Oxygen–hemoglobin dissociation curve

In the capillaries, where carbon dioxide is produced, oxygen bound to the hemoglobin is released into the blood's plasma and absorbed into the tissues.

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.

The partial pressure of oxygen in the blood at which the hemoglobin is 50% saturated, typically about 26.6 mmHg (3.5 kPa) for a healthy person, is known as the P50.

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.

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 50 mmHg).

The effect of this shift of the curve increases the partial pressure of oxygen in the tissues when it is most needed, such as during exercise, or hemorrhagic shock.

Left shift of the curve is a sign of hemoglobin's increased affinity for oxygen (e.g. at the lungs).

For example, during exercise, muscles have a higher metabolic rate, and consequently need more oxygen, produce more carbon dioxide and lactic acid, and their temperature rises.

[4] A reduction in the total binding capacity of hemoglobin to oxygen (i.e. shifting the curve down, not just to the right) due to reduced pH is called the root effect.

First, CO2 accumulation causes carbamino compounds to be generated through chemical interactions, which bind to hemoglobin forming carbaminohemoglobin.

The formation of a bicarbonate ion will release a proton into the plasma, decreasing pH (increased acidity), which also shifts the curve to the right as discussed above; low CO2 levels in the blood stream results in a high pH, and thus provides more optimal binding conditions for hemoglobin and O2.

[6][7] 2,3-Bisphosphoglycerate or 2,3-BPG (formerly named 2,3-diphosphoglycerate or 2,3-DPG) is an organophosphate formed in red blood cells during glycolysis and is the conjugate base of 2,3-bisphosphoglyceric acid.

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, which necessitate easier oxygen unloading in the peripheral tissue.

An increased concentration of BPG in red blood cells favours formation of the T (taut or tense), low-affinity state of hemoglobin and so the oxygen-binding curve will shift to the right.

The reaction HbO2 + CO → HbCO + O2 almost irreversibly displaces the oxygen molecules forming carboxyhemoglobin; the binding of the carbon monoxide to the iron centre of hemoglobin is much stronger than that of oxygen, and the binding site remains blocked for the remainder of the life cycle of that affected red blood cell.

The nitrite also acts as a vasodilator, promoting the cellular supply of oxygen, and the addition of an iron salt provides for competitive binding of the free cyanide as the biochemically inert hexacyanoferrate(III) ion, [Fe(CN)6]3−.

An alternative approach involves administering thiosulfate, thereby converting cyanide to thiocyanate, SCN−, which is excreted via the kidneys.

The effects appear to last roughly as long as the affected red blood cells remain in circulation.

The fetal dissociation curve is shifted to the left relative to the curve for the normal adult because of these structural differences: In adult hemoglobin, the binding of 2,3-bisphosphoglycerate (2,3-BPG) primarily occurs with the beta chains, preventing the binding of oxygen with haemoglobin.

This binding is crucial for stabilizing the deoxygenated state of hemoglobin, promoting the efficient release of oxygen to body tissues.

As a result, 2,3-BPG binds more strongly to adult hemoglobin, causing HbA to release more oxygen for uptake by the fetus, whose HbF is unaffected by the 2,3-BPG.