The Placenta
The temporary organ that connects fetus to mother — gas and nutrient exchange, immune tolerance, hormone production, and what happens at delivery.
The placenta is a disposable organ built jointly by mother and embryo, performing the duties of lungs, kidneys, gut, and endocrine system for nine months, and then discarded. No other organ is grown, used, and shed with such precision — or does its job so differently from the adult systems it temporarily replaces.
Structure and function
Maternal blood and fetal blood never mix. The placenta's chorionic villi bathe in maternal blood sinuses; oxygen and nutrients diffuse across a few cell layers into fetal capillaries, while fetal waste (CO2, urea) diffuses back. This countercurrent-free, flow-limited exchange is much less efficient than lungs — which is why fetal hemoglobin has a higher oxygen affinity than adult hemoglobin, extracting oxygen from the same blood the mother has already used. Nutrient transfer is selective: glucose crosses freely, large proteins and most cells do not; the fetus produces its own red blood cells and most of its proteins from early on.
Hormones and immune negotiation
The placenta is also an endocrine organ. It secretes hCG (the signal pregnancy tests detect), progesterone (which maintains the uterine lining), and placental lactogen, which restructures the mother's glucose metabolism to favor the fetus. On the immune side, the fetus is genetically half-foreign, yet the maternal immune system does not reject it: the placenta expresses HLA-G and other molecules that suppress local rejection, and regulates maternal immune responses rather than hiding from them. This immunological negotiation is a standing model for transplant tolerance.
The delivery
At birth the placenta separates and is expelled — the "afterbirth" — weighing about 500 g. Post-delivery inspection is not ritual: the placenta records gestational health, and retained placental tissue is a leading cause of postpartum hemorrhage. Related reading: homeostasis for the maternal physiology being adjusted, and circadian rhythm reset for why the fetal clock is entrained by maternal rhythms.
Tags
development physiology placenta pregnancy