Oxygen–hemoglobin dissociation curve

The oxygen–hemoglobin dissociation curve plots hemoglobin oxygen saturation against oxygen partial pressure. Its sigmoidal shape reflects cooperative oxygen binding and helps explain oxygen loading in lungs and unloading in tissues.

Hemoglobin: Hemoglobin is the iron-containing protein in red blood cells that binds and transports oxygen. Its oxygen-binding behavior generates the curve.
Oxygen saturation: Oxygen saturation is the fraction of available hemoglobin oxygen-binding sites occupied by oxygen. It is the vertical-axis quantity whose relation to partial pressure the curve describes.
Partial pressure: Partial pressure is the pressure contributed by one gas within a mixture of gases. Oxygen partial pressure forms the curve’s horizontal axis.
Oxygen–myoglobin dissociation curve: The oxygen–myoglobin dissociation curve relates oxygen partial pressure to myoglobin saturation and is hyperbolic. Its noncooperative shape highlights the effect of hemoglobin’s multiple interacting binding sites.
Cooperative binding: Cooperative binding occurs when a ligand’s binding changes a protein’s affinity for additional ligand molecules. Binding one oxygen molecule makes hemoglobin more likely to bind the next, producing a sigmoid.
Arterial blood gas: An arterial blood gas test measures oxygen, carbon dioxide, and acidity in arterial blood. Its oxygen partial pressure can be related to hemoglobin saturation using the curve.
Oxygen transport: Oxygen transport is the movement of oxygen from the lungs through blood to tissues and back as carbon dioxide is carried away. The curve describes how hemoglobin supports oxygen loading and unloading along this route.
Right shift of the oxygen–hemoglobin dissociation curve: A right shift is a change in the oxygen–hemoglobin curve toward lower saturation at a given oxygen partial pressure. It represents reduced oxygen affinity and facilitates release to tissues.
Bohr effect: The Bohr effect is the reduction in hemoglobin’s oxygen affinity caused by increased carbon dioxide or acidity. It shifts the curve rightward, promoting oxygen release in metabolically active tissues.
Pulse oximetry: Pulse oximetry estimates arterial hemoglobin oxygen saturation by analyzing light absorption through tissue. The curve helps relate a saturation estimate to blood oxygen partial pressure, with limitations.

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