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Hemoglobin dissociation curve
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If you draw a graph with oxygen pressure on one axis and, on the other, how full of oxygen hemoglobin is, you do not get a straight line: you get an S-shaped curve.
There is something behind that worth knowing. Hemoglobin, the protein that carries oxygen in the blood, is a tetramer, meaning it is made of four pieces, and each piece carries one molecule of oxygen. When the first one latches on, the other pieces change shape and the next oxygen latches on more easily. That is called cooperativity: the first molecule opens the door for the rest.
The curve flattens out where there is plenty of oxygen, and it gets steep where there is little.
The curve also moves. With more carbon dioxide, more acidity, more 2,3-DPG or a higher temperature, the curve shifts to the right, and once it shifts that way hemoglobin releases oxygen more easily. The relationship between acidity, carbon dioxide and oxygen affinity is called the Bohr effect.
In brief
Plotting oxygen pressure against saturation produces an S-shaped curve.
Hemoglobin has four subunits, each carrying one molecule; once the first binds, the others change shape and the next oxygen binds more easily: this is cooperativity.
The first molecule opens the door for the rest.
The curve flattens at high pressure and steepens at low pressure.
More carbon dioxide or more acidity shifts it to the right and more oxygen is released: this is the Bohr effect; 2,3-DPG and temperature shift it the same way.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Oxyhemoglobin Dissociation Curve, StatPearlsPublished
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