Although the placenta acts as a barrier to most substances, it allows exchange of gases, transfer of fetal nutrition, and removal of waste products in a highly effective manner. Speed of exchange and concentration of substance exchanged depends upon:

A low-molecular-weight lipid-soluble substance with a high concentration gradient across the placenta, for example, will be transferred quickly to the fetus. Actual transfer occurs by simple diffusion, facilitated diffusion, active transport, and/or endocytosis (Table 1.3).