The oxygen dissociation curve (ODC) is an essential part of respiratory physiology. The curve allows us to understand how oxygen is transported around to the blood from the lungs through the alveoli and vice-versa.
Role of Hemoglobin
To understand the ODC, we must have an idea about the structure of hemoglobin (Hb).
Hemoglobin is made up of 4 subunits - 2 alpha and 2 beta subunits. Each subunit contains a heme moiety attached to a polypeptide chain, as shown in the image given below (a).
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Heme is a porphyrin ring complex that consists of an Iron atom. The iron atom is in its ferrous (Fe2+) state. The iron atom reversibly binds to an oxygen atom, thus playing a crucial role in cellular respiration.
Oxygen Bonding
Now, when hemoglobin is not bound to any oxygen atom, the globin units are in a tight, tense configuration (T configuration). But when an oxygen atom binds to a globin chain, the tight configuration relaxes (R configuration), and the oxygen affinity increases by 500 times!
When plotted on a graph, the curve is a distinct sigmoid. We need to remember two things while reading the curve:
- When the first O2 binds to a heme molecule, it increases the affinity of heme to the second oxygen molecule. When the second oxygen binds to heme, the affinity of heme to the subsequent oxygen molecule increases. So on and so forth.
- When there are small changes at a low partial pressure of Oxygen (pO2), there is a much more significant change in SaO2, which is a measure of saturated hemoglobin.
Factors Affecting the Curve
The curve represents the amount of saturated hemoglobin plotted against the partial pressure of arterial oxygen, as shown in the image given below.
Three factors control the shift of the curve (thereby affect gas exchange). They include:
- pH
- Temperature
- 2,3-bisphosphoglycerate (BPG or 2,3-BPG)
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Effect of Temperature
An increase in temperature causes the bond between the Hb and O2 to break. The result is an increase in the partial pressure of O2, thus effectively shifting the curve to the right.
Conversely, A fall temperature would result in the leftward shift in the curve.
Effect of pH
A decrease in blood pH shifts the ODC to the right as a decrease in pH increases the amount of H+ available.
Deoxygenated Hb binds more actively than H+ and stabilizes the T form. Therefore, it becomes difficult for oxygen to bind with the Hb molecule. This phenomenon is called the Bohr effect.
Conversely, an increase in pH shifts the curve to the left.
Effect of 2,3-BPG
2,3-Bisphosphoglycerate (earlier called 2,3-diphosphoglycerate) is a by-product of glycolysis in red blood cells. It binds to the beta chains of hemoglobin, and hence Oxygen gets liberated.
Therefore, an increase in the BPG will result in the shift of the curve to the right. Conversely, a decrease in BPG will shift the ODC to the left.
Fetal Hemoglobin-oxygen Saturation Curve
A major difference between fetal and adult hemoglobin is that fetal hemoglobin consists of 2 alpha chains and 2 gamma chains (in contrast with adult Hb that consists of 2 beta chains).
It is interesting to note that the gamma subunits have a lesser affinity towards BPG.
The fetal curve is typically situated to the left to the adult curve, as shown below.
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References:
- Ganong’s Review of Medical Physiology 26th Edition - Page no: 87 - 92
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6025097/
Author’s footnote
The oxygen dissociation curve and the structure of various types of hemoglobin are frequently tested concepts in various competitive exams.
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