The Placenta Has Maternal and Fetal Circulations That Allow Exchange Between the Mother and Fetus

The development of a human fetus depends on nutrient, gas, water, and waste exchange in the maternal and fetal portions of the placenta. The human fetal placenta is sup plied by two umbilical arteries, which branch from the internal iliac arteries, and is drained by a single umbilical vein (Fig. 17.6). The umbilical vein of the fetus returns oxygen and nutrients from the mother's body to the fetal cardiovascular system, and the umbilical arteries bring in blood laden with carbon dioxide and waste products to be transferred to the mother's blood. Although many liters of oxygen and carbon dioxide, together with hundreds of grams of nutrients and wastes, are exchanged between the mother and fetus each day, the exchange of red blood cells or white blood cells is a rare event. This large chemical exchange without cellular exchange is possible because the fetal and maternal blood are kept completely separate, or nearly so.

The fundamental anatomical and physiological structure for exchange is the placental villus. As the umbilical arteries enter the fetal placenta, they divide into many branches that penetrate the placenta toward the maternal system. These small arteries divide in a pattern similar to a fir tree, the placental villi being the small branches. The fetal capillaries bring in the fetal blood from the umbilical arteries

Fetal lung

High-resistance pulmonary vessels

Arteries to upper body

Superior-vena cava

Fetal lung

High-resistance pulmonary vessels

Arteries to upper body

Superior-vena cava

Spiral Artery Placenta

Iliac arteries

Fetal placenta

Fetal Syncytial capillary knot

Spiral artery Endometrial vein

Iliac arteries

Syncytiotrophoblast

Cytotrophoblast

Fetal placenta

Fetal Syncytial capillary knot

Spiral artery Endometrial vein

The fetal and placental circulations. Schematic representation of the left and right sides of the fetal heart are separated to emphasize the right-to-left shunt of blood through the open foramen ovale in the atrial septum and the right-to-left shunt through the ductus arteriosus. Arrows indicate the direction of blood flow. The numbers represent the percentage of saturation of blood hemoglobin with oxygen in the fetal circulation. Closure of the ductus venosus, foramen ovale, ductus arteriosus, and placental vessels at birth and the dilation of the pulmonary vas-culature establish the adult circulation pattern. The insert is a cross-sectional view of a fetal placental villus, one of the branches of the tree-like fetal vascular system in the placenta. The fetal capillaries provide incoming blood, and the sinusoidal capillaries act as the venous drainage. The villus is completely surrounded by the maternal blood, and the exchange of nutrients and wastes occurs across the fetal syncytiotrophoblast.

and then blood leaves through sinusoidal capillaries to the umbilical venous system. Exchange occurs in the fetal capillaries and probably to some extent in the sinusoidal capillaries. The mother's vascular system forms a reservoir around the tree-like structure such that her blood envelops the placental villi.

As shown in Figure 17.6, the outermost layer of the pla-cental villus is the syncytiotrophoblast, where exchange by passive diffusion, facilitated diffusion, and active transport between fetus and mother occurs through fully differentiated epithelial cells. The underlying cytotrophoblast is composed of less differentiated cells, which can form additional syncytiotrophoblast cells as required. As cells of the syncytiotrophoblast die, they form syncytial knots, and eventually these break off into the mother's blood system surrounding the fetal placental villi.

The placental vasculature of both the fetus and the mother adapt to the size of the fetus, as well as to the oxygen available within the maternal blood. For example, a minimal placental vascular anatomy will provide for a small fetus, but as the fetus develops and grows, a complex tree of placental vessels is essential to provide the surface area needed for the fetal-maternal exchange of gases, nutrients, and wastes. If the mother moves to a higher altitude where less oxygen is available, the complexity of the placental vascular tree increases, compensating with additional areas for exchange. If this type of adaptation does not take place, the fetus may be underdeveloped or die from a lack of oxygen.

During fetal development, the fetal tissues invade and cause partial degeneration of the maternal endometrial lining of the uterus. The result, after about 10 to 16 weeks gestation, is an intervillous space between fetal placental villi that is filled with maternal blood. Instead of microves-sels, there is a cavernous blood-filled space. The intervillous space is supplied by 100 to 200 spiral arteries of the maternal endometrium and is drained by the endometrial veins. During gestation, the spiral arteries enlarge in diameter and simultaneously lose their vascular smooth muscle layer—it is the arteries preceding them that actually regulate blood flow through the placenta. At the end of gestation, the total maternal blood flow to the intervillous space is approximately 600 to 1,000 mL/min, which represents about 15 to 25% of the resting cardiac output. In comparison, the fetal placenta has a blood flow of about 600 mL/min, which represents about 50% of the fetal cardiac output.

The exchange of materials across the syncytiotro-phoblast layer follows the typical pattern for all cells. Gases, primarily oxygen and carbon dioxide, and nutrient lipids move by simple diffusion from the site of highest concentration to the site of lowest concentration. Small ions are moved predominately by active transport processes. Glucose is passively transferred by the GLUT 1 transport protein, and amino acids require primarily facilitated diffusion through specific carrier proteins in the cell membranes, such as the system A transporter protein.

Large-molecular-weight peptides and proteins and many large, charged, water-soluble molecules used in pharmacological treatments do not readily cross the placenta. Part of the transfer of large molecules probably occurs between the cells of the syncytiotrophoblast layer and by pinocytosis and exocytosis. Lipid-soluble molecules diffuse through the lipid bilayer of cell membranes. For example, lipid-soluble anesthetic agents in the mother's blood do enter and depress the fetus. As a consequence, anesthesia during pregnancy is somewhat risky for the fetus.

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Responses

  • Hassan
    Where is the exchange of nutrient occur between the mother and the fetus?
    8 years ago
  • Pamphila
    What is the surface area for diffusion of gases water and nutrients between the mother and the fetus?
    8 years ago
  • Juhana
    What process exchanges gases, nutrients, and wastes between the placenta?
    8 years ago
  • jasmine
    What materials can be exchanged between endometrium and placenta?
    7 years ago

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