Functional system of mother-placenta-fetus

According to modern concepts, the single mother-placenta-fetus system that emerges and develops during pregnancy is a functional system. According to the theory of PK Anokhin, a dynamic system of structures and processes of an organism is considered to be a functional system, which involves individual components of the system, regardless of their origin. This is an integral formation that includes the central and peripheral links and operates on the principle of feedback. Unlike others, the mother-placenta-fetus system is formed only from the beginning of pregnancy and ends its existence after the birth of the fetus. It is the development of the fetus and its bearing up to the term of birth and is the main goal of the existence of this system.

Functional activity of the mother-placenta-fetus system has been studied for many years. At the same time, the individual links of this system were studied - the state of the mother's organism and adaptation processes in it, occurring during pregnancy, the structure and functions of the placenta, the processes of growth and development of the fetus. However, only with the advent of modern methods of intravital diagnosis (ultrasound, dopplerometry in the blood vessels in the mother's vessels, the placenta and fetus, a thorough evaluation of the hormonal profile, dynamic scintigraphy), as well as the improvement of morphological studies, were possible to establish the main stages of the Establishment and the principles of the functioning of a single fetoplacental system.

Features of the emergence and development of the new functional system of mother-placenta-fetus are closely related to the features of the formation of the provisional organ - the placenta. The human placenta refers to the hemochoric type, characterized by the presence of direct contact of the maternal blood and chorion, which contributes to the fullest implementation of complex interrelations between the mother and fetus organisms.

One of the leading factors that ensure the normal course of pregnancy, growth and development of the fetus, are hemodynamic processes in a single system of mother-placenta-fetus. The restructuring of the hemodynamics of the mother's body during pregnancy is characterized by an intensification of blood circulation in the vascular system of the uterus. Blood supply to the uterus with arterial blood is carried out by a number of anastomoses between the arteries of the uterus, the ovaries and the vagina. The uterine artery approaches the uterus at the base of the broad ligament at the level of the internal pharynx, where it divides into the ascending and descending branches (of the first order) located along the ribs of the vascular layer of the myometrium. From them, almost perpendicular to the uterus, there are 10-15 segmental branches (second order), due to which numerous radial arteries (of the third order) branch off. In the main layer of the endometrium, they are divided into basal arteries, supplying the lower third of the main part of the endometrium with blood, and spiral arteries that run up to the surface of the uterine mucosa. The outflow of venous blood from the uterus occurs through the uterine and ovarian plexuses. Morphogenesis of the placenta depends on the development of uteroplacental blood circulation, and not on the development of blood circulation in the fetus. The leading value is attached to the spiral arteries - the terminal branches of the uterine arteries.

Within two days after implantation the crushing blastocyst is completely immersed in the mucous membrane of the uterus (nidation). Nidation is accompanied by the proliferation of trophoblast and its transformation into a two-layered formation consisting of a cytotrophoblast and syncytial multinucleated elements. In the early stages of implantation, trophoblast, without significant cytolytic properties, penetrates between the cells of the superficial epithelium, but does not destroy it. Histophilic properties of trophoblast acquires in the process of contact with the mucous membrane of the uterus. Destruction of the decidua occurs as a result of autolysis, caused by the active activity of lysosomes of the uterine epithelium. On the 9th day of ontogenesis, small cavities appear in the trophoblast - lacunae, in which, due to the erosion of small vessels and capillaries, the blood of the mother enters. Heavy parts and trophoblast partitions that separate lacunas are called primary. By the end of the 2nd week of pregnancy (12-13th day of development), the connective tissue grows from the chorion to the primary villi, resulting in the formation of secondary villi and intervorsitis space. From the 3rd week of the development of the embryo, a placentation begins, characterized by the vascularization of villi and the transformation of secondary villi into tertiary vessels containing vessels. The transformation of secondary villi into tertiary is also an important critical period in embryo development, since gas exchange and transport of nutrients in the mother-fetus system depend on their vascularization. This period ends by 12-14 weeks of pregnancy. The main anatomical and functional unit of the placenta is the placenta ,. The constituent parts of which from the fruit side is cotyledon, and from the maternal side - kuruncul. Cotyledon, or placenta lobule, is formed by the trunk nap and its numerous branches, containing fruit vessels. The base of cotyledon is fixed on the basal chorionic plate. Individual (anchor) villi are fixed on the basal decidual membrane, but the vast majority of them freely float in the intervillous space. Each cotyledon corresponds to a definite part of the decidua, separated from the neighboring partitions by septa. At the bottom of each curculum, spiral arteries are opened that carry blood supply to the intervillous space. In view of the fact that the septa do not reach the chorionic plate, the individual chambers are connected to each other by a subchorial sinus. From the side of the intervillous space, the chorionic plate as well as the placenta is lined with a layer of cytotrophoblast cells. Due to this, the maternal blood also does not touch the decidual membrane in the intervillous space. In the placenta formed by the 140th day of pregnancy there are 10-12 large, 40-50 small and 140-150 rudimentary cotyledons. In these terms, the thickness of the placenta reaches 1.5-2 cm, a further increase in its mass occurs, mainly due to hypertrophy. At the boundary of the myometrium and endometrium, the spiral arteries are provided with a muscle layer and have a diameter of 20-50 μm; after passing through the main plate, when they enter the intervillous space, they lose muscle elements, which increases their lumen to 200 μm or more. The blood supply of the intervillous space occurs on average through 150-200 spiral arteries. The number of functioning spiral arteries is relatively small. In the physiological course of pregnancy, spiral arteries develop with such intensity that they can provide blood supply to the fetus and the placenta 10 times more than necessary, their diameter increases to 1000 μm or more by the end of pregnancy. The physiological changes that spiral arteries undergo as the pregnancy progresses consist in elastolysis, muscular layer degeneration and fibrinoid necrosis. This reduces peripheral vascular resistance and, accordingly, blood pressure. The process of trophoblast invasion ends completely by the 20th week of pregnancy. It is during this period that the systemic arterial pressure decreases to the lowest values. There is practically no resistance to blood flow from the radial arteries to the intervillous space. The outflow of blood from the intervillous space is carried out through 72-170 veins located on the surface of the terminal villi and, in part, into the marginal sinus bordering the placenta and communicating with both the veins of the uterus and the intervillous space. The pressure in the vessels of the utero-placental contour is: 80/30 mmHg in the radial arteries, 12-16 mmHg in the decidual part of the spiral arteries, and about 10 MMHg in the intervillous space. Thus, the loss of the musculo-elastic cover by the spiral arteries leads to their insensitivity to adrenergic stimulation, the capacity for vasoconstriction, which ensures an unhindered blood supply to the developing fetus. By the method of ultrasonic dopplerometry, a sharp decrease in the resistance of the uterine vessels to the 18-20 week of pregnancy, i.e., to the period of completion of trophoblast invasion, was revealed. In the subsequent periods of pregnancy, resistance remains at a low level, providing a high diastolic blood flow.

The proportion of blood flowing to the uterus during pregnancy increases by 17-20 times. The volume of blood flowing through the uterus is about 750 ml / min. In myometrium15% of the blood flowing to the uterus is distributed, 85% of the blood volume goes directly to the utero-placental circulatory system. The volume of intervillar space is 170-300 ml, and the rate of blood flow through it is 140 ml / min per 100 ml volume. The rate of uteroplacental blood flow is determined by the ratio of the difference in uterine arterial and venous pressure (ie perfusion) to peripheral vascular resistance of the uterus. Changes in uteroplacental blood flow are determined by a number of factors: the effect of hormones, changes in the volume of circulating blood, intravascular pressure, changes in peripheral resistance, determined by the development of intervillar space. As a result, these effects are reflected in the peripheral vascular resistance of the uterus. Intervorsing space is subject to changes under the influence of varying blood pressure in the mother and fetus vessels, pressure in the amniotic fluid and contractile activity of the uterus. With uterine contractions and hypertension, the uterine-placental blood flow decreases in the uterus as a result of an increase in uterine venous pressure and an increase in intra-venous pressure in the uterus. It was established that the constancy of the blood flow in the intervillous space is supported by a multistage chain of regulatory mechanisms. These include the adaptive growth of uteroplacental vessels, the system of autoregulation of the organ blood flow, conjugated placental hemodynamics on the maternal and fetal sides, the presence of a circulatory fetal system, including the vascular network of the placenta and umbilical cord, the ductal duct and the pulmonary vasculature of the fetus. The regulation of blood flow on the maternal side is determined by the movement of blood and uterine contractions, on the side of the fetus - by the rhythmic active pulsation of the chorion capillaries under the influence of the fetal heart contractions, the influence of the smooth muscles of the villi and the periodic liberation of the intervillaceous spaces. The regulatory mechanisms of uteroplacental blood circulation include strengthening the contractile activity of the fetus and increasing its blood pressure. Development of the fetus and its oxygenation are largely determined by the adequacy of the functioning of both utero-placental and placental-placental blood circulation.

The umbilical cord is formed from the mesenchymal strand (amniotic leg) into which the allantois, carrying umbilical vessels, grows. When the branches of the umbilical vessels growing from the allantois are connected, the circulation of embryonic blood in the tertiary villi is established with the local circulation network, which coincides with the onset of cardiac contractions of the embryo on the 21st day of development. In the early stages of ontogeny, the umbilical cord contains two arteries and two veins (merge into one at later stages). Umbilical vessels form about 20-25 turns in a spiral due to the fact that the vessels exceed the umbilical cord in length. Both arteries are the same size and supply half of the placenta. Arteries are anastomosed in the chorionic plate, passing through the chorial plate into the stem nap, they give rise to a second and third order arterial system, repeating the structure of cotyledon. Cotyledon arteries are terminal vessels with three orders of division and contain a network of capillaries, the blood from which is collected into the venous system. Due to exceeding the capacity of the capillary network, the floor of the arterial blood vessels of the fruit part of the placenta creates an additional blood pool that forms a buffer system that regulates the blood flow velocity, blood pressure, cardiac activity of the fetus. This structure of the fetal vascular bed is fully formed already in the first trimester of pregnancy.

The second trimester of pregnancy is characterized by the growth and differentiation of the fetal circulation (fetalization of the placenta), which are closely related to changes in the stroma and trophoblasts of the branching chorion. In this period of ontogeny, the growth of the placenta is faster than the development of the fetus. This is manifested in the convergence of maternal and fetal blood flow, the improvement and increase in surface structures (syncytiotrophoblasm). From 22 to 36 weeks of gestation, the increase in the mass of the placenta and fetus occurs evenly, and by the 36th week the placenta reaches full functional maturity. At the end of pregnancy, the so-called "aging" of the placenta occurs, accompanied by a decrease in the area of its exchange surface. In more detail it is necessary to dwell on the peculiarities of the fetal circulation. After implantation and establishment of connection with maternal tissues, the delivery of oxygen and nutrients is carried out by the circulatory system. Distinguish consistently developing circulatory system in the intrauterine period: yolk, allantoic and placental. The yolk period of the development of the circulatory system is very short - from the moment of implantation to the end of the first month of life of the embryo. Nutrients and oxygen, contained in the embryotroph, penetrate the embryo directly through the trophoblast forming the primary villi. Most of them fall into the yolk sac formed by this time, which has foci of hematopoiesis and its own primitive vascular system. Hence, nutrients and oxygen through the primary blood vessels enter the embryo.

Allantoid chorial circulation begins at the end of the first month and lasts 8 weeks. Vascularization of the primary villi and their transformation into the true villi of the chorion marks a new stage in the development of the embryo. Placental circulation is the most developed system providing ever increasing fetal needs, and begins with the 12th week of pregnancy. The embryo's heart is formed at week 2, and its formation basically ends at 2 months of pregnancy: it acquires all the features of a four-chambered heart. Along with the formation of the heart, the vascular system of the fetus arises and differentiates: by the end of the second month of pregnancy the formation of the main vessels terminates, in subsequent months the vascular network develops further. Anatomic features of the fetal cardiovascular system is the presence of an oval aperture between the right and left atria and the arterial (botallova) duct connecting the pulmonary artery to the aorta. The fetus receives oxygen and nutrients from the mother's blood through the placenta. In accordance with this, the blood circulation of the fetus has significant features. Blood enriched in the placenta with oxygen and nutrients enters the body through the vein of the umbilical cord. Having penetrated through the umbilical ring into the abdominal cavity of the fetus, the umbilical vein approaches the liver, gives it twigs, then goes to the inferior vena cava into which the arterial blood flows. In the inferior vena cava, arterial blood is mixed with the venous blood from the lower half of the body and the internal organs of the fetus. The site of the vein of the umbilical cord from the umbilical ring to the inferior vena cava is called the venous (arantzium) duct. Blood from the inferior vena cava enters the right atrium, which also receives venous blood from the superior vena cava. Between the place of the confluence of the lower and upper hollow veins is the flap of the inferior vena cava (eustachian), which prevents mixing of blood coming from the upper and lower hollow veins. The damper directs the blood flow of the inferior vena cava from the right atrium to the left through the oval aperture located between the two atria; from the left atrium blood enters the left ventricle, from the ventricle into the aorta. From the ascending aorta, blood containing relatively much oxygen enters the blood vessels supplying the head and upper body with blood. Venous blood entering the right atrium from the superior vena cava is directed to the right ventricle, and from it to the pulmonary arteries. From the pulmonary arteries, only a small part of the blood enters the non-functioning lungs; The main mass of blood from the pulmonary artery comes through the arterial (botalla) duct and the descending aorta. The fetus, unlike the adult, is dominated by the right ventricle of the heart: its ejection is 307 + 30 ml / min / kg, and the left ventricle 232 + 25 ml / min / kg. The descending aorta, which contains a significant portion of the venous blood, supplies the lower half of the trunk and lower limbs with blood. Fetal blood, poor in oxygen, enters the arteries of the umbilical cord (branches of the iliac arteries) and through them into the placenta. In the placenta, blood receives oxygen and nutrients, is released from carbon dioxide and metabolic products and returns to the fetal body via the umbilical vein. Thus, the pure arterial blood of the fetus is contained only in the vein of the umbilical cord, in the venous duct and branchlets going to the liver; the lower vena cava and the ascending aorta have mixed blood, but contain more oxygen than the blood in the descending aorta. Due to these features of the blood circulation, the liver and upper body of the fetus are supplied with arterial blood better than the lower one. As a result, the liver reaches a large size, the head and upper body in the first half of pregnancy develop faster than the lower body. It should be emphasized that the placental-placental system has a number of powerful compensatory mechanisms ensuring the maintenance of fetal gas exchange in conditions of reduced oxygen supply (predominance of anaerobic metabolic processes in the fetus and in the placenta, a large minute volume of the heart and fetal blood flow velocity, the presence of fetal hemoglobin and polycythemia, increased fetal oxygen affinity for the fetal tissues). As the fetus develops, a certain narrowing of the oval aperture and a decrease in the flap of the inferior vena cava occur; in this regard, arterial blood is more evenly distributed throughout the fetal organism and the lag in the development of the lower half of the body is leveled.

Immediately after birth, the fetus takes the first breath; from this moment pulmonary respiration begins and there is an extrauterine type of blood circulation. At the first inhalation, the pulmonary alveoli spread out and the flow of blood to the lungs begins. Blood from the pulmonary artery now enters the lungs, the arterial duct collapses, and the venous duct also desolates. The blood of the newborn, enriched in the lungs with oxygen, flows through the pulmonary veins into the left atrium, then into the left ventricle and the aorta; The oval aperture between the atria is closed. Thus, the newborn has an extrauterine type of circulation.

In the process of fetal growth, the systemic arterial pressure and the volume of circulating blood constantly increase, the vascular resistance decreases, and the pressure in the umbilical vein remains relatively low - 10-12 mmHg. The pressure in the artery increases from 40/20 MMHg in the 20 weeks of pregnancy to 70/45 mm MMHg at the end of pregnancy. The increase in umbilical cord blood flow in the first half of pregnancy is achieved mainly by reducing the vascular resistance, and then mainly due to an increase in the arterial pressure of the fetus. This is confirmed by ultrasound dopplerometry data: the greatest decrease in resistance of the placenta-vascular vessels occurs at the beginning of the second trimester of pregnancy. For the umbilical artery, the translational movement of blood is characteristic both in the phase of the systole and in the diastole phase. From the 14th week on dopplerograms, the diastolic component of blood flow in these vessels begins to be recorded, and from the 16th week - it is constantly detected. There is a directly proportional relationship between the intensity of uterine and umbilical blood flow. Cord blood flow is regulated by perfusion pressure, determined by the ratio of pressure in the aorta and umbilical vein of the fetus. Cord blood circulation receives approximately 50-60% of the total cardiac output of the fetus. The amount of umbilical cord blood flow is influenced by the physiological processes of the fetus - respiratory movements and motor activity. Rapid changes in umbilical cord blood flow occur only due to changes in fetal arterial pressure and its cardiac activity. The results of studying the effect of various drugs on utero-placental and placental-placental blood circulation deserve attention. To reduce the blood flow in the mother-placenta-fetus system can lead to the use of various anesthetics, narcotic analgesics, barbiturates, ketamine, halothane. Under experimental conditions, increased uteroplacental blood flow causes estrogens, however, in clinical settings, the administration of estrogens for this purpose sometimes turns out to be ineffective. When studying the effect of tocolytics (beta-adrenomimetics) on uteroplacental blood flow, it was found that beta-mimetics expand arterioles, reduce diastolic pressure, but cause fetal tachycardia, increased blood glucose levels and are effective only in functional placental insufficiency. Functions of the placenta are diverse. Through it, nutrition and gas exchange of the fetus, release of metabolic products, formation of hormonal and immune status of the fetus are carried out. In the process of pregnancy, the placenta replaces the missing functions of the blood-brain barrier, protecting the nerve centers and the entire fetal organism from the effects of toxic factors. It also has antigenic and immune properties. An important role in the performance of these functions is played by amniotic fluid and fetal membranes that form a single complex together with the placenta.

Being an intermediary in the creation of the hormonal complex of the mother-fetus system, the placenta plays the role of an endocrine gland and synthesizes hormones, using the mother and fruit predecessors. Together with the fetus the placenta forms a single endocrine system. Hormonal function of the placenta contributes to the preservation and progression of pregnancy, changes in the activity of the mother's endocrine organs. In it, there are processes of synthesis, secretion and transformation of a number of hormones of the protein and steroid structure. There is a relationship between the body of the mother, the fetus and the placenta in the production of hormones. Some of them are secreted by the placenta and transported to the blood of the mother and fetus. Others are derived from precursors that enter the placenta from the mother or fetus. Direct dependence of the synthesis of estrogens in the placenta from androgen precursors produced in the fetus allowed E. Diczfalusy (1962) to formulate the concept of the fetoplacental system. Through the placenta can be transported and unchanged hormones. Already in the pre-plantation period at the blastocyst stage, the germ cells secrete progesterone, estradiol and chorionic gonadotropin, which are of great importance for nidation of the fetal egg. In the process of organogenesis, the hormonal activity of the placenta increases. Among the hormones of protein nature, the fetoplacental system synthesizes chorionic. Gonadotropin, placental lactogen and prolactin, thyrotropin, corticotropin, somatostatin, melanocyte-stimulating hormone, and from steroids - estrogens (estriol), cortisol and progesterone.

Ambulatory water (amniotic fluid) is a biologically active surrounding fetal medium, intermediate between it and the mother's body and performing diverse functions throughout pregnancy and childbirth. Depending on the term of pregnancy, water is formed from various sources. In the embryotrophic esteriode, the amniotic fluid is a trojoblast transudate, in the yolk-feeding period, by the transudate of the villus of the chorion. By the 8th week of pregnancy, there is an amniotic sac that is filled with a liquid that is similar to extracellular. Later, amniotic fluid is an ultrafiltrate of maternal blood plasma. It is proved that in the second half of pregnancy and up to the end of it, the source of the amniotic fluid, in addition to the blood filtrate of the mother, is the secret of the amniotic membrane and the umbilical cord, after 20 weeks - the product of the kidneys of the fetus, as well as the secret of its lung tissue. The volume of amniotic fluid depends on the weight of the fetus and the size of the placenta. So, at 8 weeks of pregnancy, it is 5-10 ml, and by 10 weeks it is increased to 30 ml. In the early stages of pregnancy, the amount of amniotic fluid increases by 25 ml / week, and in the period from 16 to 28 weeks - by 50 ml. By 30-37 week, their volume is 500-1000 ml, reaching a maximum (1-1.5 liters) by the 38th week. By the end of pregnancy, the volume of amniotic fluid can decrease to 600 ml, decreasing by about 145 ml every week. The quantity of an amniotic fluid less than 600 ml is considered to be anhydrous, and its quantity is more than 1.5 liters - by polyhydramnios. At the beginning of pregnancy, amniotic fluid is a colorless transparent liquid that during pregnancy changes its appearance and properties, becomes unclear, opalescent due to the release of the sebaceous glands of the fetus skin, fleece hairs, epidermis scales, amniotic epithelium products, including fat droplets . The quantity and quality of suspended particles in the water depend on the gestational age of the fetus. The biochemical composition of amniotic fluid is relatively constant. There are slight fluctuations in the concentration of mineral and organic components, depending on the period of pregnancy and the state of the fetus. Amblerous waters have a slightly alkaline or close to neutral reaction. The composition of amniotic fluid includes proteins, fats, lipids, carbohydrates, potassium, sodium, calcium, microelements, urea, uric acid, hormones (chorionic gonadotropin, placental lactogen, estriol, progesterone, corticosteroids), enzymes (thermostable alkaline phosphatase, oxytocinase, lactate - and succinate dehydrogenase), biologically active substances (catecholamines, histamine, serotonin), factors affecting the blood coagulation system (thromboplastin, fibrinolysin), group fetal blood antigens. Consequently, amniotic fluid is a very complex environment and functions. In the early stages of fetal development, amniotic fluid participates in its nutrition, promote the development of the respiratory tract and digestive tract. Later they perform the functions of the kidneys and skin. The rate of exchange of amniotic fluid is of paramount importance. Based on radioisotope studies, it was found that when the pregnancy is complete, about 500-600 ml of water is exchanged for 1 hour, that is, 1/3 of them. Their complete exchange occurs within 3 hours, and the complete exchange of all dissolved substances - for 5 days. Placental and paraplacental pathways of amniotic fluid exchange (simple diffusion and osmosis) are established. Thus, the high rate of formation and re-absorption of amniotic fluid, a gradual and constant change in their quantity and quality, depending on the gestational age, the condition of the fetus and the mother, indicate that this environment plays a very important role in the metabolism between the mother and fetus organisms. The amniotic fluid is an important part of the protective system that protects the fetus from mechanical, chemical and infectious effects. They protect the embryo and fetus from direct contact with the inner surface of the fetal sac. Due to the presence of a sufficient amount of amniotic fluid, the fetal movements are free. So, a deep analysis of the formation, development and functioning of the unified mother-placenta-fetus system makes it possible to revise from a modern point of view some aspects of the pathogenesis of obstetric pathology and, thus, to develop new approaches to its diagnosis and treatment tactics.

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