The primary function of the placenta in all species is to promote selective transport of nutrients and waste products between mother and fetus. Such transport is facilitated by the close approximation of maternal and fetal vascular systems within the placenta.
It is important to recognize that there normally is no mixing of fetal and maternal blood within the placenta. Entry of small amounts of fetal blood into the maternal circulation does occasionally occur, and can evoke an immune response in the mother that affects that fetus after birth or fetuses in subsequent pregnancies that are sired by the same father.

The placenta is a complex tissue and should not be envisioned as simple permiable membrane. In addition to transporting some molecules unaltered between fetal and maternal blood, it also consumes a large fraction of certain types of cargo – glucose and oxygen being good examples. Additionally, a number of molecules that cross the placenta are metabolized to other things during passage.
There are a number of differences among species in the characteristics of transport across the placenta, which should not be a big surprise considering the differences in structure of the placental interface. The following discussions reflect general principles of placental transport.
Gases like oxygen and carbon dioxide diffuse through and across tissues in response to differences in partial pressure.
In late pregnancy, the mean partial pressure of oxygen [P02] in maternal blood is considerably higher than in fetal blood. As a consequence, oxygen readily diffuses across the placenta from maternal to fetal blood. Despite its low PO2, fetal blood is able to transport essentially the same quantity of oxygen to tissues as maternal blood. This is because the hemoglobin concentration in fetal blood is about 50% higher than in maternal blood, and the majority of hemoglobin in the fetus is fetal hemoglobin, which has a higher oxygen carrying capacity than adult hemoglobin.
Carbon dioxide is produced abundantly in the fetus, and the PCO2 of fetal blood is higher than maternal blood. Carbon dioxide therefore diffuses from fetal blood, through the placenta, into the maternal circulation, and is disposed of by expiration from the mother’s lungs.
Glucose is the major energy substrate provided to the placenta and fetus. It is transported across the placenta by facilitated diffusion via hexose transporters that are not dependent on insulin [GLUT3 and GLUT1]. Although the fetus receives large amounts of intact glucose, a large amount is oxidized within the placenta to lactate, which is used for fetal energy production.
Amino acid concentrations in fetal blood are higher than in maternal blood. Amino acids are therefore transported to the fetus by active transport. A family of at least 10 sodium-dependent amino acid transporters have been identified in placenta that serve this function. There is substantial metabolism of some amino acids as they cross the placenta – for example, much of the serine taken up by the placenta is converted to glycine prior to delivery to the fetus.
There is much more variability among species in placental permiability to fatty acids than to glucose or amino acids. In some animals, there is little transport of fatty acids from mother to fetus, while in others a significant amount of transport takes place.
There are marked differences among species in whether immunoglobulins are transported across the placenta. In primates and rodents, there is substantial transfer of immunoglobulin G from maternal to fetal circulations prior to birth. This process requires immunoglobulin-binding proteins in the placenta.
In contrast, there is no transplacental transfer of immunoglobulins in animals like cattle, sheep, horses and pigs. In those species, the neonate is essentially devoid of circulating antibodies until it absorbs them from colostrum [first milk].
Bilirubin is a waste product derived from the heme in hemoglobin. This lipophilic molecule is conjugated in the liver to make it water-soluble, and eliminated by excretion into bile. The fetus also produces bilirubin, but conjugates only a small fraction. This is good because conjugated bilirubin is transported across the placenta very poorly. In contrast, unconjugated fetal bilirubin is readily transported from the fetal circulation, across the placenta, for elimination by the mother.
Many drugs are eliminated in bile through pathways similar to bilirubin. The relative inability of the fetal liver to metabolize and conjugate means that it is impaired for eliminating such molecules compared to adults.

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