HL 1 Biology

11.4d Pregnancy & Birth


  • depends on production of progesterone and estrogen
  • at early pregnancy: HCG (human chorionic gonadotropin) produced in embryo
  • HCG stimulates corpus luteum to produce more progesterone and estrogen
  • placenta: needed to help fetus grow
  • 9 weeks: placenta starts to secrete estrogen + progesterone to sustain pregnancy
  • Amniotic fluid: supports and protects developing fetus

<<Structure and function of the Placenta>>

  • placental villus: basic funtional unit of the placenta ; increase in number during pregnancy to meet increasing demands of fetus for exchange of materials with mother
  • fetal blood circulates in blood capillaries
  • distance between fetal and maternal blood is small (5µm)
  • placental barrier:  the wall formed by cells that separate maternal and fetal blood (must be permeable)
  • microvilli from placental cells into the maternal blood increase in Surface Area


  • at the end of pregnancy: hormones from fetus tell the placenta to stop secreting progesterone = oxytocin secreted
  • oxytocin: stimulates contractions of muscle fibres in the myometrium & signal pituitary glands to increase oxytocin secretion
  • increased oxytocin = increase contractions more,causing oxytocin secretion
  • advantage of causing gradual increase in myometrial contractions = help baby be born with minimum intensity of contraction
  • uterine contraction release amniotic sac and amniotic fluid out
  • After hours of uterine contractoins: baby pushed out through cervix and vagina
  • Placenta + umbilical cord expelled from mothers body


Electron Micrograph of Placenta (p.283)

1) (a)  microvilli

(b) The microvilli help transport nutrients by the process of active transport of food (e.g. glucose) and also function to absorb water.The microvilli increase in surface area as it undergoes the process of projecting form placental cells into the maternal blood. As the fetal blood enters, the placental villi is oxygenated and filled with nutrients and removes excretory products.

2) As the sER has a function of synthesizing lipids, including steroids, it may have a function of producing progesterone in the placenta. This is because progesterone is a steroid hormone, and is present in females. As pregnancy depends on the production of progesterone and estrogen, the progesterone must continuously be produced to stimulate the development of the uterus wall, an essential supply for the embryo.

3) The structure in the lower left part of the micrograph can be identified as a nucleus because it is a large structured organelle. Also, the placement of this structure next to the sER makes the structure to be the nucleus. Also, a nucleus must be present in order for activity to be functioned in cells.

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