The placental barrier is another impediment to chemical substances. Besides providing for the nutritional, gas exchange, and excretory needs of the developing fetus, the placenta also protects the fetus from toxicants absorbed and subsequently distributed in the maternal circulation. The barrier is formed from cell layers between the maternal and fetal circulatory vessels in the placenta. Lipids present in the plasma membranes of these additional cells serve to limit the diffusion of watersoluble toxicants. As with the blood-brain barrier, the placental barrier only slows down the diffusion of toxicants from maternal circulation into fetal circulation, reducing exposure of developing fetal tissues to toxicants.