Oxygen placenta

The quantity P50, a measure of O2 concentration, is the partial pressure of Oj that half-saturates a given hemoglobin. Depending on the organism, P50 can vary widely, but in all instances it will exceed the PO2 of the peripheral tissues. For example, values of P50 for HbA and fetal HbF are 26 and 20 mm Hg, respectively. In the placenta, this difference enables HbF to extract oxygen from the HbA in the mother s blood. However, HbF is suboptimal postpartum since its high affinity for O2 dictates that it can deliver less Oj to the tissues. 

Relative affinities of different hemoglobins for oxygen are expressed as P5Q, the POj that half-saturates them with Oj. Hemoglobins samrate at the partial pressures of their respective respiratory organ, eg, the lung or placenta. 

A similar adaptive mutation is found in human fetal hemoglobin. This has a lower oxygen affinity than the adult in the absence of phosphate, but it also has a lower affinity for DPG, due mainly to the replacement His H21(143)/3 — Ser. As a result, the oxygen affinity of fetal blood exceeds that of the adult, which helps the transfer of oxygen from the maternal to the fetal circulation across the placenta (5). 

BCRP (ABCG2) is similar to pGp but has only one ATP-binding domain. It is found in stem cells, where it may function to protect them against toxicants, and also in placenta and the canalicular membrane in hepatocytes. It is upregulated by low oxygen levels. 

Two umbilical arteries from the foetus carry blood to the placenta and a single umbilical vein returns blood from the placenta back to the foetus. The functions of the placenta in pregnancy are to supply oxygen and nutrients from the maternal circulation to the foetus and to remove waste materials, such as urea and carbon dioxide, from foetal blood. 

Cells need oxygen, water, and food for life. These are first supplied, via the placenta, by the mother. The subsequent, highly complicated evolving program is devoted to ensuring independence from the mother. 

A reduced affinity for BPG in Hb F gives it an increased oxygen affinity. This allows 02 to transfer from Hb A to Hb F in the placenta and so provides good oxygenation to fetal tissues. 

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