Placenta introduction.
[1]. Placenta is a temporary membranous vascular organ that develops in females during pregnancy. It is expelled after childbirth. Placenta forms a link between the fetus and mother. It is considered as an anchor for the growing fetus.
[2]. It is not only the physical attachment between the fetus and mother, but also forms the physiological connection between the two. Placenta is implanted in the wall of the uterus.
[3]. It is formed from both embryonic and maternal tissues. So, it consists of two parts namely the fetal part and the mother’s part. It is connected to the fetus by umbilical cord, which contains blood vessels and connective tissue.
[4]. The delivery of fetus is followed by the expulsion of placenta. After expulsion of the placenta, the umbilical cord is cut. The site of attachment of placenta in the center of anterior abdomen of fetus is called navel or umbilicus.
Functions of Placenta.
Nutritive Function of Placenta.
Nutritive substances, electrolytes and hormones necessary for the development of fetus diffuse from mother’s blood into fetal blood through placenta.
Excretory Function.
Metabolic end products and other waste products from the fetal body are excreted into the mother’s blood through placenta.
Respiratory Function.
[1]. Fetal lungs are nonfunctioning and placenta forms the respiratory organ for fetus.
[2]. Oxygen necessary for fetus is received by diffusion from the maternal blood and carbon dioxide from fetal blood diffuses into the mother’s blood through placenta.
Exchange of Respiratory Gases between Fetal Blood and Maternal Blood.
[1]. Exchange of respiratory gases between fetal blood and maternal blood occurs mainly because of pressure gradient. Partial pressure of oxygen in the maternal blood is 50 mm Hg.
[2]. In fetal blood, the partial pressure of oxygen is 30 mm Hg. This pressure gradient of 20 mm Hg causes the diffusion of oxygen into the fetal blood.
[3]. This pressure gradient is very low, compared to the gradient existing between partial pressure of oxygen in arterial blood and alveoli in adults. Still, an adequate quantity of oxygen is available for fetus.
[4]. It is because of two reasons:
1. The hemoglobin in fetal blood has 20 times more affinity for oxygen than the adult hemoglobin .
2. The concentration of hemoglobin is about 50% more in fetal blood than in adult blood.
Bohr effect and Double Bohr effect.
[1]. Bohr effect is the decrease in the affinity of hemoglobin for oxygen due to increased carbon dioxide tension. When carbon dioxide tension decreases, the affinity of hemoglobin for oxygen is increased.
[2]. All the metabolic end products including carbon dioxide are completely excreted from fetus into the maternal blood. This develops low partial pressure of carbon dioxide in fetal blood. So, the affinity of fetal hemoglobin for oxygen increases resulting in diffusion of more amount of oxygen from mother’s blood into fetal blood.
[3]. At the same time, because of entrance of fetal carbon dioxide into maternal blood, partial pressure of carbon dioxide is very high in mother’s blood. It decreases the affinity of mother’s hemoglobin for oxygen resulting in diffusion of more amount of oxygen into the fetal blood.
[4]. Double Bohr effect is the operation of Bohr effect in both fetal blood and maternal blood.
Endocrine Functions of Placenta.
Hormones secreted by placenta are:
1. Human chorionic gonadotropin.
2. Estrogen.
3. Progesterone.
4. Human chorionic somatomammotropin.
5. Relaxin.
1. Human Chorionic Gonadotropin.
Human chorionic gonadotropin (hCG) is a glycoprotein. Its chemical structure is similar to that of LH.
Actions of hCG hormone.
a. On corpus luteum:
hCG is responsible for the preservation and the secretory activity of corpus luteum. Progesterone and estrogen secreted by corpus luteum are essential for the maintenance of pregnancy. Deficiency or absence of hCG during the first 2 months of pregnancy leads to termination of pregnancy (abortion), because of involution of corpus luteum.
b. On fetal testes:
Action of hCG on fetal testes is similar to that of LH in adults. It stimulates the interstitial cells of Leydig and causes secretion of testosterone. The testosterone is necessary for the development of sex organs in male fetus.
2. Estrogen.
Placental estrogen is similar to ovarian estrogen in structure and function.
Actions of placental estrogen.
i. On uterus: Causes enlargement of the uterus so that, the growing fetus can be accommodated.
ii. On breasts: Responsible for the enlargement of the breasts and growth of the duct system in the breasts.
iii. On external genitalia: Causes enlargement of the female external genitalia.
iv. On pelvis: Relaxes pelvic ligaments. It facilitates the passage of the fetus through the birth canal at the time of labor.
3. Progesterone.
Placental progesterone is similar to ovarian progesterone in structure and function.
Actions of placental progesterone.
i. On endometrium of uterus: Accelerates the proliferation and development of decidual cells in the endometrium of uterus. The decidual cells are responsible for the supply of nutrition to the embryo in the early stage.
ii. On the movements of uterus: Inhibits the contraction of muscles in the pregnant uterus. It is an important function of progesterone as it prevents expulsion of fetus during pregnancy.
iii. On breasts: Causes enlargement of breasts and growth of duct system of the breasts. Progesterone is responsible for further development and preparation of mammary glands for lactation.
4. Human Chorionic Somatomammotropin.
[1]. Human chorionic somatomammotropin (HCS) is a protein hormone secreted from placenta. It is often called placental lactogen. It acts like prolactin and growth hormone secreted from pituitary.
[2]. So, it is believed to act on mammary glands and to enhance the growth of fetus by influencing the metabolic activities. It increases the amount of glucose and lipids in the maternal blood, which are transferred to fetus.
Actions of HCS.
i. On breasts: In experimental animals, administration of HCS causes enlargement of mammary glands and induces lactation. That is why, it is named as mammotropin. However, the action of this hormone on the breasts of pregnant women is not known.
ii. On protein metabolism: HCS acts like GH on protein metabolism. It causes anabolism of proteins and accumulation of proteins in the fetal tissues. Thus, the growth of fetus is enhanced.
iii. On carbohydrate metabolism: It reduces the peripheral utilization of glucose in the mother leading to availability of large quantity of glucose to the growing fetus.
iv. On lipid metabolism: It mobilizes fat from the adipose tissue of the mother. A large amount of free fatty acid is made available as the source of energy in the mother’s body. It compensates the loss of glucose from the mother’s blood to fetus.
5. Relaxin.
Relaxin is a polypeptide, which is secreted by corpus luteum. It is also secreted in large quantity by placenta and mammary glands at the time of labor .
Fetoplacental unit.
[1]. Fetoplacental unit refers to the interaction between fetus and placenta in the formation of steroid hormones.
[2]. The interaction between fetus and placenta occurs because some of the enzymes involved in steroid synthesis present in fetus are absent in placenta and those enzymes, which are absent in fetus are present in placenta. Due to this interaction during synthesis of steroid hormones, fetus and placenta are together called fetoplacental unit .
Functions of Fetoplacental unit.
Placenta and fetus interact with each other in the synthesis of steroid hormones in the following manner:
1. Cholesterol, which is the precursor for steroid hormones, is obtained by placenta from mother’s blood .
2. Placenta synthesizes pregnenolone from cholesterol.
3. From pregnenolone, progesterone is formed.
4. Some amount of the pregnenolone from placenta enters fetus. Fetal liver also produces a small quantity of pregnenolone.
5. Pregnenolone from placenta and fetal liver forms the substrate for the formation of two substances in the adrenal gland of the fetus:
a. Dehydroepiandrosterone sulfate (DHEAS).
b. 16hydroxydehydroepiandrosterone sulfate (16OHDHEAS).
Some of the DHEAS is also hydroxylated into 16-OH-DHEAS in fetal liver.
6. DHEAS and 16OHDHEAS are transported back into the placenta to form estrogen.
7. Estradiol is synthesized from DHEAS and estriol from 16OHDHEAS. These two forms of estrogen enter mother’s blood.
8. Some amount of the progesterone enters the fetus from placenta.
9. From this progesterone, cortisol and corticosterone are synthesized in fetal adrenal glands.
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