What Is Placenta?
The placenta is an organ that forms in the womb of a woman during pregnancy. The placenta's primary function is to supply oxygen and nutrients to the developing fetus. It also helps in removing waste products from the blood of the fetus. It also produces hormones necessary for the fetus's growth and development.
The baby's umbilical cord arises from the placenta attached to the wall of the uterus. In the walls of the uterus, the placenta can be positioned anywhere in the front, back, top, or side. It is rare to find the placenta to be attached in the lower area of the uterus, but when this happens, it is referred to as a low-lying placenta. By the end of pregnancy, the placenta grows about nine inches in diameter, and from the center, it can be about an inch thick.
Why Is Placental Circulation Important?
The placenta is a unique organ with two separate circulatory systems for blood that receives blood supplies from both the fetal and the maternal systems. The two different circulatory systems for the placenta are:
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Uteroplacental Blood Circulation - Blood circulation from the mother to the placenta.
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Fetoplacental Blood Circulation - Blood circulation from the fetus to the placenta.
The uteroplacental circulation begins with the mother's blood flow and enters the intervillous space through decidual spiral arteries. The oxygen and nutrient exchange occurs as the maternal blood flows enter the intervillous space. The maternal blood flowing in the spiral arteries pushes deoxygenated blood into the uterine veins back to the maternal circulation.
In fetoplacental blood circulation, the umbilical arteries carry the blood without nutrients and oxygen from the fetus to the villous core fetal vessels. After swapping nutrients and oxygen, the umbilical vein carries fresh, nutrient-rich, and oxygenated blood circulating back to the fetal systemic circulation.
How Does Maternal Placental Circulation Take Place?
The complete establishment of uteroplacental circulation usually takes place at the end of the first trimester. However, the exact mechanism of establishing uteroplacental circulation is unclear, and two theories have been proposed.
First theory - In the first trimester, endovascular trophoblasts migrate through the decidual spiral arteries and occupy the vessel walls to create a passage for the placenta intervillous space perfusion by the maternal blood. This theory is mostly appreciated.
The second theory - It is put forward that trophoblasts occupy decidual spiral arteries to become trophoblastic plugs and obstruct maternal blood flow into the intervillous space until the end of the first trimester of pregnancy. Later, the plugs loosen and permit uninterpreted maternal blood flow into the intervillous space.
Although both theories have opinions on invading trophoblast plugs, it is clear that the origin of uteroplacental blood flow during the first trimester is a dynamic and progressive process.
During the early development of the placenta, there is a transformation in the extension of spiral arteries from the layer of tissue lining the uterus to the muscle layer. The maternal blood enters the placenta through the spiral arteries and perfuses intervillous spaces to flow around the villi. It is estimated that around 120 spiral arteries invade the intracellular space. Here the oxygen and nutrients exchange takes place with fetal blood. Maternal blood travels through the intervillous space of the placenta and leaves through venous orifices in the basal plate, and goes back to the maternal systemic circulation through uterine veins. Maternal arterial pressure drives the maternal-placental blood flow, and it has a unique nature to fit the massive increase in uterine perfusion during pregnancy.
How Does Fetoplacental Blood Circulation Take Place?
During the development of the fetus in the uterus, the umbilical cord is the lifeline that supplies nutrients and oxygen to the baby, and it attaches the placenta to the fetus. The umbilical cord is from the same zygote as the fetus. It extends from the umbilicus of the fetus to the fetal surface of the placenta. The length of the umbilical cord in a full-term neonate will be approximately around 50 to 70 centimeters long and two centimeters in diameter. The cord connects to the mother's circulatory system through the placenta, which helps in transferring materials to and from the mother's blood. The umbilical cord contains one vein and two arteries embedded within Wharton's jelly.
Umbilical cord vessels are very sensitive to many vasoactivators, such as serotonin, oxytocin, and angiotensin II. The vessel wall's smooth muscle contractility is also influenced by substances in the neighboring endothelial cells. Several potent vasodilators like prostacyclin (PGI2) and Prostaglandin E2 (PGE2) are produced by the umbilical cord. Both PGI2 and PGE2 are inhibitors for platelet aggregation and potent vasodilators. The synthesis and production of these vasodilators are significantly less in smoking and diabetic pregnant women compared to normal pregnant women.
Placental Villous Capillaries - The umbilical arteries branch to form chorionic arteries at the umbilical cord and placenta junction. It enters the chorionic plate by crossing the fetal surface of the placenta and branching further before entering into the villi. These arteries are responsive to vasoactive substances. About two-thirds of the chorionic arteries supply the cotyledons, and the remaining radiate to the edge of the placenta and get down to a network. Each umbilical cord artery usually provides eight or more terminal chorionic plate arteries called stem arteries of the peripheral trunci chorii. The first-order branches divide into four to eight horizontal cotyledonary vessels of the secondary order. These again branch into the third order as they curve toward the basal plate. In the villi, the third-order branches form the arteriocapillary venous system to the villous capillaries. Here the fetal blood is very close to the maternal blood, but no mixing of fetal and maternal blood occurs.
The capillary networks are the functional unit of maternal and fetal blood exchange. The blood pressure in the umbilical arteries is about 50 mmHg, and the blood pressure falls to 30 mmHg as it flows through smaller vessels. In the umbilical vein, the pressure is 20 mmHg. The fetal vessels and their branch's pressure is always higher than that within the intervillous space to protect the fetal vessels against collapse.
Conclusion:
The placental circulation is distinctive, as they share two different circulatory systems, both the mother and the fetus. This circulation plays a significant role in supplying nutrients and oxygen to the baby from the mother without combining the mother's and baby's blood.
