BIOLOGY Volume 3 - A Guide to General Biology - 2004

21. REPRODUCTION

21.8. Human sexual reproduction

21.8.9. Maternal-fetal exchange

Water. Water can cross the Placenta by osmosis.

NUTRIENTS. Glucose, Amino Acids, Lipids, inorganic salts, and Vitamins are transferred from the mother to the fetus across the placenta. Glucose is transferred via Facilitated Diffusion using specific carrier Proteins described in section 5.9.8, whereas ions such as sodium, potassium, and calcium are transported primarily via Active Transport, although some diffusion may also occur in the case of ions. Amino acids, iron, and vitamins cross the placenta through active transport. Structure/19.html">The Importance of maintaining a proper diet during Pregnancy is discussed in Chapter 8.

RESPIRATORY GASES. Oxygen is essential for fetal aerobic Respiration. This gas diffuses down a concentration gradient from a region of higher concentration to one of lower concentration, i.e., from maternal Blood to fetal blood. Fetal Hemoglobin has a higher affinity for oxygen than adult hemoglobin, thereby enhancing gas exchange efficiency. Carbon dioxide, the end product of aerobic respiration, diffuses in the opposite direction.

EXCRETORY PRODUCTS. In addition to carbon dioxide (see above), the fetus produces nitrogenous waste products, primarily urea, which diffuses from the fetus to the mother across the placenta. Urea is then excreted via the mother's Kidneys.

Antibodies. Antibodies can pass from the mother to the fetus across the placenta. As a result, the fetus is protected against the same diseases as the mother. This phenomenon is known as passive Immunity. After birth, this immunity gradually fades because the fetal immune system is not yet capable of producing its own antibodies (active immunity). Occasionally, maternal antibodies can be harmful to the fetus, as occurs in rhesus incompatibility (see below).

ENDOCRINE Organs. The placenta also Functions as an endocrine organ. From the third month of pregnancy onward, the placenta completely takes over all the Functions of the corpus luteum as the primary source of estrogen and progesterone. The roles of these Hormones are outlined in Table 21.5.

Throughout pregnancy, estrogen and progesterone are secreted in progressively increasing amounts, initially by the corpus luteum and subsequently, and chiefly, by the placenta. During the final three months of pregnancy, estrogen secretion increases at a faster rate than progesterone secretion; immediately prior to childbirth, progesterone levels drop while estrogen levels rise.

The placenta—specifically the chorion, a component of it—also secretes human chorionic gonadotropin and placental lactogen, the functions of which are listed in Table 21.5.

Class="center">Table 21.5. Hormones secreted by the placenta during pregnancy and their functions