The umbilical cord extends from the fetal umbilicus to the fetal surface of the placenta, that is, the chorionic plate. Blood flows from the umbilical vein toward the fetus. Blood then takes a path of least resistance via two routes within the fetus. One is the ductus venosus, which empties directly into the inferior vena cava (Fig. 7-9, p. 130). The other route consists of numerous smaller openings into the hepatic circulation. Blood from the liver flows into the inferior vena cava via the hepatic vein. Resistance in the ductus venosus is controlled by a sphincter that is situated at the origin of the ductus at the umbilical recess and is innervated by a vagus nerve branch.
Blood exits the fetus via the two umbilical arteries. These are anterior branches of the internal iliac artery and become obliterated after birth to form the medial umbilical ligaments.
FIGURE 5-17 Cross-section of umbilical cord. The large umbilical vein carries oxygenated blood to the fetus (right). To its left are the two smaller umbilical arteries, carrying deoxygenated blood from the fetus to the placenta. (Used with permission from Dr. Mandolin
S. Ziadie.)
The production of steroid and protein hormones by human trophoblasts is greater in amount and diversity than that of any single endocrine tissue in all of mammalian physiology. A compendium of average production rates for various steroid hormones in nonpregnant and in near-term pregnant women is given in Table
It is apparent that alterations in steroid hormone production that accompany normal human pregnancy are incredible. he human placenta also synthesizes an enormous amount of protein and peptide hormones, summarized in Table 5-2. Another remarkable feature is the successful
TABLE 5-1 . Steroid Production Rates in Nonpregnant and Near-Term Pregnant Women aEstrogens and progesterone are produced by placenta. Aldosterone is produced by the maternal adrenal in response to the stimulus of angiotenoin II. Deoxycorticosterone is produced in extraglandular tissue sites by way of the 21-hydroxylation of plasma progesterone. Cortisol production during pregnancy is not increased, even though the blood levels are elevated because of decreased clearance caused by increased cortisol-binding globulin.
physiological adaptations of pregnant women to the unique endocrine milieu, and this is discussed throughout Chapter 4 (p. 49).
Chorionic gonadotropin is a glycoprotein with biological activity similar to that of LH, and both act via the same LH-hCG receptor. hCG has with a molecular weight of 36,000 to 40,000 Da and has the highest carbohydrate content of any human hormone-30 percent. The carbohydrate component, and especially the terminal sialic acid, protects the molecule from catabolism. As a result, the 36-hour plasma half-life of intact hCG is much longer than the 2 hours for LH. he hCG molecule is composed of two dissimilar subunits termed . and � subunits. hese are noncovalently linked and are held together by electrostatic and hydrophobic forces. Isolated subunits are unable to bind the LH-hCG receptor and thus lack biological activity.
hCG is produced almost exclusively in the placenta, but low levels are synthesized in the fetal kidney. Other fetal tissues produce either the �-subunit or intact hCG molecule (McGregor, 1981,i1983).
The hCG hormone is structurally related to three other glycoprotein hormones-LH, FSH, and TSH. All four glycoproteins share a common a-subunit. However, each of their 3-subunits, although sharing certain similarities, is characterized by a distinctly diferent amino acid sequence.