The placenta is shaped like a disc or pancake, with the umbilical cord connecting the fetus on the smooth side, and the exposed villi on the other side tightly attached to the inner surface of the uterine wall. The placenta is mainly composed of countless root structures, with trunks and branches, and branches branch again until they are divided into very fine branches. Because it is shaped like villi at the beginning of development, it is called placental villi. There are villous epithelial cells on the surface of the villi, and there are small arteries and veins inside the villi. These blood vessels converge along the villus branches to the main trunk, and finally become the veins and arteries of the umbilical cord and enter the fetus. The outside of the villi is flowing maternal blood.
At the beginning of pregnancy, villous cells invade the endometrial capillaries and cause them to bleed, forming blood cavities around the villi, called villous lumens. Each lumen has a small arterial opening for supply. Blood flows in; there are also small vein openings that allow blood to flow back to the mother\’s body. The villi are soaked in the mother\’s blood between the villi, exchanging substances, absorbing nutrients, and expelling waste. Fetal blood and maternal blood are not in direct contact. The exchanged substances pass through the villous epithelial cells, villus blood vessels and materials surrounding the blood vessels.
The blood flow of a full-term fetus flows through the placental villi at a flow rate of 500 ml per minute, while the blood flow of the full-term pregnant uterus can reach 500-700 ml per minute. In addition to part of it supplying the uterus itself, most of the blood flows through the placental villi. Some blood flows through the placental intervillous space. With such a huge amount of blood, as someone said, the growth of the fetus is watered by the mother\’s blood. Therefore, fetal development depends on the function of the placenta. Just like a plant has roots, the roots of the fetus are villi, but they are buried in the maternal blood. Oxygen in maternal blood and carbon dioxide in fetal blood can simply diffuse and exchange through villi, and the placenta replaces the fetal respiratory organ function. Substances with small molecular weights such as water, potassium, sodium, magnesium, amino acids, and glucose enter the placenta through relatively simple diffusion. For iron, calcium, phosphorus, iodine, and water-soluble vitamins, despite the low content in maternal blood, the villi actively choose to absorb them.
The products of fetal metabolism are excreted through the villi into the maternal blood in the intervillous space, then enter the maternal blood circulation for processing, and are finally excreted from the maternal body. The placenta serves as a \”transfer station\” for essential substances and metabolites of the fetus.
Macromolecule substances such as protein and fat cannot pass through the placenta directly, but the villi cells have a variety of enzymes that can break down these substances into small simple substances. It waits until it is resynthesized in the fetus, and the placenta acts as a \”processing factory\” again.
Pathogens such as bacteria and protozoa cannot pass through the villus epithelium. At this time, the placenta plays a role inActs as a \”defensive barrier\”. However, viruses and drugs can get through. The mother\’s immunoglobulin G can also pass through, so the fetus can obtain certain antibodies from the mother to resist infection.
The placenta also produces hormones. Villous epithelial cells produce a large amount of hormones, such as estrogen (estradiol, estriol) and progesterone (also called progesterone, progesterone). The production of these hormones increases with the months of pregnancy. Taking estriol as an example, the content in maternal blood during full-term pregnancy can be as high as 1,000 times that of non-pregnancy. The villi also produce a pituitary-like gonadotropin called chorionic gonadotropin. Pregnancy can be confirmed by detecting the presence of this hormone in maternal blood and urine in early pregnancy. It also produces human chorionic growth lactogen, which regulates maternal fat metabolism and is beneficial to fetal growth and development. In addition, many enzymes and specific proteins are produced. The hormones, proteases and other substances produced by the \”factory\” of the placenta, on the one hand, supply and promote the growth and development of the fetus, on the other hand, they change the environment in the mother\’s body to adapt to the progress of pregnancy and prepare for delivery.
The reason why pregnancy does not cause rejection and can continue to develop to term is related to the placenta producing a large amount of hormones and specific proteins.
This article is provided by Baidu Reading and is excerpted from \”The Clear \”Conception\” Plan\” Author: Sun Jianqiu and Xie Yingbiao