A level Biology exam revision resources written by A level Examiners


The placenta grows tightly fused to the wall of the uterus. Its blood vessels, supplied by the fetal heart, are literally bathed in the mother's blood. Although there is normally no mixing of the two blood supplies, the placenta does facilitate the transfer of a variety of materials between the fetus and the mother, listed in table 1.

Mother to Fetus Fetus to Mother

  • Oxygen
  • Glucose
  • Amino acids
  • Lipids, fatty acids and glycerol
  • Vitamins
  • Ions; Na+, Cl-, Ca2+
  • Alcohol, nicotine + other drugs.
  • Viruses
  • Antibodies
  • Carbon dioxide
  • Urea
  • Other waste products

The placenta is an organ of exchange and therefore has a large surface area. To achieve this it has chorionic villi (the cells of which have microvilli and many mitochondria). The cells also have large numbers of transport proteins present in their membranes. The maternal blood capillaries that surround the chorionic villi break down creating lacunae. These are blood filled spaces. This has the effect of reducing the diffusion distances and increasing the surface area for exchange. Blood is supplied to the fetus by an umbilical vein and taken from the fetus to the placenta by umbilical arteries.

The metabolic activity of the placenta is very high - this is due to the amount of active transport taking place.

The placenta is also an endocrine organ and it secretes hCG, progesterone and oestrogen.

During pregnancy, prenatal diagnosis of genetic disorders can be made, using the procedures of amniocentesis and chorionic villus sampling (CVS) - (to be considered in a later section on screening).

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