Nutritive and excretory functions
The supply of nutrients to the fetus, which is essential for its growth and its energy use, is guaranteed by the maternal organism.
Water diffuses into the placenta along an osmolar gradient. The water exchange increases during the pregnancy up to the 35th week (3.5 liter / day).
The electrolytes follow the water, whereby iron and calcium only go from mother to child.
Glucose is the fetus' main source of energy and passes the placenta via simplified transport. The fetal blood sugar concentration is 2/3 of the mother's and depends on it. At the level of the trophoblast the placenta can synthesize and store glycogen in order to satisfy local glucose requirements through glycogenolysis. Important alterations in carbohydrate metabolism arise in the mother during the pregnancy in order to cope with the fetal needs and to prepare for lactation. For the mother pregnancy is "diabetogenous" due to the impaired insulin sensibility of her tissues, which can be reduced up to 80%. This effect can be partially explained by the increase of specific insulin-antagonists, of which the most important are the human placental lactogene hormone HPL, also known as HCS (human chorionic somatotrophic hormone) (see endocrinal function).
Proteins (also see protein transfer) are too large to pass through the placental barrier. Peptides and amino acids, though, pass through the placental barrier via active transport and thus insure the fetus' own protein synthesis.
Amino acids, precursors of fetal protein synthesis, stem from the metabolism of the maternal proteins. The placental transport is facilitated by the influence of hormones, e.g., GH (growth hormone) and TSH (thyroid stimulating hormone) against a concentration gradient (2-3 times higher in the fetus as in the mother).
Lipids and triglycerides are decomposed in the placenta, where new lipid molecules are synthesized.
Cholesterol passes through the placental membrane easily, just like its derivates: e.g., steroid hormones.
Water-soluble vitamins easily pass through the placental membrane. The amount of the fat soluble vitamins (A,D,E and K) in the fetal circulation is, on the other hand, quite low. Vitamin K plays an important role in blood coagulation and is applied to the child immediately after birth, in order to prevent hemorrhages.
Placental exchange processes are also involved in the removal of waste products from the fetal metabolism. They cross over into the maternal blood in order to be excreted by the mother (urea, creatinine, ureic acid).