Anatomy & Physiology

Anatomy & Physiology Study Guide pregnancy . This occurrence is a medical emergency as a growing embryo and, subsequent, fetus can rupture uterine tubes and other structures and lead to massive internal bleeding. Implantation sets in motion the necessary activities for the formation of the support structures for a viable pregnancy. Placentation occurs as blood vessels form around the periphery of the blastocyst, and as the placenta—a complex organ that permits exchange between the maternal and embryonic circulatory systems—develops. The fetus and placenta remain connected by the umbilical cord . Germ layers also participate in the formation of four extra embryonic membranes : the yolk sac endoderm and mesoderm, the amnion, the allantois (endoderm and mesoderm), and the chorion mesoderm. Embryogenesis is the formation of a viable embryo. After implantation, the hormone human chorionic gonadotropin (hCG) appears in the maternal bloodstream. As the pregnancy continues through the first trimester, the presence of hCG in urine or blood samples provides a reliable indicator of pregnancy. Over-the-counter pregnancy tests are sensitive to the presence of hCG. Embryogenesis After multiple cell divisions to form a blastula, embryogenesis is the next occurrence. First, the body of the embryo begins to separate itself from the rest of the embryonic disc. The formation of the body and organs begins. This process is called embryogenesis . The first trimester is a critical for development as the first 12 weeks establish the basis for organ formation called organogenesis . During the second and third trimesters, maternal organ systems support the developing fetus, and the uterus undergoes structural and functional changes. By the end of the first trimester, the basic structure of all the major organ systems are formed. The fetus will grow to a weight of about 0.64 kg (1.4 lb.) over the next three months. During the third trimester, the organ systems become ready to perform their normal functions. For example, the kidneys are functional, and the fetus regularly urinates in utero. The rate of growth begins to slow; yet, this trimester sees the largest weight gain. The fetus gains about 2.6 kg (5.7 lb.), reaching a full-term (at 37 weeks gestation), weight of approximately 3.2 kg (7 lb.). At full term pregnancy, the uterus and fetus displace many of the maternal abdominal organs out of their normal positions. Pregnancy and Maternal Sy tems The developing fetus is completely dependent on maternal organ systems for its survival. The maternal organs provide nourishment, respiration, and waste removal for the fetus. Maternal systems perform these functions in addition to their normal operations. The major changes that occur in maternal systems include the following: • Maternal respiratory rate increases and tidal volume increases. • Maternal blood volume increases. By the end of gestation, maternal blood volume has increased by almost 50 percent. • Maternal nutrient requirements increase 10–30 percent. Pregnant women must nourish both their fetus and themselves; hence, they tend to have increased hunger sensations.

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