Oxygen & CO2

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)

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 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

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

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

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

Oxygen and carbon dioxide transport