- sigmoid shape of the oxy-Hb dissociation curve results from the allosteric interactions of the globin monomers that make up the haemoglobin tetramer as each one binds O2
- multiple factors can affect the affinity of Hb for oxygen, thus causing the curve to shift to the left (increased oxygen affinity) or to the right (decreased O2 affinity)
OXY-HB DISSOCIATION CURVE
- pulmonary veins
PO2 95 -> SO2 97%
- pulmonary arteries
PO2 40 -> SO2 75%
SpO2 (%) PO2 (mmHg)
- the amount of O2 bound to Hb is determined by the PO2 in a relationship termed the oxy-Hb dissociation curve.
- though atmospheric O2 concentration changes markedly, the buffering of Hb maintains constant tissue PO2.
Flat, upper portion of curve
-> if PO2 in alveolar falls, loading of O2 will be unaffected
-> a large partial pressure difference between blood & alveolar gas once most of gas has been transferred (diffusion hastened).
Middle, steep lower portion of curve
-> peripheral tissues can withdraw large amounts of O2 for a small drop in PO2 (assists O2 diffusion into tissue).
Utilization co-efficient = % of blood that gives up its O2 as it passes through tissue
- normal is 25%
- during exercise 75 to 85% (even higher)
FACTORS THAT SHIFT THE DISSOCIATION CURVE
- interferes with O2 transport
- 240 x the affinity for Hb than O2 -> small amounts of CO can tie up large portions of Hb -> decreases O2 concentration
- oxy-Hb dissociation curve shifts to left -> favours uploading of O2
- can test for on co-oximetry
- increased temperature shifts curve to right.
- the Bohr effect
- high CO2 & H+ ion concentration
-> as O2 is given up in tissues
-> CO2 begins to bind & form carbonic acid
-> shifts curve to right
-> enhancing O2 off loading.
- blood passing through lungs gives up CO2 & H+ ions in the form of carbonic acid
-> shifts O2 dissociation curve to left
-> quantity of O2 binding increases at any given PO2
-> increased O2 transport to tissues.
Hydrogen ion concentration
- see above
- produced in response to hypoxia (after a few hours) or anaemia.
-> right shift in curve
- formed in RBCs from 3-phosphoglyceraldehyde (a product of glycolysis)
- binds to the beta chains of deoxy Hb
- more O2 released into tissues, but also more difficult for O2 to bind with Hb in lungs.
- O2 released to tissues at as much as 10mmHg higher tissue O2 pressure than would be without increase in 2,3 DPG.
- in stored blood: 99% @ 7 days, 50% @ 14 days, 5% @ 28 days.
- can test for in blood
- at present there is not enough data to support manipulation of O2-Hb curve to improve O2 delivery
- if arterial PO2 is critically low then O2 binding in the lungs may be impaired by a shift to the right -> this may seriously impair tissue oxygenation
References and Links
- Morgan TJ. The oxyhaemoglobin dissociation curve in critical illness. Crit Care Resusc. 1999 Mar;1(1):93-100. PubMed PMID: 16599868. [ ]