In the later stages of pregnancy, the cellular layer disappears. The fetal and maternal blood do not mix within the placenta. In the human, the fetal blood is contained entirely within small vessels and capillaries, whereas the maternal blood within the placenta leaves arterial vessels and gains free access to the space surrounding the fetal villi. Thus, what is termed the maternal blood lake is formed. It is across this interface of trophoblastic epithelium and maternal blood that oxygen, carbon dioxide, nutrients, and waste products are exchanged.
Maternal blood is drained by veins from the intervillous spaces of the placenta.
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