Haemoglobin

·         Contained in red blood cells, haemoglobin (Hb) is a large protein with a quaternary structure consisting of four polypeptide chains.

o   Each chain has a haem group which contains iron.

o   Hb has a high oxygen affinity and can carry up to 4 O₂ molecules.

·         In the lungs, oxygen joins to the iron in the haem group to form oxyhaemoglobin.

·         This is a reversible reaction:

Hb + 4O₂ à HbO₈

o   Oxyhaemoglobin turns back into Hb when oxygen dissociates near the body cells.

·         The partial pressure of oxygen, pO₂, is a measure of oxygen concentration.

·         The partial pressure of carbon dioxide, pCO₂, is a measure of the concentration of CO₂ in a cell.

·         Hb’s affinity for oxygen varies depending on the pO_2.

o   At a high pO₂ – oxygen loads onto Hb to form oxyhaemoglobin.

o   At a lower pO₂, - oxyhaemoglobin unloads oxygen.

·         Oxygen enters the blood capillaries at the alveoli.

o   High pO₂ so oxyhaemoglobin formed.

·         Red blood cells deliver oxyhaemoglobin to respiring tissues

o   Lower pO₂ as cells respiring uses oxygen – unload oxygen

·         The Hb returns to lungs to pick up more oxygen.

·         A dissociation curve shows how saturated the Hb is with oxygen at any given partial pressure.

·         Where pO₂ is high (e.g. the lungs), Hb has a high affinity to oxygen – so it has a high saturation of oxygen.

·         Where pO₂ is low (e.g. in respiring tissues), Hb has a low affinity to oxygen – so it has a low saturation of oxygen.

·         As Hb becomes saturated, it is harder for more O₂ molecules to join.

·         When Hb combines with the first O₂ molecule, Hb changes its shape so other molecules can join easier.

·         Foetal haemoglobin has a higher affinity for oxygen than adult.

·         The oxygen saturation of the mother’s Hb has decreased by the time it reaches the placenta.

·         The higher oxygen affinity enables the foetus to get enough oxygen to makes its blood saturated enough.

·         Hb gives up oxygen more readily at high pCO₂.

o   Cells can get more O₂ during activity.

·         Cells produce CO₂ upon respiration – increases pCO₂ and the rate of O₂ unloading.

o   10% of CO₂ binds to Hb and is carried to the lungs.

·         CO₂ is converted into carbonic acid by carbonic anhydrase enzyme.

·         The carbonic acid splits up into:

o   Hydrogen, H⁺, ions

o   Hyrdrogencarbonate ions, HCO₃^-

·         More H⁺ ions causes the oxyhaemoglobin to give up O₂ so that the Hb can take up the H⁺ ions.

o   This forms haemoglobinic acid and stops the H⁺ ions increasing cell activity.

·         The HCO₃^- ions diffuse out of the red blood cells and are transported in the blood plasma.

·         Upon reaching the lungs, the low pCO₂ causes the H⁺ and HCO₃^- to recombine into CO₂.

·         The CO₂ diffuses out of the alveoli and is breathed out.