Oxygen Carriage and Storage

• 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

• 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

• 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