Carriage of 02 and C02

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.

• 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.

• 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