The World of Pregnancy

What Is the Function of the Placenta?

What is the function of the placenta? How does the placenta work? What happens to the placenta after birth? These should be important questions for any future mother if she wants to have complete control over her pregnancy.

The placenta is the organ through which the maternal organism connects with the fetus in the maternal-placental-fetal unit.

The placental membrane divides the 2 circulations:

  • The maternal placental circulation which runs from the basal plate.
  • The villous fetal circulation which runs in a capillary system with an arterial path and a venous one.

The transport of the blood at the fetal level is made during the cardiac systole, this being the most important controller of maternal circulation in the placenta.

The physical factors that condition the changes are represented by: the placenta`s weight, placenta`s size, blood flow, hydrostatic and osmotic pressure. The placental surface is estimated as being around 11 – 13m². The hydrostatic pressure within the intervillous chamber is equal with the amniotic pressure: 10 – 30mm Hg, and the pressure at the level of the fetal capillaries is of 30 – 35mm Hg.

During pregnancy, the total membrane of the placental exchange increases along with its volume.

The placental hemodynamic conditions can be modified by:

  • The normal uterine contractions that reduce the maternal placental flow through the compression exerted against intramyometrial vessels. The vascular tone increases due to medulla hypersecretion, induced by pain. When membranes brake, for 20 to 40 minutes the uterine flow decreases due to the lack of the uterine contractions.
  • The pregnant woman`s position is important for the placental blood flow. Thus, when standing, the uterine flow decreases with 15% compared with the lateral decubitus position.
  • Variations of the maternal blood pressure can influence the uterine blood flow. Thus, hypotension induces the decrease of blood flow, just as hypertension which can decrease the flow up to 50%.

Definition of Placenta

The placenta is a structure that develops in your uterus during pregnancy. It produces oxygen and nutrients which help in the development of your baby and eliminate waste produced by the baby`s blood. It attaches by one of the uterine walls and the baby`s umbilical cord rises from it. In most pregnancies, the placenta attaches to the top or one of the sides of the uterus.

What Is the Function of the Placenta?

 The placenta performs an essential function for the fetal development due to the mediation role of the transfer between the maternal and fetal circulation.

The apical membrane, also known as sincitiotrofoblasto, is morphologically specialized in achieving the transplacental transfer.

There are the following types of transplacental transfer:

  1. The simple diffusion realized after the physical-chemical laws which has a tendency to equal the concentration of substances and is influenced by blood flow. The transplacental transfer of the oxygen, carbon dioxide and anesthetic gases is made through the simple diffusion.
  2. The facilitated diffusion is subject to the general law of equalization of concentration, but presents particular characteristics: the level of transfer is higher which requires the problem of existence of a competition between various products with similar chemical structures.
  3. The active transport realized with transport molecules can be made in terms of a counter-current of relative concentrations. This type of transport can be achieved with the help of certain transport molecules that act at the level of cellular membranes.
  4. Pinocytosis represents the process of taking a small quantity of intracellular liquid by a cell membrane (valid for macromolecules). This property is owned by the placenta at the beginning of the pregnancy for maternal red blood cells.
  5. Direct passage through solutions of continuity of the placental membrane.

The Transfer of Some Important Elements

  1. Oxygen – The transfer of the oxygen is made through the mechanism of simple diffusion. The consumption of oxygen of the uterus and its content at term is of around 20ml/min. from which 2/3 transferred to the fetus. This transfer can be influenced by several factors:
  • Vascular geometry;
  • The oxygen pressure in the uterine and umbilical venous blood;
  • Maternal and fetal blood flow;
  • The characteristics of the maternal and fetal hemoglobin;
  • The consumption of oxygen at the placental level;
  • Placental diffusion capacity.

The fetus has a great ability of maintaining the consumption of oxygen. The fetal hemoglobin presents an affinity for oxygen, and if the umbilical flow is reduced, the fetus has an increased ability of extracting the oxygen. In case that the released of the oxygen towards the fetus is reduced, the most part from the umbilical venous blood is diverted towards myocardium and the upper side of the heart. The secretion of catecholamines will increase simultaneously along with the achievement of vasoconstriction.

  1. Carbon Dioxide – At the placental membrane`s level, the permeability to carbon dioxide is increased.
  2. Water – The water flow at the placental level is influenced by the osmotic and hydrostatic force as well as the electrolyte gradient, the fetus drinking a large quantity of water on a daily basis.
  3. Glucose – The main energy substrate for the fetus is formed from maternal glucose. The normal transfer of this nutrient is an essential condition for the normal development of the fetus, so at the term, the placental transfer must be of around 20mg/min, around 30g/day. The main way of the transfer is the facilitated diffusion made through certain proteins. The placenta is also big consumer of glucose, and the rate of the transfer is influenced by this aspect as well.

The control of the glucose transfer is influenced by: maternal and fetal insulin, glucocorticoids, progesterone, estrogen and concentration gradients.

The placental deposits of glycogen are in equilibrium with maternal glycemia.

  1. Amino Acids – The fetal protein synthesis starts from the transferred amino acids through the placenta and present in the fetal plasma in higher concentrations than the maternal one.

The transfer of amino acids is provided through 3 categories of systems:

  • Passive diffusion
  • Na-dependent system
  • Na-independent system
  1. Proteins – The placenta is impermeable at proteins, but there`s an exception regarding lg G, which confers immunity to the newborn through the placental transfer of these antibodies, transfer that is realized through endocytosis mediated by receptors or through solutions of continuity of the membranes.
  1. Lipids – The lipid concentration at the fetus is smaller. The one that is important is arachidonic acid – an important element for fetal development.
  1. Monovalent Cations – The sodium pump is found in the trophoblast, the transport of the sodium being important in regulating the trophoblastic pH.
  1. Anions – Since the thyroid hormones don`t cross the placenta, providing iodine to the fetus is essential. The transportation is made actively and the fetal concentrations are higher than the maternal ones.

The sulfates also have an important nutritional role.

  1. Calcium – The calcium`s transfer is made through active transport. During the third trimester of pregnancy, the calcium`s concentration at the fetus exceeds maternal concentration.

The regulators of this transport are:

  • The metabolites of vitamin D
  • The parathyroid hormone
  • Calcitonin
  • Prolactin
  • Somatotrupe hormone.

The transport is dependent by: ATP, magnesium and the pump of calcium.

  1. Metals – The transfer of iron reaches its peak during the third trimester of pregnancy, receptor-mediated endocytosis (clathrin-mediated endocytosis) being the type of transfer. Zinc is involved in enzymatic activity.
  1. Substances that Ensures the Maintenance of Pregnancy &Modifies the Fetal Growth – These substances pass through a mechanism of slow diffusion: steroid and protein hormones.

From steroid hormones: estrogen, progesterone and androgens, pass slowly through the placental barrier, and their transplacental diffusion is influenced by their fixation on the carrier proteins.

From protein hormones: ACTH doesn`t cross the placenta, the insulin passes but in low quantities, and for hCG hasn`t been yet described a mechanism for passage.

The Endocrine Function of the Placenta

It`s known that the human placenta produces over 30 hormones and has receptors for almost all factors of regulation, being the main major endocrine organ during pregnancy. Syncytiotrophoblast and villous cytotrophoblast are the main places of the hormonal production.

The placenta produces steroid hormones, protein hormones, cytokines and growth factors.

  1. Steroidogenesis during Pregnancy – During pregnancy, the placenta synthesizes large quantities of progesterone and estrogen, hormones that are secreted in the maternal and fetal blood compartment. They act against the maternal genital tract and metabolic systems, and assure a normal evolution of the pregnancy.

The essential condition for maintaining the pregnancy is the functionality of the pregnancy body and the extension of the progesterone`s secretion, especially until the placental steroids are able to support the pregnancy on their own. The steroids that are produced by the placenta derive from precursors provided by the mother or fetus, are metabolized and conjugated. This particular way of biosynthesis of the steroids by the placenta explains the usage of the “fetoplacental unit” or “maternal-placental-fetal unit” terminology.

The progesterone is synthesized from the maternal cholesterol. During the last phase of the pregnancy, the placenta produces 250 – 600mg of progesterone on a daily basis. The production of this hormone depends firstly by the LDL associated cholesterol and an adequate blood flow. Estrogens increase the formation of progesterone from the cholesterol.

The estrogens are secreted, as progesterone, by the pregnancy yellow body during the first weeks of pregnancy, and their role extends until the end of the second month when their synthesis is provided by the fetoplacental unit.

The estrogens synthesized by the fetoplacental unit are in most part channeled towards the maternal compartment and partially towards the amniotic liquid.

The secretion in the maternal blood is of 10 – 15mg/day of estradiol and 40 – 50mg/day for estriol. Most estrogens produced by the fetoplacental unit are secreted by the maternal kidney, and this way of elimination requires the transformation of estrogens in metabolites, conjugation that happens at the liver`s level, in the small intestine and kidney, and the renal elimination occurs in different ways for different estrogens.

In the last trimester of pregnancy, the radio-immunological estrogens from the maternal compartment are dosed to assess the functional status of the placenta and fetus. Still, these results are modified by certain circumstances: posture or dieresis`s volume.

The secretion`s control is made through the gonadotropins. Prolactin plays an inhibitory effect against the secretion of estrogen.

The main biological actions of estrogens are:

  • Regulation of protein synthesis and secretion in the first trimester;
  • The ductal development of the mammary gland;
  • Cervical ripening;
  • Adjuvant factors in triggering labor: stimulates the formation of myometrial junctions, positive effects against receptors, stimulates the synthesis and release of prostaglandins, stimulates the synthesis of contractile proteins, favors the enzymatic synthesis, positive influences against the permeability of the membrane.
  1. The Protein Hormones – The placenta secretes several proteins in the maternal blood: hormones, proteins with enzymatic activity and proteins with biological functions.

hCG (human chrionic gonadotropin), produced by sincitotrofoblast is secreted into the maternal circulation, and small quantities in the fetal one. The placenta contains a similar quantity of hCG with the one from the serum, which suggests that this hormone isn`t stored in the placenta, but is continually released into circulation. Gonadotrophin`s functions are: it stimulates steroidogenesis and various tissues, sustains the luteal function during the course of fertile cycles stimulating the synthesis of progesterone and estrogen in the gestation body.

The main use in practice is related to the diagnosis of the pregnancy during the first stages of evolution, diagnosis and monitoring subsequent of the molar pregnancy and choriocarcinoma.

Somatotroph chorionic hormone (HCS, human placental lactogen HPL) is synthesized and produced in the syncytiotrophoblast. The placental secretion rate is very high, being a major product in the placenta at term. The serum is proportional to the placenta`s weight, it increases from 5 – 6 weeks, reaching at maximum 36 weeks.

Hormone roles: regulates fetal growth interfering with the metabolism of carbs and fats, owns proliferative effects on mammary gland.

The adjustment of its production is semiautonomous, and its dosage can be achieved through radio-immunological methods, being considered as one of the best markers of placental function and of the fetus`s status indirectly.

Thyroid stimulating hormone is a protein with big molecular weight of which activity increases during pregnancy. It owns antigenic properties due to its structure, thus making its immunological dosage possible. During pregnancy, its levels are higher during the first 2 trimesters, reaching minimal values at term.

Corticotropin chorionic hormone (corticotropin-releasing hormone) has similar properties to ACTH, its structure not being well defined. Its levels increase gradually during pregnancy.

The Protective Role of the Placenta

The trophoblast unit represents the interface between the fetoplacental unit and the maternal endometrium.

The trophoblast`s antigens are grouped in:

  • The major histocompatibility complex antigens (MHC): represented by antigens of class I of the major histocompatibility complex; they are controlled by genes found on the chromosomes 6. The HLA molecules are found on citotrofoblast during the 5th week, but they aren`t found in the villous trophoblastic, being expressed in the non villous trophoblastic.
  • The transferrin and its receptor: they were highlighted in the microvilli- maternal blood interference.
  • TLX antigens (the antigens of the trophoblast-lymphocyte reaction): they are antigens widely spread in the human tissues, identified in the trophoblast`s tissues from the placental bed, in the lymphocytes and platelets. They are allotypic and can be treated to the minor histocompatibility

One of the aspects of the immunological processes is in relation with the presence of cytokines in the fetoplacental unit. Cytokines represent a wide group of polypeptides that include interleukins, TNS alpha and beta interferons, representing chemical messengers of the manifest cellular interactions in the immune response and effectors in the immune processes.

The Non Endocrine Metabolic Function of the Placenta

The placental metabolism reaches high levels comparable to those of adult organs, presenting some characteristics of the liver, lung, kidney or of some endocrine organs, but the placental metabolism isn`t totally autonomous.

The placenta ensures the thermoregulation of the fetus during prenatal life. Normally, the temperature of the human fetus is 0.5 ° C higher than the mother, and this gradient is determined by the level of the placental perfusion as well as the direction of the blood in the placenta.

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