Breathing function
The placenta, which plays the role of «fetal lungs», is 15 times less efficient (with equivalent weight of tissue) than the real lungs.
The exchange takes place between maternal (oxygen-rich) blood and the blood of the Aa. umbilicales (mixing of arterial and venous blood, oxygen-poor). The oxygen goes via diffusion from the maternal into the fetal circulation system (PO2 maternal > PO2 fetal).
The carbon dioxide, the partial pressure of which is elevated in fetal blood, follows a reversed gradient.
Fetal, oxygen saturated blood, returns to the fetus via the umbilical vein, while maternal, oxygen-poor blood, flows back into the uterine veins.
The supply of the fetus with oxygen is facilitated by three factors:
- difference of oxygen concentration and partial pressure within the feto-maternal circulation system
- higher affinity of fetal hemoglobin (HbF) for oxygen
- l'effet Bohr
Expression of the human hemoglobin chains as a function of age.
Fig. 36Dissociation curve of fetal (HbF) and adult (HbA) hemoglobin. In the placenta the hemoglobin A of the maternal erythrocytes frees up its oxygen in favor of the fetal hemoglobin F, that possesses a higher affinity for oxygen.
Fetal hemoglobin and 2,3-DPG (2,3 diphosphoglycerate)
The large affinity for oxygen of fetal hemoglobin in comparison with adult hemoglobin facilitates oxygen transfer from the mother to fetus. The reason for this elevated affinity is the weaker binding of the g-chain (specifically for HbF) to the 2,3 DPG, the chain that replaces the b-chain in fetal hemoglobin. 2,3-diphosphoglycerate lowers the affinity of hemoglobin A for oxygen by binding to and stabilizing deoxyhemoglobin. The cavity in the center of the four globin chaines is capable of binding one molecule of 2,3-DPG between the two b-chains of the hemoglobin A whereas this is not possible for the g-chains of the hemoglobin F.
P50 (**) of the HbF is reduced in comparison to adult Hb (figure)
**P50 = PO2 n which the hemoglobin is 50% saturated with oxygen.
The hemoglobin binds oxygen in order to form oxyhemoglobin. The affinity of the hemoglobin for oxygen depends on the temperature, the pH, the pCO2 and the 2,3 DPG concentration in the erythrocytes. An elevation of the temperature, of PCO2 (partial pressure of CO2) or 2,3 DPG blood value or a lowering of the pH (acidosis) shifts the dissociation curve towards the right, reduces the affinity of hemoglobin for oxygen and favors its dissociation in the blood. This phenomenon is known as the Bohr effect
Oxygen enrichment of fetal blood is promoted by concentration differences and partial pressure differences in the feto-maternal circulation, as well as by the larger affinity for oxygen to fetal HbF hemoglobin and the Bohr effect
.