Placental physiology
- The placenta has several functions, including the maternal–fetal transfer of nutrients and oxygen, the clearance of fetal waste, and the synthesis of proteins and hormones.
- The human placenta is classified as hemochorioendothelial, because only three cell layers separate the maternal and fetal circulations: fetal trophoblast, fetal villous stroma, and fetal capillary endothelium. Fetal villi are suspended in intervillous spaces bathed with maternal blood (Figure 36.1).
- Placental villi create a high surface area/volume ratio with a total surface area at term of around 10 m2.
- Transfer across the placenta occurs by passive diffusion (oxygen, CO2, electrolytes, simple sugars), active transport (iron, vitamin C), or carrier-mediated facilitated diffusion (immunoglobulins).
- There is a large placental reserve; 30–40% of placental villi can be lost without evidence of placental insufficiency.
Fetal physiology
Nutrition
- The embryo consists almost entirely of water. After 10 weeks, however, the fetus is dependent on nutrients from the maternal circulation via the developing placenta.
- The average term fetus weighs 3,400 g. Birthweight is influenced by race, socioeconomic status, parity, genetic factors, diabetes, smoking, and fetal gender. At term, the fetus grows around 30 g/day.
Cardiovascular system
- The fetal heart starts beating at 4–5 weeks’ gestation.
- The fetoplacental blood volume at term is 120 mL/kg (or a total of approximately 420 mL).
- After birth, the fetal circulation undergoes profound hemodynamic changes (Figure 36.3). The umbilical vessels, ductus arteriosus, foramen ovale, and ductus venosus constrict. This is thought to be due to a change in oxygen tension within minutes of birth. The distal portions of the umbilical arteries atrophy within 3–4 days to become the umbilical ligaments
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