Carriage of 02 and C02

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Oxygen is required by all respiring cells along with glucose in order to provide ATP. 

 It is the role of haemoglobin within red blood cells to transport oxygen from the alveoli in the lungs to the capillaries where it enters the tissue fluid and diffuses into cells.

 Haemoglobin is a globular protein which features a haem prosthetic (non-protein) group in the centre of each of its four subunits. 

The haem prosthetic group features an Fe2+ ion capable of binding with O2.Therefore each haemoglobin molecule can bind with up to 4 O2 molecules. With millions of haemoglobin molecules inside of each red blood cell that’s a lot of O2! Haemoglobin binds to oxygen (loads) in areas of high oxygen concentration. i.e. in the lungs.  It releases oxygen (unloads) in areas of low oxygen concentration, i.e. respiring tissues.

 This is often represented in graphs called ‘Oxygen Dissociation Curves’.

Dissociation- The breakdown of a molecule into two molecules, atoms or ions.  For example the release of oxygen from oxyhaemoglobin.

Partial Pressure (pp)- This is the individual pressure exerted independently by a particular gas within a mixture of gases.


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Red blood cell

  • CO2 from respiring tissues enters the red blood cell by diffusion and combines with water, forming carbonic acid.  The reaction is accelerated by the enzyme carbonic anhydrase.

     Carbonic acid dissociates into H+ ions and HCO3- ions.  HCO3- is transported out of the RBC by active transport, in exchange for Cl-.  (This is the CHLORIDE SHIFT)

  •  H+ displaces 02 from haemoglobin, forming HHb - reduced haemoglobin.  The O2 is released to the tissues.

  • The more CO2, the greater the displacement of O2 from Hb.  CO2 reduces the affinity of Hb for O2.  This explains the Bohr effect. 

  • All these reactions are reversible.  In the lung capillaries ppO2 is higher and ppCO2 is lower and so the reverse happens.

  • In the lung capillaries ppO2 is higher and ppCO2 is lower, so now 02 binds to Hb and this results in the release of C02.

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Benefits of Bohr effect to respiring tissue

  • Actively respiring tissue requires more oxygen
  • For aerobic respiration to release more energy
  • Actively respiring tissue produces more CO2
  • Haemoglobin involved in transport of CO2
  • Less haemoglobin available to combine with O2
  • Bohr shift causes more oxygen to be released
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