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The oxygen saturation curve for fetal hemoglobin (blue) appears left-shifted when compared to adult hemoglobin (red) since fetal hemoglobin has a greater affinity for oxygen.
Both mother and fetus share a common blood supply. In particular, the fetus's blood supply is delivered via the umbilical vein from the placenta, which is anchored to the wall of the mother's uterus. As blood courses through the mother, oxygen is delivered to capillary beds for gas exchange, and by the time blood reaches the capillaries of the placenta, its oxygen saturation has decreased considerably. In order to recover enough oxygen to sustain itself, the fetus must be able to bind oxygen with a greater affinity than the mother.
Fetal hemoglobin's affinity for oxygen is substantially greater than that of adult hemoglobin. Notably, the P50 value for fetal hemoglobin (i.e., the partial pressureThe partial pressure of a gas in a mixture is a measure of thermodynamic activity of gas molecules. The pressure a gas exerts is proportional to the temperature and the concentration of the gas. John Dalton's law of partial pressures The pressure of an id of oxygen at which the protein is 50% saturated; lower values indicate greater affinity) is roughly 19 mmHg, whereas adult hemoglobin has a value of approximately 26.8 mmHg. As a result, the so-called "oxygen saturation curve", which plots percent saturation vs. pO2, is left-shifted for fetal hemoglobin in comparison to the same curve in adult hemoglobin.
After the first 10 to 12 weeks of development, the fetus' primary form of hemoglobin switches from embryonic hemoglobin to fetal hemoglobin. At birth, fetal hemoglobin comprises 50-95% of the child's hemoglobin. These levels decline after six months as adult hemoglobin synthesis is activated while fetal hemoglobin synthesis is deactivated. Soon after, adult hemoglobin ( hemoglobin A in particular) takes over as the predominant form of hemoglobin in normal children.
Most types of normal hemoglobin, including hemoglobin A , hemoglobin A2 , hemoglobin S , and hemoglobin F, are tetramer s composed of four protein subunitIn structural biology, a protein subunit or subunit protein is a single protein molecule that assembles (or coassembles ) with other protein molecules to form a multimeric or oligomeric protein. Many naturally occurring proteins and enzymes are multimerics and four hemeA haem or heme is a metal-containing cofactor that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin''. Although porphyrins do not necessarily contain iron, a substantial fraction of porphyrin-contain prosthetic groups. Whereas adult hemoglobin is composed of two alpha and two beta subunits, fetal hemoglobin is composed of two alpha and two gamma subunits, commonly denoted as α2γ2. Because of its presence in fetal hemoglobin, the gamma subunit is commonly called the "fetal" hemoglobin subunit.
In humans, each chromosome 11 contains two identical copies of the geneDNA and to a chromosome (right). Introns are regions often found in eukaryote genes which are removed in the splicing process: only the exons encode the protein. This diagram labels a region of only 40 or so bases as a gene. In reality many genes are much that encodes the gamma subunit. The gene that codes for the alpha subunit is located on chromosome 16 and is also present in duplicate.