OXYGEN DISSOCIATION CURVE
- Plot between amount of oxygen in association with hemoglobin (oxyhemoglobin) against PO2 of blood
- Explains hemoglobin’s affinity for oxygen.
“Sigmoid shape” –
- Molecular basis:
- “Due to phenomenon “Cooperative binding of Oxygen to hemoglobin”.
Steps involved:
Hemoglobin, a tetramer
- Four O2 molecules binds 1 Hb molecule.
- 1st O2 molecule bonds with greatest difficulty.
- Increases affinity to next O2 molecule.
With rising PO2 levels, Hb – O2 saturation increases very slowly.
- Saturation increases steeply between PO2 15mm Hg & 40 mm Hg.
Upon increasing PO2, Hb-O2 saturation remains unaltered.
- Due to reduced scope for excess O2 binding with hemoglobin.
- Since, Hb-O2 bonding ratio is 4:1
- Beyond PO2 60mm Hg, curve becomes almost flat.
Approximate saturation at
- 10mm Hg = 10%
- 15mm Hg = 20%
- 40mm Hg = 75%
- 60mm Hg = 90%
FACTORS AFFECTING OXYGEN HEMOGLOBIN DISSOCIATION CURVE
- Hb-O2 dissociation curve shifted to left/right by various factors,
– PO50 mark is baseline for determining curve shift.
– PO50 – PO2 at which hemoglobin 50% saturated.
– PO50 of normal adult hemoglobin – 26mm Hg (3.47 kpa).
I) CURVE SHIFTING TO RIGHT:
- PO50 higher.
– i.e., 50% hemoglobin saturation happens at higher PO2
- Indicating decrease in affinity of hemoglobin for oxygen.
Conditions associated:
- Low pH Hypoxia
- Increase in H+ ion concentration (acidosis)
- Reduced PO2
- High PCO2
- Increased body temperature
- Increased 2,3-bisphosphoglycerate (2,3 – BPG)/2,3-diphosphoglycerate (DPG)
- Exercise
- Within systemic capillaries.
II) CURVE SHIFTING TO LEFT:
- PO50 lower.
– i.e., 50% hemoglobin saturation happens at lower PO2
- Indicating increase in affinity of hemoglobin for oxygen.
Conditions associated:
- High pH.
- Decreased H+ ion concentration (alkalosis).
- Reduced PCO2
- Reduced body temperature.
- Reduced 2,3-bisphosphoglycerate (2,3 -BPG)/2,3-diphosphoglycerate (DPG)
- Fetal hemoglobin.
- CO poisoning.
RELATED IMPORTANT EFFECTS:
1. BOHR EFFECT:
- Increase in partial pressure of carbon dioxide/Hypercarbia
- Curve shifts to right.
Effects:
- Main effect of raised PCO2 mediated by,
– Increased hydrogen ion concentration
- Deoxygenated hemoglobin (Deoxyhemoglobin) binds H+ more actively
- Hence, reduced oxygen accessibility to hemoglobin.
Steps involved:
- High metabolic activity of tissue causes,
– Rise in PCO2 & temperature.
– Fall in PO2 & pH.
- Hb-O2 affinity is reduced causes,
– Increased tissue O2 delivery.
2. EFFECT OF 2, 3 – DI-(BIS-)PHOSPHOGLYCERATE (2,3 – DPG/BPG)
- DPG –
– Optional by-product of glycolytic pathway.
– Present in RBC.
- Increased DPG concentration shifts curve to right.
Steps involved:
- One molecule of DPG binds with one mole of deoxyhemoglobin
– Doesn’t bind with oxygenated hemoglobin
- Raised DPG concentration releases oxygen from oxyhemoglobin.
- Resulting in O2 delivery/unloading to tissues.
2a). Factors increasing DPG & effect on O2 – Hb curve:
Increased DPG-
- Lowers oxygen affinity of hemoglobin.
- Increases delivery of oxygen to tissue
Conditions:
- Hypoxia
- High altitude
- Exercise
- Anemia
- Chemical agents (inosine, pyruvate, PEP, phosphate, dihydroxyacetone)
- Alkalosis
- Pregnancy
2b). Factors decreasing DPG & effect on O2 – Hb curve
Decreasing DPG-
- Increases oxygen affinity of hemoglobin.
- Decreases delivery of oxygen to tissue
Conditions:
- Acidosis
- Stored blood
- Fetal hemoglobin (HbF)
3. COMPARISON BETWEEN FETAL Hb & ADULT Hb:
- HbF –
– Has low affinity to DPG.
– Greater affinity for oxygen, than adult Hb.
- In human blood, affinity of fetal Hb for 2’3-DPG is only about 40% that of adult hemoglobin.
– Fetal hemoglobin behaves as if 2,3-DPG levels are low.
- Poor binding of DPG to fetal Hb.
– Important factor facilitating transplacental oxygen transport between mother & fetus.
- Curve shift variations at tissue level v/s at Lungs:
In muscles & other tissues:
- Curve shifts to right
- Facilitates O2 delivery to tissues.
In lungs:
- Curve shifts to left
- Facilitates O2 uptake in muscles.
- More oxygen is released from alveoli
- Hyperbola in shape.
Molecular basis:
- Different mechanism between myoglobin & hemoglobin.
1. Myoglobin:
- Monomer.
– Binds with only one molecule of oxygen.
– Hence no phenomenon of cooperative binding.
- Has much higher affinity for O2 than hemoglobin.
– Hence O2-myoglobin binding is tighter than with hemoglobin.
- P50 – Only 5mm Hg for myoglobin.
– P50 – 26mm Hg for hemoglobin.
- Myoglobin dissociation curve is hyperbola.
– Compared to sigmoid shape Hb-O2 curve.
– Curve is shifted far to left than Hb-O2 dissociation curve.
- Myoglobin binds O2 at very low PO2.
- Release them at even lower PO2.
- Eg: As in exercising muscles.
Exam Important
- Sigmoid shaped-
– “Due to phenomenon “Cooperative binding of Oxygen to hemoglobin”.
- Hemoglobin is a tetramer
– Four O2 molecules binds 1 Hb molecule.
– 1st O2 molecule bonds with greatest difficulty.
– Increasing affinity to next O2 molecule
– Saturation increases steeply between PO2 15mm Hg & 40 mm Hg.
Beyond PO2 60mm Hg, curve becomes almost flat.
- PO50 is PO2, at which 50% hemoglobin saturated.
– PO50 of normal adult hemoglobin – 26mm Hg (3.47 kpa).
I) CURVE SHIFTING TO RIGHT:
- PO50 higher
– Indicating decreased hemoglobin affinity for oxygen.
Conditions associated:
- Low pH/Increased H+ ion concentration/acidosis.
- Hypoxia/Reduced PO2.
- High PCO2.
- Increased body temperature.
- Increased 2,3 – BPG/DPG.
- Exercise.
- Within systemic capillaries.
- PO50 lower.
- Indicating increase in affinity of hemoglobin for oxygen.
Conditions associated:
- High pH/Decreased H+ ion concentration/alkalosis.
- Reduced PCO2.
- Reduced body temperature.
- Reduced 2,3 BPG/DPG.
- Fetal hemoglobin.
- CO poisoning.
BOHR EFFECT:
- Increase in partial pressure of carbon dioxide/Hypercarbia
– Curve shifts to right.
– Main effect of raised PCO2 mediated by increased hydrogen ion concentration.
– Deoxyhemoglobin binds H+ more actively.
- DPG is an optional by-product of the glycolytic pathway, present in RBC.
– Increased DPG concentration shifts curve to right.
– One molecule of DPG binds with one mole of deoxyhemoglobin.
– Binds with deoxygenated hemoglobin but not with oxygenated hemoglobin.
- Raised DPG concentration releases oxygen from oxyhemoglobin.
- Resulting in O2 delivery/unloading to tissues.
Factors increasing DPG & effect on O2 – Hb curve:
- Hypoxia.
- High altitude.
- Exercise.
- Anemia.
- Chemical agents (inosine).
Factors decreasing DPG & effect on O2 – Hb curve:
- Acidosis.
- Stored blood.
- Fetal hemoglobin (HbF).
- HbF has low affinity of DPG.
- In human blood, affinity of fetal Hb for 2’3-DPG is only about 40% that of adult hemoglobin.
- Fetal hemoglobin behaves if 2,3-DPG levels are low.
- Poor binding of DPG to fetal Hb.
– Important factor in facilitating transplacental transport of oxygen between mother & fetus.
- Exercise shifts curve to right in muscles & other tissues.
- Dissociation curve for myoglobin is “Hyperbola” in shape.
- Myoglobin binds with only one molecule of oxygen.
– Due to no phenomenon of cooperative binding.
- Myoglobin has much higher affinity for O2 than hemoglobin.
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