending aortic arch, so the liver and upper body of the fetus are supplied with arterial blood better than the lower half of the body. In the future, as pregnancy progresses, there is a slight narrowing of the foramen ovale 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 the fetal circulation are important not only from the point of view of supplying it with oxygen. The fetal circulation is of no less importance for the implementation of the most important process of removing CO2 and other metabolic products from the body of the fetus. The anatomical features of the fetal circulation described above create the prerequisites for the implementation of a very short path for removing CO2 and metabolic products: aorta - umbilical cord arteries - placenta.

Fetal cardiovascular system has pronounced adaptive responses 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 of the heart, 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, in the event of a syndrome of compression of the inferior vena cava. The essence of this syndrome lies in the fact that in some women at the end of pregnancy there is compression of the inferior vena cava by the uterus and, apparently, partially of the aorta. As a result, in the position of a woman on her back, her blood is redistributed, while a large amount of blood is 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 the syndrome of compression of the inferior vena cava clearly demonstrates the presence of a close relationship between the vascular system of the mother, hemodynamics of the placenta and fetus.

Fetal circulation has certain peculiarities (Fig. 51).

Figure 51. Fetal circulation scheme: 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 the atmospheric air first enters 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 intrauterine period, the fetus breathes through the placenta - placental respiration .

Wherein fetal blood and mother's blood do not mix . Through the placenta, the fetus receives nutrients and removes toxins. From the placenta, blood flows to the fetus through the umbilical vein. As we know, veins are blood vessels. In this case flows through the umbilical vein not venous, but arterial blood is the only exception to the rule. In the body of the fetus, vessels (venous capillaries of the liver) depart from the umbilical vein, feeding the liver, which receives the blood richest in oxygen and nutrients. Most of the blood from the umbilical vein 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 an open oval hole (window) between the atria. The valve of the inferior vena cava prevents mixing of blood in the right atrium. Then the mixed blood enters the left ventricle and aorta. The coronary arteries supply the heart from the aorta. In the ascending part of the aorta, the brachiocephalic trunk, subclavian and carotid arteries depart. The brain and upper limbs receive adequately 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. The internal organs (except the liver and heart) and the lower extremities receive the least oxygenated blood with a low nutrient content. Therefore, the lower part of the trunk and legs are developed in a newborn child to a lesser extent. from the common iliac arteries umbilical arteries through which flows deoxygenated blood to the placenta.

Between the large and small circles of blood circulation there are two anastomoses (connections) - the venous (Arantsiev) duct and the arterial (Botallov) duct. Through this anastomosis blood is shed along the pressure gradient from the pulmonary circulation to the systemic . Since in the intrauterine period fetal lungs do not function , 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 .

After birth the child begins to breathe, with the first breaths the lungs straighten out, the resistance of the vessels of the pulmonary circulation decreases, the blood pressure in the circulatory circles levels off. Therefore, the discharge of blood no longer occurs, the anastomoses between the circles of blood circulation are closed first functionally, and then anatomically. From the umbilical vein, the round ligament of the liver is formed, from the venous (Arantsiev) duct - the venous ligament, from the arterial (Botallov) duct - the arterial ligament, from the umbilical arteries - the medial umbilical ligaments. The oval hole overgrows and turns into an oval hole. Anatomically, the arterial (Botallov) duct closes by 2 months of life, the oval window - by 5-7 months of life. If these anastomoses do not close, a heart defect is formed.

The heart in a newborn occupies a fairly large volume of the chest, and a higher position than in adults, which is associated with a high standing of the diaphragm. The ventricles are underdeveloped 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).

Myocardium in newborns has signs embryonic structure : muscle fibers are thin, poorly separated, have a large number of oval nuclei, no striation. The connective tissue of the myocardium is weakly 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, respiratory arrhythmia.

With age, striation of myofibrils appears, connective tissue develops intensively, muscle fibers thicken, and myocardial development, as a rule, ends by the onset of puberty.

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

Cardiovascular Examination Plan

I. Complaints.

Pain in the region of the heart (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 appearance - at rest or during physical exertion, inhalation and (and) exhalation is difficult);

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

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

The presence of rashes (annular erythema, rheumatic nodules, rash in the form of a butterfly on the face);

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

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

Lagging behind in physical development;

Frequent colds, pneumonia;

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

II. Objective research.

1.Inspection:

Assessment of physical development;

The proportionality of the development of the upper and lower halves of the body;

-skin examination:

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

Presence of rashes (annular erythema, rheumatic nodules, butterfly symptom on the face);

Ø the severity of the venous network on the head, chest, abdomen, limbs;

Inspection of the fingers (the presence of "drumsticks", "watch glasses");

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

Pulsation of the vessels of the neck (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);

-top push:

Ølocalization (along intercostal spaces and lines);

Ø area (in square centimeters);

The presence of edema (localization, prevalence).

2. Palpation:

Cardiac impulse (presence, localization, prevalence);

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

Systolic or diastolic trembling (presence, localization, prevalence);

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

Ø radial arteries;

Ø carotid arteries;

Ø femoral arteries;

Charters of the rear of the foot;

Examination 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, texture);

Pulsation of the vessels of 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);

The width of the vascular bundle (symptom of Philosophov's bowl);

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

4. Auscultation.

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

When listening to 5 standard points, the entire region of the heart, left axillary, subscapular, interscapular regions needs to be characterized:

Heart rate;

Rhythm of tones;

Number of tones;

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

The presence of splitting, bifurcation of I or (and) II tone (at what points, what position of the child);

-in the presence of pathological noises, characterize them:

Ø systolic or (and) diastolic;

Ø strength, duration, timbre, character (increasing or waning);

Ø prevalence and places of the best listening;

Shirradiation outside the heart - to the left axillary, subscapular, interscapular region, to the region of the vessels of the neck;

Ø dependence on the position of the body;

Ø dynamics after physical activity;

Rubbing noise of the pericardium (presence, localization, prevalence).

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

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

Jugular veins.

B. Blood pressure measurement(systolic and diastolic):

On the hands (left and right);

Legs (left and right).

5. Carrying out functional tests:

Klino-orthostatic (Martinet);

Orthostatic (Shellong);

Differentiated samples according to Shalkov;

Samples with breath holding on inhalation (Bar) and on exhalation (Gencha).

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Fetal circulation has several features.

• One of them is that the function of the lung is performed by the placenta.
• Oxygenated blood flows from the placenta to the fetus through the umbilical vein.
• Approximately 50% of the blood passes through the liver, and from there, through the venous duct characteristic of the fetus, it enters the inferior vena cava. The rest of the umbilical vein blood (with high oxygen saturation) flows directly into the inferior vena cava
• From the last divided crista dividens part of the blood through the oval window inherent in the fetus goes 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 respiration, the pulmonary arterioles create great resistance to blood flow. As a result, blood from the pulmonary trunk enters the aorta through the wide arterial (botallian) duct, 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 perfuse the organs and tissues of the newborn. (Fig. 18.4).

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

PP and RV - right atrium and ventricle;
LP and LV - left atrium and ventricle;
SVC - superior vena cava;
IVC - inferior vena cava;
AP - ductus arteriosus;
VP - venous duct;
OO - oval hole.

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

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As for the regulation of fetal 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 the blockade of cholinergic fibers by it, contributes to a progressive increase in the heart rate in the fetus. This means that in the process of maturation, the cholinergic regulation of the heart is enhanced.

From the moment of the first breath, the resistance in the 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 foramen ovale usually occurs by 3 months of age, but in 25% of adults, cardiac catheterization can lead the probe through the tissue covering it. In response to hypoxia of the newborn, the vessels of the lungs 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 a deepening of hypoxia. In addition, it causes cleft 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 arterial duct. The functional closure of the latter is usually completed by the 10th-15th hour of life.

The arterial duct differs from the aorta of the pulmonary trunk by a large number of circularly arranged muscle fibers. In the fetus, keeping 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 RO 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 arterial duct of premature babies are less sensitive to oxygen, even with a developed muscular layer. Therefore, in premature or hypoxic children, the risk of non-closure of the ductus arteriosus and oval cona 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 high energy metabolism of the child, the future respiratory rate and heart rate. The decrease with age in the relative value of the IOC 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 newborns is also affected by the features of his physique. The relative dimensions of the head (in relation to the size of the body) are 4 times those 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 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.

The reaction of the cardiovascular system of the newborn to the orthostatic test(rapid change in body position from horizontal to vertical) is different from the reaction of an adult. If in an adult the transition to a vertical position is accompanied by an 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 twice as high as in adults. In premature newborns, it can be even more. There are several causes of high capillary filtration in neonates: dilated arterioles, high capillary density, high venous pressure, relatively large plasma volume, low protein content, and high tissue metabolism. The central venous pressure in a newborn is higher than in an adult, which is due to the weak extensibility of the veins, their narrow lumen, large plasma volume, high heart rate (the heart does not have time to fill with blood as with a rarer heart rate and, accordingly, prolonged diastole) .

In the early stages of postnatal ontogenesis, the heart continues to be dominated by sympathetic nerves. However, parasympathetic influences gradually increase during the development of the child. So, with the introduction of atropine to a newborn child, the heart rate increases by 15%, while in adults with 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 the central regulation, but also with the instability of the synthesis of acetylcholine in presynaptic plaques.

The decrease in heart rate observed with age is based on increased influence of parasympathetic fibers, stimulation of vascular mechanoreceptors by an increasing level of blood pressure, increasing activity of skeletal muscles, leading to increased influence of the vagus nerve. So, the heart rate of a child 7-8 months old 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 posture at 9-12 months.

In the process of age-related development, the thickness of the wall of large elastic arteries increases, the walls of the vessels of the muscular type thicken. As a result, the stiffness of the 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 they have the same excitability in the neonatal period as in an adult; the other is that the chemoreceptors, which are sensitive to the tension of carbon dioxide in the blood, mature gradually.

Increasing constriction of arterioles underlies the characteristic trend of ontogenetic development - a gradual increase in blood pressure from birth to adolescence. The determinants of AD in the age aspect are also the features of the genotype, the phenomenon of acceleration, the level of puberty. The most significant determinants of BP 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.

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

From 7-8 years old, children have a pre-launch reaction of the circulatory system: even before the start of muscle work, heartbeats become more frequent and blood pressure rises. This indicates the appearance of conditioned reflex reactions in the circulatory system, 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, which is typical for adults.

Circulatory changes during adolescence

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Pronounced changes in blood circulation occur in 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 the 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 valvular growth, leading to transient valvular 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 the blood flow and contribute to the appearance of functional heart murmurs. In connection with the phenomenon of acceleration 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 the high rates of physical development, the 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 the regulation of the heart and blood vessels. For example, the gonadotropic function of the pituitary gland and the level of sex hormones in the blood contribute to the proper development of the heart (hypophysectomy in experimental animals leads to a decrease in heart mass relative to body weight). In adolescence, gender differences in the cardiovascular system increase - the myocardium of adolescent boys is characterized by greater functionality than that of girls. In girls, in connection with the menstrual cycle, there is a premenstrual rise in systolic blood pressure and a decrease in heart rate. The value of blood pressure in girls reaches the adult level earlier than in boys (approximately 3.5 years after the onset of the first menstruation).

During the adolescent spurt of body length, a transient increase in heart rate may be observed. Its adult level is established at the end of adolescence; girls have a 10% higher heart rate than boys. The slower rate of heart contractions 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 rearrangements of the cardiovascular system associated with muscle load are improved in adolescents mainly due to an increase in heart rate, while stroke volume changes slightly.

Despite the fact that by adolescence the role of the muscle pump increases and the phases of the cardiac cycle, especially diastole, are lengthened, and thus favorable conditions are created 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 arterioles and a delay in relation to the size of the heart of an increase in the diameter of 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 the IOC does not keep pace with the increase in body weight.

Development of the heart. 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, the anterior arterial, or ventricular, part is distinguished, which continues into the truncus arteriosus, which divides into two primary aortas, and the posterior venous, or atrial, into which the yolk-mesenteric veins flow, vv. omphalomesentericae. At this stage, the heart is single-cavity, dividing it into the 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 hollow veins is in the right, and the pulmonary veins - 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 enters directly into the left. The ventricle is also divided into two halves by a septum, which grows from below towards the atrial septum, without completing, however, the complete separation of the ventricular cavities. Outside, according to the boundaries of the septum of the ventricles, furrows appear, sulci interventriculares. The 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.

Circulation of the fetus and newborn. During intrauterine development, the fetal circulation goes through three successive stages: yolk, allantoid and placental.

The yolk period of the development of the circulatory system in humans 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 the cells of the trophoblast, which do not yet have vessels during this period of embryogenesis. Much of the nutrients are stored in the yolk sac, which also has its own meager supply of nutrients. From the yolk sac, oxygen and the necessary nutrients travel through the primary blood vessels to the embryo. This is how the yolk circulation is carried out, which is inherent in the earliest stages of ontogenetic development.

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

The placental circulation replaces the allantoid one. It begins at the 3-4th month of pregnancy and reaches its peak at the end of pregnancy. The formation of placental 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 the blood from the pulmonary artery through the arterial (botallian) duct is directed to the descending aortic arch. The blood of the descending aortic arch supplies the lower half of the trunk and lower limbs. After that, blood, poor in oxygen, through the branches of the iliac arteries enters the paired arteries of the umbilical cord and through them into the placenta.

The volumetric distributions of blood in the fetal circulation are as follows: approximately half of the total blood volume from the right parts of the heart enters the left parts of the heart through the foramen ovale, 30% is discharged through the ductus arteriosus into the aorta, 12% enters the lungs. Such a distribution of blood is of great physiological importance from the point of view of obtaining oxygen-rich blood by individual organs of the fetus, namely, purely arterial blood is found only in the vein of the umbilical cord, in the venous duct and vessels of the liver; mixed venous blood, containing a sufficient amount of oxygen, is located in the inferior vena cava and the ascending aortic arch, so the liver and upper body of the fetus are supplied with arterial blood better than the lower half of the body. In the future, as pregnancy progresses, there is a slight narrowing of the foramen ovale 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.