Describe the process of oxygen transport via haemoglobin.

Haemoglobin is a complex metal ion consisting of hexa-coordinated Fe²⁺ ions. With 6 coordinate bonds, haemoglobin forms an octahedral shape where 4 nitrogen ligands form a haem disc, a globin molecule forms the 5th bond, and the final bond can be interchanged between H₂O and O₂. An equilibrium is established by this 6th bond, following the equation: Hb(H₂O)₄ + 4O₂ <=> Hb(O₄) + 4H₂O. In the lungs, oxygen concentration is higher, or increased, so equilibrium shifts right to oppose the change, meaning the H₂O ligands are substituted for O₂ ligands to form oxyhaemoglobin to carry oxygen through the blood. As the oxyhaemoglobin arrives at respiring tissues, oxygen concentration is low, while water concentration is higher due to respiration. Equilibrium shifts left to oppose the change, and oxygen is exchanged with water molecules. When haemoglobin reaches the lungs again, the process repeats.