Thursday, January 29, 2015

Study group discussion: Fetal hemoglobin

What is the difference between fetal Hb (HbF) and adult Hb (HbA)? In term of the structures?

HbA has two beta chains and HbF has two gamma chains.

The gamma fraction allows fetal Hb to have higher affinity of it for oxygen which allows it to extract oxygen from maternal blood.

And inducing Hb F production is also used in the treatment of sickle cell anemia.

Hydroxyurea is the drug used for the same.

Hb F production starts from 8th week of intra uterine life.

Fetal Hb has more affinity towards oxygen than adult Hb, meaning the oxygen dissociation curve is shifted left compared to normal adult Hb.

Left doesn't leave the oxygen! (

The primary structural differences between HbF and HbA are located in or near the 2,3-BPG binding site between the γ1-γ2 interface of HbF and the β1-β2 interface of HbA. The net effect of these structural differences is that 2,3-BPG binds less tightly to deoxyHbF by comparison to deoxyHbA. Thus, 2,3-BPG does not stabilize the deoxyHbF as effectively as it stabilizes deoxyHbA, thus accounting for the leftward shift of the O2 saturation curve of HbF compared to HbA when tested with the same concentration of 2,3-BPG.

Fetal hb has more affinity because of poor binding of 2,3-DPG by the gamma polypeptide chain so it can take larger volume of oxygen than adult hb at low oxygen pressure

A higher affinity for oxygen allows higher concentrations of oxygen into fetal circulation, however this also inhibits oxygen dissociation into fetal tissue where the oxygen is needed. To overcome this, other mechanisms are in place to ensure oxygen delivery to fetal tissue: Increased Crit – higher number of red blood cells per blood volume. This is a common reaction to reduced oxygen availability. Exacerbated Bohr effect – acidic pH has a greater effect on oxygen unloading in fetal tissues allowing better oxygen delivery. Acidic pH shifts the fetal oxygen-haemoglobin dissociation curve to the right, so that oxygen unloading can occur at higher oxygen partial pressures.

Adult haemoglobin starts to be produced in utero, at around the 13th week of gestation.

I think it's 30the week the switch over from fetal Hb to adult Hb.

Its by 6th month of life.. That's when majority of a child's RBC shifts to adult haemoglobin. It's important in case of thalasaemia.. Because that's when most symptoms start showing.

What I meant to say was - Initiation of production of HbA starts from 13th week gestation.

At first, there is gradual increase in concentration of HbA until it reaches 20-30% of total Hb.

And the switch is not completed until 6 months of age.

What my hemaologist taught was HbA production will be started from 30th week of gestation and by 6th month of life only HbA is produced.

And the main function of HbF is delivering oxygen in hypoxic condition like immediately after birth.

But don't we need HbF to take oxygen from the maternal blood in the first place?

Because the partial pressures in the placenta aren't enough to allow the transfer from what I've studied.
Everything takes part in delivering oxygen depending upon tissue tension of O2. Before 7th week, there are embryonic Hb like portland, gower, etc.

Embryonic erythropiesis takes place from yolk sac.

From 7th week to 30th week liver.

After 30th week, long bones start erythropoesis.

Gene for both HbA and HbF are present since birth but Its all epigenetics that make these genes to produce one kind in one period and other type in other period.

Oh I didn't know the Hb concept in such depth.

My sir told these details. Epigenetics is given in Robbins 5th chapter.

That's all!
Be sure to let us know if there are any errors or corrections.

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