Haemoglobin
- Consists of four polypeptide chains joined to a porphyrin ring
- Haemoglobin A1 consists of two alpha and two beta chains
- The porphyrin ring contains iron
- Only when the iron is in the ferrous (reduced) state does it binds
with oxygen
- Haemoglobin with ferric (oxidised) iron is known as methaemoglobin
- It cannot take part in oxygen transport
- Each haemoglobin molecule can attach up to four molecules of oxygen
- Oxygen is primarily transported bound to haemoglobin
- Small amount transported dissolved free in the plasma
- Haemoglobin bound to oxygen is known as oxyhaemoglobin
- 1 gram of haemoglobin can bind with 1.34 millilitres of oxygen
- 100 millilitres of plasma will have about 20 millilitres of oxygen
bound to haemoglobin
- The amount of oxygen bound to haemoglobin is expressed as oxygen
saturation (sO2)
- This is dependent upon the partial pressure of oxygen (pO2)
Oxygen dissociation curve
- The relationship between pO2 and sO2 is expressed as the oxygen
dissociation curve
- It is sigmoid in shape
- The sigmoid shape of the curve is due to facilitative binding of
oxygen to haemoglobin
- Oxygen bound to haemoglobin initially increases its affinity to
oxygen
- This facilitates further binding which gives the curve its sharp
- Haemoglobin has the least affinity for the final O2 molecule
- This explains the flattening of the curve towards the end
- The p50 is the partial pressure of oxygen at which 50% of the
haemoglobin is saturated
- The p50 of adult haemoglobin is about 3.5 kPa or 26 mm Hg
- The affinity of haemoglobin to oxygen is variable
- When the affinity reduces the curve shifts to the right
- When the curve shifts to the right p50 increases
- As the affinity increases the curve shifts to the left
- The curve for haemoglobin in sickle cell anaemia is shifted to the
right
- The curve for foetal haemoglobin is shifted to the left
Factors affecting haemoglobin affinity
- Factors reducing the affinity and thus shifting the curve to the
right are an increase in:
- pCO2( partial pressure of carbon dioxide)
- Reduced pH
- Increase in temperature
- Increase in 2,3-biphosphoglycerate
- Similarly a reduction in these variables results in an increase in
the affinity of haemoglobin
- Conditions which increase the affinity, exist in the lungs
- Thus oxygen gets bound to haemoglobin
- Conditions which reduce the affinity, exist in the body tissues
- Thus oxygen gets unbound from haemoglobin
- The shift in the dissociation curve due to changes in pCO2 is known
as the Bohr effect
Role of 2,3-biphosphoglycerate
- It is the bi-product of anaerobic respiration
- It reduces the affinity of haemoglobin to oxygen
- Thus it shifts the oxygen dissociation curve to the right
- Levels of 2,3-biphosphoglycerate increase in chronic hypoxia
- Thus more oxygen is delivered to tissues
- Levels are reduced in stored blood
- Thus oxygen delivery to tissues is sub-optimal with transfused blood
- Foetal haemoglobin is not affected by 2,3-BPG
- Thus it has a higher affinity to oxygen than has maternal
haemoglobin.
- This ensures transfer of oxygen across the placenta
Affect of Carbon Monoxide
- Affinity of haemoglobin to Carbon monoxide is 250 times more than
that towards oxygen
- Its binding with haemoglobin is competitive with Oxygen
- Carboxyhaemoglobin has a hyperbolic oxygen-dissociation curve
Transport of carbon dioxide
- Carbon dioxide is 20 times more soluble in plasma than oxygen
- Transported in three forms
- Dissolved in plasma
- Bound to haemoglobin
- Bicarbonate ion
- Haemoglobin combined with carbon dioxide is known as
carbaminohaemoglobin
- The majority of carbon dioxide produced is transported as
bicarbonate
- Carbon dioxide diffuses into red cells and reacts with water to
produce bicarbonate and carbonic acid
- The reaction is catalysed by carbonic anhydrase
- The bicarbonate produced inside red cells diffuses out into the
plasma in exchange with chloride ions
- This is known as chloride shift
- Chloride moves into red cells and bicarbonate moves out
Carbon dioxide equilibrium curve
- Over the physiological concentrations of carbon dioxide this curve
is a straight line
- The curve is shifted to the left in venous blood
- This improves carbon dioxide transport
- This is known as Haldane effect
- It occurs because deoxygenated haemoglobin is a weaker acid
- This allows more carbon dioxide to combine with haemoglobin
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