8.4 Transport of oxygen in the blood

8.4 Transport of oxygen in the blood

1. Role of haemoglobin

   • Oxygen bind to haemoglobin to form oxyhaemoglobin
     • In respiring tissue O2 dissociates from oxyhaemoglobin
       • O2 then diffuses out of the erythrocytes to respiring cells
         • Haemoglobin can readily associate with O2 when there's are high conc of O2 and readily dissociate when there's a low conc.
   • Haemoglobin is comprised of 4 sub-units ( 2 alpha glucose, 2 beta glucose)
     • Each sub-unit is comprised of 1 polypeptide chain and 1 haem group with an iron ion at the centre
       • Each iron ion can reversably bind to O2
         • The haem group has an affinity for O2
         • Therefore O2 + haemoglobin = oxyhaemoglobin is a reversable reaction

2. Loading O2

   • Partial pressure = amount of pressure exerted by a gas relative to the total pressure exerted by all the gases in the mixture.
     • Measured in KPa and written as P(x)
   • How O2 is loaded
     • Haemoglobin doesn't absorb evenly
       • At low conc's the sub-units are tightly packed making it difficult for the first O2 to bind.
         • After the first O2 has bound there is a conformational change of the haemoglobin - making it easier for the 2nd and 3rd O2's to bind
           • Then its difficult for the 4th O2 to associate as the haemoglobin molecule is 'full'
   • Oxygen dissociation curve (Sigmoid curve)
     • • Tissues(low Kpa)
         • P(O2) = 2-4 Kpa
           • A small decrease in P(O2) leads to a large decrease in % saturation
             • Haemoglobin has a low affinity for O2
       • Lungs (high KPa)
         • P(O2) = 10-12 KPa
           • A large decrease in P(O2) leads to a small decrease in % saturation
             • Haemoglobin has a high affinity for O2
     • Remember to read the graph from right to left

3. Foetal haemoglobin

   • Blood arrives at the placenta, low P(O2) (2-4 Kpa)
     • Haemoglobin has a low affinity for O2
       • At this point foetal haemoglobin has a higher affinity for O2
         • The adult haemoglobin dissociates with O2 releasing it to the placenta tissue when it will readily associate with foetal haemoglobin

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