Amniotic fluid

Amniotic fluid can be considered as the largest part of the fetal extracellular fluid, since its osmotic parameters, electrolyte and biochemical composition are identical to fetal plasma.

Normally, the amount of amniotic fluid is 0.5-1.5 liters, and varies depending on the gestational age. It should be noted that the physiological development of the fetus largely depends on the adequacy of the amniotic fluid.

Functions of amniotic fluid

Amniotic fluid performs a number of important functions in fetal development, namely:

• creates conditions for the unimpeded movement of the fetus and the development of its muscular-skeletal system;
• the waters that the fetus swallows contribute to the development of the digestive tract;
• supplies components vital for the nutrition of the fetus;
• maintains constant intrauterine pressure, thus reducing the loss of pulmonary fluid, a component necessary for lung development (Nicolini, 1998);
• protects the fetus from numerous exogenous influences;
• protects the umbilical cord from compression;
• constant temperature of amniotic fluid helps maintain fetal body temperature;
• The bacteriostatic properties of amniotic fluid reduce the risk of possible infection of the fetus.

Physiology of amniotic fluid

The main sources of amniotic fluid formation are the respiratory system, fetal kidneys, umbilical cord, exfoliated skin epithelium, mucous membrane of the cheeks, urinary and genital organs of the fetus, fetal surface of the amnion and chorionic cells.

The process of formation and evacuation of amniotic fluid from the amnion is quite fast. Thus, the complete exchange of water occurs in 3 hours, and the exchange of dissolved substances - in 5 days.

It should be noted that the factors that regulate the exchange of amniotic fluid have not yet been fully studied. Brace (1997) identifies 6 possible pathways for the movement of amniotic fluid.

The movement of amniotic fluid occurs in the direction from the mother to the fetus, from the fetus into the amniotic cavity and again into the mother's body.

The excretion of amniotic fluid occurs through the placental and paraplacental pathways. In the first case, they are excreted through the fetus's body during respiratory movements (by swallowing). Thus, approximately 600-800 ml of fluid per day passes through the fetus's lungs. Some of the fluid is utilized by the skin and respiratory tract and is subsequently excreted through the umbilical cord vessels and placenta. Approximately 40% of amniotic fluid is excreted in this way.

Pathways of movement of amniotic fluid

Pathways of movement of amniotic fluid	Volume, ml/day
	To the fruit	Into the amniotic fluid
Ingestion by the fetus	500-1000	-
Oral secretion	-	25
Secretion through the respiratory tract	170	170
Fetal urination	-	800-1200
Intramembranous movement through the placenta, umbilical cord and fetus	200-500	-
Transmembranous movement from the amniotic cavity into the uterine bloodstream	-	10

Most of the amniotic fluid is excreted paraplacental, namely through the intercellular spaces into the blood vessels of the smooth chorion, the decidua and the venous system of the mother.

Chemical composition of amniotic fluid

Amniotic fluid contains 98-99% water, 1-2% is a solid residue, half of which is organic compounds and half is inorganic.

A study of the composition of amniotic fluid showed that amniotic fluid contains 27 amino acids and 12 protein fractions.

All lipid fractions were also found in amniotic fluid: mono-, di-, triacylglycerides, cholesterol and its esters, fatty acids and all classes of phospholipids.

Carbohydrate metabolism in amniotic fluid is not fully understood.

In addition, amniotic fluid contains acid hydrolase, alkaline and acid phosphatase, beta-glucuronidase, hyaluronidase, hexosamidine amidase, lactate dehydrogenase, isocitrate dehydrogenase, carbonic anhydrase, glucose-6-phosphatase, glucose-6-phosphate dehydrogenase and other enzymes.

Also, a significant amount of biologically active substances, in particular histamine, dopamine, catecholamines, and serotonin, have been identified in amniotic fluid.

Histamine is synthesized both in the mother's and the fetus's body and takes part in the regulation of embryonic growth processes. In turn, dopamine is found in significant quantities in the amniotic fluid immediately before birth. The concentration of catecholamines increases with increasing gestational age, which reflects the maturation of the sympathetic autonomic nervous system of the fetus. Serotonin is actively secreted into the amniotic fluid by the fetus and its concentration increases with increasing gestational age. The content of serotonin in amniotic fluid has diagnostic significance: its decrease is one of the early manifestations of compensatory reactions to intrauterine hypoxia.

Amniotic fluid has high hormonal activity. It contains the following hormones: chorionic gonadotropin, placental lactogen, adrenocorticotropic hormone, prolactin, somatotropic hormone, thyroxine, insulin, and steroid hormones. The main source of steroids is the placenta. It is where cholesterol is successively converted into pregnanolone, then into progesterone, from which fetal corticosteroids are synthesized.

In the amniotic fluid, all estrogen fractions are determined in a concentration that significantly exceeds the latter in the blood of the mother and fetus. During the physiological course of pregnancy, the amount of estrogens in the amniotic fluid increases progressively. The increase in estrogen levels occurs due to estriol, which enters the amniotic fluid with the urine of the fetus by direct absorption from the circulatory system.

Amniotic fluid contains almost all corticosteroids. Most of them are of fetal origin. It is known that an increase in the concentration of cortisol in amniotic fluid is an integral part of the mechanism of normal labor and an indicator of fetal maturity.

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