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·         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.

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Haemoglobin Saturation

·         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 pO2.

o   At a high pO2oxygen loads onto Hb to form oxyhaemoglobin.

o   At a lower pO2, - oxyhaemoglobin unloads oxygen.

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Haemoglobin Saturation

·         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.

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Dissociation Curves

·         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.

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Dissociation Curves


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Adult and Foetal Haemoglobin

·         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.

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The Bohr Effect

·         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.

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The Bohr Effect

·         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.

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The Bohr Effect


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