Oxygen-haemoglobin dissociation curve

Overview
The oxygen-haemoglobin dissociation curve plots the proportion of haemoglobin in its saturated form on the vertical axis against the prevailing oxygen tension on the horizontal axis. The oxyhemoglobin dissociation curve is an important tool for understanding how our blood carries and releases oxygen. Specifically, the oxyhemoglobin dissociation curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2), and is determined by what is called "hemoglobin's affinity for oxygen," that is, how readily hemoglobin acquires and releases oxygen molecules from its surrounding tissue.

Background
Hemoglobin (Hb), an intracellular protein, is the primary vehicle for transporting oxygen in the blood. Oxygen is also carried (dissolved) in plasma, but to a much lesser degree. Hemoglobin is contained in erythrocytes, more commonly referred to as red blood cells. Under certain conditions, oxygen bound to the hemoglobin is released into the body tissue, and under others, it is absorbed from the tissue into the blood. Each hemoglobin molecule has a limited capacity for holding oxygen molecules. How much of that capacity that is filled by oxygen bound to the hemoglobin at any time is called the oxygen saturation. Expressed as a percentage, the oxygen saturation is the ratio of the amount of oxygen bound to the hemoglobin, to the oxygen carrying capacity of the hemoglobin. The oxygen carrying capacity is determined by the amount of hemoglobin present in the blood. The amount of oxygen bound to the hemoglobin at any time is related, in large part, to the partial pressure of oxygen to which the hemoglobin is exposed. In the lungs, at the alveolar-capillary interface, the partial pressure of oxygen is typically high, and therefore the oxygen binds readily to hemoglobin that is present. As the blood circulates to other body tissue in which the partial pressure of oxygen is less, the hemoglobin releases the oxygen into the tissue because the hemoglobin cannot maintain its full bound capacity of oxygen in the presence of lower oxygen partial pressures.

Sigmoidal shape
It is usually a sigmoid plot. A hemoglobin molecule can bind up to four oxygen molecules in a reversible way.

The shape of the curve results from the interaction of bound oxygen molecules with incoming molecules. The binding of the first molecule is difficult. However, this facilitates the binding of the second and third molecules, and it is only when the fourth molecule is to be bound that the difficulty increases, partly as a result of crowding of the hemoglobin molecule, partly as a natural tendency of oxygen to dissociate.

In its basic form, the oxyhemoglobin dissociation curve describes the relation between the partial pressure of oxygen (x axis) and the oxygen saturation (y axis). Hemoglobin's affinity for oxygen increases as successive molecules of oxygen bind. More molecules bind as the oxygen partial pressure increases until the maximum amount that can be bound is reached. As this limit is approached, very little additional binding occurs and the curve levels out as the hemoglobin becomes saturated with oxygen. Hence the curve has a sigmoidal or S-shape. At pressures above about 60 mmHg, the standard dissociation curve is relatively flat, which means that the oxygen content of the blood does not change significantly even with large increases in the oxygen partial pressure. To get more oxygen to the tissue would require blood transfusions to increase the hemoglobin count (and hence the oxygen carrying capacity), or supplemental oxygen that would increase the oxygen dissolved in plasma. Although binding of oxygen to hemoglobin continues to some extent for pressures below about 60 mmHg, as oxygen partial pressures decrease in this steep area of the curve, the oxygen is unloaded to peripheral tissue readily as the hemoglobin's affinity diminishes. The partial pressure of oxygen in the blood at which the hemoglobin is 50% saturated, typically about 26.6 mmHg for a healthy person, is known as the P50. The P50 is a conventional measure of hemoglobin affinity for oxygen. In the presence of disease or other conditions that change the hemoglobin's oxygen affinity and, consequently, shift the curve to the right or left, the P50 changes accordingly. An increased P50 indicates a rightward shift of the standard curve, which means that a larger partial pressure is necessary to maintain a 50% oxygen saturation. This indicates a decreased affinity. Conversely, a lower P50 indicates a leftward shift and a higher affinity.

The 'plateau' portion of the oxyhemoglobin dissociation curve is the range that exists at the pulmonary capillaries (minimal reduction of oxygen transported until the p(O2) falls below 60 mmHg).

The 'steep' portion of the oxyhemoglobin dissociation curve is the range that exists at the systemic capillaries (a small drop in systemic capillary p(O2) can result in the release of large amounts of oxygen for the metabolically active cells).

Factors shifting curve
Many factors influence the affinity of this binding and alter the shape of the curve:

Left shift of the curve is a sign of hemoglobin's increased affinity for oxygen (eg. at the lungs). Similarly, right shift shows decreased affinity, as would appear with an increase in body temperature, hydrogen ion, 2,3-bisphosphoglycerate (not diphosphoglycerate) or carbon dioxide concentration (the Bohr effect)

Carbon monoxide has a much higher affinity for haemoglobin than oxygen does. In carbon monoxide poisoning, oxygen cannot be transported and released to body tissues thus resulting in hypoxia.

With fetal hemoglobin, the shift facilitates diffusion of oxygen across the placenta. The oxygen dissociation curve for myoglobin exists even further to the left.

Factors that Affect the Standard Dissociation Curve
The effectiveness of hemoglobin-oxygen binding can be affected by several factors. The factors can be viewed as having the effect of shifting or reshaping the oxyhemoglobin curve ("the standard curve") of a typical, healthy person. The standard curve is shifted to the right by an increase in temperature, 2,3-BPG, or PCO2, or a decrease in pH. The curve is shifted to the left by the opposite of these conditions. A rightward shift, by definition, causes a decrease in the affinity of hemoglobin for oxygen. This makes it harder for the hemoglobin to bind to oxygen (requiring a higher partial pressure to achieve the same oxygen saturation), but it makes it easier for the hemoglobin to release bound oxygen. Conversely, a leftward shift increases the affinity, making the oxygen easier for the hemoglobin to pick up but harder to release.

Variation of the hydrogen ion concentration
This changes the blood's pH. A decrease in pH shifts the standard curve to the right, while an increase shifts it to the left. This is known as the Bohr effect.

Effects of carbon dioxide
Carbon dioxide affects the curve in two ways: first, it influences intracellular pH (the Bohr effect), and second, CO2 accumulation causes carbamino compounds to be generated through chemical interactions. Low levels of carbamino compounds have the effect of shifting the curve to the right, while higher levels cause a leftward shift.

Effects of 2,3-BPG
2,3-Bisphosphoglycerate, or 2,3-DPG, is an organophosphate, which are created in erythrocytes during glycolysis. The production of 2,3-BPG is likely an important adaptive mechanism, because the production increases for several conditions in the presence of diminished peripheral tissue O2 availability, such as hypoxemia, chronic lung disease, anemia, and congestive heart failure, among others. High levels of 2,3-BPG shift the curve to the right, while low levels of 2,3-BPG cause a leftward shift, seen in states such as septic shock and hypophosphatemia.

Temperature
Temperature does not have so dramatic effect as the previous factors, but hyperthermia causes a rightward shift, while hypothermia causes a leftward shift.

Carbon Monoxide
Hemoglobin binds with carbon monoxide 240 times more readily than with oxygen, and therefore the presence of carbon monoxide can interfere with the hemoglobin's acquisition of oxygen. In addition to lowering the potential for hemoglobin to bind to oxygen, carbon monoxide also has the effect of shifting the curve to the left. With an increased level of carbon monoxide, a person can suffer from severe hypoxemia while maintaining a normal PO2.

Effects of Methemoglobinemia (a form of abnormal hemoglobin)
Methemoglobinemia causes a leftward shift in the curve.

Fetal Hemoglobin
Fetal hemoglobin (HbF) is structurally different from normal hemoglobin (Hb). The fetal dissociation curve is shifted to the left relative to the curve for the normal adult. Typically, fetal arterial oxygen pressures are low, and hence the leftward shift enhances the placental uptake of oxygen.