Oxygen Carriage and Storage

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  • Dissolved O2:
  • Obeys Henry's Law - amount dissolved is proportional to partial pressure
  • For every mmHg of PO2, 0.003ml O2/100ml blood
  • Normal arterial blood with a PO2 of 100mmHg contains 0.3ml O2/100ml
  • Hb bound O2:
  • Heme - an iron-porphyrin compound
  • Joined to the protein globin made up of 4 polypeptide chains
  • Alpha chains and beta chains
  • Normal adult haemoglobin is A
  • Haemoglobin F (fetal) is part of the haemoglobin in the newborn infant and is gradually replaced in the first year of life. It produces a dissociation curve well to the left of adult haemoglobin, binding oxygen with greater affinity than the adult form, giving the foetus better access to oxygen from the mother's bloodstream
  • Haemoglobin S (sickle) has valine instead of glutamate in the beta chains, therefore having reduced O2 affinity and a right shifted dissociation curve
  • Deoxygenated form of HbS is poorly soluble and crystallizes in the red cell
  • This causes the red cell shape to change from biconcave to crescent or sickle shaped with increased fragility and tendency to thrombose
  • Normal HbA can have its ferrous ion oxidized to the ferric form by various drugs and chemicals - incl. nitrites, sulfonamides and acetanilid
  • The ferric form is methemoglobin - not useful for O2 carriage

O2 Carriage

  • O2 capacity - the maximum O2 the can be combined with Hb
  • O2 saturation - the percentage of available binding sites that have O2 attached to them
  • O2 saturation = O2 combined with Hb/O2 capacity x 100
  • 1g of pure Hb can combine with 1.39ml O2, therefore 15g Hb (100mls of blood) can combine with 20.8
  • O2 saturation of arterial blood with PO2 of 100 is about 97.5%, O2 saturation of mixed venous blood with PO2 40mmHg is 75%
  • Change from fully oxygenated state to deoxygenated state involves conformational change of the molecule:
  • The oxygenated form is R (relaxed) and has an affinity for oxygen 500x higher than the deoxy form T (tense), which has relatively low affinity for oxygen

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  • The oxygen carriage equation calculates amount of oxygen per 100mls of blood

The O2 Dissociation Curve

  • Flat upper portion of O2 dissociation curve means that even if PO2 in alveolar gas falls a little, O2 loading is little affected
  • A large PO2 difference exists between alveoli and blood even when most O2 has been transferred
  • Steep lower part of the dissociation curve means that the peripheral tissues withdraw a large amount of O2 for only a small drop in PO2
  • P50 = the partial pressure of oxygen at which 50% of Hb molecules are saturated

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  • Reduced Hb is purple in colour, resulting in the cyanosis seen at low O2 saturations
  • O2 affinity of Hb is reduced by:
  • H+ concentration increase, PCO2 (via the Bohr effect), increasing temperature, and by concentration of 2,3 DPG in red cells
  • The Bohr Effect
  • The Bohr effect describes the alteration in haemoglobin oxygen affinity that arises from changes in H+ ion or CO2 concentrations
  • Changes in pH affect numerous electrostatic bonds that maintain the quaternary structure of Hb, stabilising the molecule in the T conformation, reducing its affinity for oxygen
  • Most of the effect of PCO2 ( the Bohr effect) is due to its action on H+ concentration
  • There is also the effect of CO2 binding to the N-terminal amino acid residues to form carbaminohaemoglobin, which stabilises the T conformation and facilitates release of the oxygen molecule from Hb
  • These effects result in increased O2 unloading in exercising environments. 25% of oxygen release and uptake is due to the Bohr effect, even though the arteriovenous pH difference is only 0.033
  • 2,3-Diphosphoglycerate
  • A number of organic phosphates in the RBC affect P50, with 2,3-DPG being the most important
  • A 2,3 DPG molecule becomes bound by electrostatic bonds between two Beta chains, stabilising the T conformation, thereby reducing oxygen affinity and displacing the dissociation curve to the right
  • Percentage of Hb molecules containing a DPG molecule governs overall P50 of a blood sample within the range of 15-34mmHg
  • Despite this, the significance of changes in P50 mediated by DPG are only marginal in practice
  • DPG is formed in the Rapoport-Luebering shunt off the glycolytic pathway, and its level determined by the balance between synthesis and degradation


  • 2,3 DPG has a half-life of 6 hours
  • Production Increased in:
  • Chronic anaemia
  • ↓pO2 - eg. at altitude or with chronic lung disease
  • ↑pH
  • Exercise
  • Pregnancy
  • Altitude causes increased 2,3 DPG both by the direct effect of hypoxia, and by raised pH secondary to hyperventilation which also promotes DPG production
  • Storage of blood in a blood bank may cause depletion of 2,3 DPG, impairing oxygen unloading by shifting the oxygen dissociation curve left
  • Foetal Hb lacks β-Units and therefore 2,3DPG is unable to bind, resulting in foetal Hb having high O2 affinity and decreased P50 to 18mHg
  • Stored blood has a shelf life of 35 days. 2,3 DPG in stored blood is depleted due to decreased production from:
  • ↓ 2,3 DPG production due to
  • ↓ pH of stored blood inhibiting glycolytic pathyway.
  • ↓ Temperature
  • 2,3 DPG levels restored after 48 hours post transfusion.
  • Additives to minimize 2,3 DPG depletion
  • Dextrose as substrate for glycolytic pathway
  • Adenosine + phosphate to maintain ATP levels.
  • Citrate to anticoagulate
  • CO combines the Hb to form carboxyhaemoglobin, & has 240x the affinity that O2 has
  • The dissociation curve of CO is the same shape as that for O2, but greatly compressed against the Y-axis
  • As a result of this, small amounts of CO can tie up large amounts of Hb, preventing O2 carriage