Oxygen Transport


Haemoglobin and Oxygen Transport

  • Haemoglobin is a conjugated protein- four polypeptide chains two alpha chains and two beta chains- each chain has a haem group attached containing iron
  • Oxygen molecule can associate with each haem group to form oxyhaemoglobin:
  • Hb + 4O2 <--> HbO2
  • Equation shows that one molecule of oxyhaemoglobin can carry four oxygen molecules
  • Oxygen can also dissociate from haemoglobin
  • When one oxygen is taken up there is conformational change in haemoglobin molecule resulting in a faster uptake of the remaining O2 molecules- cooperative loading
  • If every molecule of O2 in blood is carried then blood is 100% saturated- only two being carried blood is 50% saturated
  • Degree of saturation of haemoglobin is dependent on amount of O2 available in the environment in which haemoglobin is in at the time
  • O2 concentration in environment is its partial pressure (pO2) measured in kPa
  • If haemoglobin molecules are exposed to a range of pp their % saturation can be plotted on a graph called an oxygen dissociation curve
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Oxygen Dissociation Curve

  • Graph shows a sigmoidal pattern of O2 dissociation curves- high pO2 such as 14kPa oxyhaemoglobin is readily formed and at full saturation
  • Low pp eg 2-5kPa dissociation takes place and O2 is released into respiring tissue cells
  • S-shape makes process more efficient as over time range pp typical of respiring tissues is in rapid dissociation, making large quantities of O2 available to tissues, even though there is relatively small fall in pp
  • Loading tension- PP at which haemoglobin is 95% saturated with O2
  • Unloading tension- PP of O2 at which haemoglobin is 50% saturated 
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Bohr Effect

  • pp of CO2 (ppCO2) also effect ability of haemoglobin to combine with oxygen
  • In higher concentrations of CO2, O2 dissociation curve shifts to the right- Bohr Effect
  • Bohr Effect allows O2 to be released more readily from haemoglobin at a particular pO2- haemoglobin has a reduced affinity for O2
  • Unloading tension shifts to the right and occurs ah higher pO2- more O2 available to respiring tissues
  • In Bohr Effect haemoglobin is 40% meaning 60% of oxygen transported by haemoglobin can be released to tissues
  • Bohr Effect occurs when CO2 levels increase, such as during high rates of respiration eg exercise- means increased O2 becomes available to tissues at time of greatest need
  • Degree of shift to the right depends on pCO2, curve will shift further to the right if pCO2 levels are high- even more rapid oxygen dissociation occurs to match very high respiration rate- particular a very high pCO2
  • Uptake of O2 by haemoglobin is little affected at high PO2 levels, thereby ensuring that loading of oxygen by haemoglobin in lungs is still very efficient during Bohr Effect
  • Higher temps and lower pH also produce Bohr Effect and exercising muscles generate more heat so more O2 is released
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  • Another respiratory pigment
  • Does not circulate in blood- only found in "red" muscle
  • Consists of one polypeptide chain with a single haem group
  • Curve for myoglobin is situated to the left
  • Difference in curve is due to:

1. Myoglobin has a greater affinity for O2 than haemoglobin- will remain saturated with O2 even at relatively low pO2

2. Myoglobin will only release O2 if pO2 becomes very low- lower than normally found in respiring tissues means it serves as O2 store- does not transport O2- only releases it when blood O2 levels become very low- less than 1kPa eg strenuous exercise 

  • In period following exercise, as blood flows through muscle, with pO2 being in normal range of 2-5kPa some O2 dissociates from haemoglobin
  • O2 freed from haemoglobin readily combines with any unsaturated myoglobin  molecules until myoglobin store is fully saturated
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Effect of Altitude

  • Graph shows that if lungs have a pO2 environment around 7kPa, which is typical at high altitudes, compared to around 13kPa in more lowland altitudes, haemoglobin can't be fully saturated
  • At high elevations eg 3,500m above sea level reduced O2 levels can affect normal activity and sickness can result
  • After four days above sea level acclimisation occurs where there is an increase in erythrocytes allowing for more efficient transport of O2 available in the atmosphere and increase ventilation to maximise diffusion of O2 into blood
  • Many athletes train at high altitudes to increase their RBC count
  • Graph can also show that a llama is to the left of the human curve- this allows the llama haemoglobin to be fully saturated at pO2 values that exist high in the Andes where llamas live
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