Transport of oxygen and carbon dioxide

Adaptations of red blood cells

  • biconcave shape

    • increased surface area

  • no nucleus

    • able to transport more oxygen

Haemoglobin

  • 4 polypeptide chains

  • each contains a haem prosthetic group

  • oxygen binds to these haem groups

Taking up oxygen

  • oxygen enters lungs

  • oxygen diffuses into blood plasma and enters the erythrocytes (RBC)

  • oxygen molecules bind to haemoglobin and are taken out of solution

    • oxyhaemoglobin

  • maintains a steep diffusion gradient so more oxygen can enter the bloood

Releasing oxygen

  • in the tissues oxygen is needed for respiration

  • oxyhaemoglobin must be able to release oxygen

    • dissociation

  • % saturation is a measure of how many oxygen molecules are bound

Oxyhaemoglobin dissociation curve

  • partial pressure

    • the relative pressure that a gas contributes to a mixture of gases

      • the more of a gas there is, the higher the partial pressure

    • units kPa

  • uptake/dissociation of oxygen produces an S shaped curve

  • oxyhaemoglobin dissociation curve

  • low pO2

    • difficult for first oxygen molecule to bind

  • pO2 increases

    • oxygen eventually binds

    • causes conformational change in haemoglobin molecule shape

    • easier for further oxygen molecules to bind

    • affinity of haemoglobin for oxygen increases

    • positive co-operativity

  • high pO2

    • curve levels off as last O2 molecule is more difficult to bind

    • eventually haemoglobin is fully saturated

  • oxygen dissociation curve detailed

  • pO2 in lungs is enough to allow almost 100% saturation

  • pO2 in respiring tissues is low enough for oxygen to dissociate easily from the oxyhaemoglobin

Bohr shift

  • low blood pCO2

    • haemoglobin picks up O2 easily

  • high blood pCO2

    • haemoglobin gives up O2 easily

  • curve shifts right with increasing pCO2

  • Bohr shift

Foetal haemoglobin

  • foetal haemoglobin has a higher affinity for oxygen than maternal haemoglobin

  • oxygen passes to the foetus from the mother

  • foetal haemoglobin

Transport of CO2

  • carbon dioxide produced by the respiring tissues is transported to the lungs to be removed during exhalation

    • 5% dissolved in plasma

    • 10-20% as carbaminohaemoglobin

    • 75-85% as hydrogen carbonate ions in the cytoplasm of RBC

Transport of CO2 has hydrogen carbonate ions

  • at respiring tissues (pCO2 is high)

    • CO2 reacts with water \lrArr carbonic acid (H2CO3)

      • catalysed by carbonic anhydrase

    • H2CO3 dissociates to form H+ and HCO3- ions

    • H+ binds to haemoglobin forming haemoglobinic acid

      • causes haemoglobin to release oxygen

    • prevents blood from becoming too acidic (H+ ions lower pH) so act as buffer

    • causes the Bohr effect

      • haemoglobin releases oxygen so it can diffuse into respiring tissues

    • chloride shift

      • HCO3- ions leave RBC and are transported via plasma while chloride ions enter RBC

      • maintains charge balance by preventing excessive positive charge in RBC

  • at the lungs (low pCO2)

    • HCO3- and H+ ions reform CO2

    • CO2 diffuses out of body via expiration

  • transport of CO2 as hydrogen carbonate ions