Haemoglobin, the Dissociation Curve and the Bohr Shift
The oxygen dissociation curve is S-shaped because binding the first oxygen makes the next one easier. Carbon dioxide shifts the whole curve to the right, so tissue that is working hard gets unloaded first.
What a learner can do afterwards
- Reads percentage saturation off the curve at a given partial pressure.
- Explains the S shape in terms of what happens to haemoglobin after the first oxygen binds.
- Predicts which way the curve moves in an active muscle and what that does for unloading.
1 · Read
Almost all the oxygen in your blood rides on haemoglobin, a protein packed inside your red blood cells. Each haemoglobin molecule has four subunits, and each subunit holds one iron atom that one oxygen molecule can grab. Where oxygen pressure is high, as in the lungs, haemoglobin loads up fast. Where pressure is low, as in the tissues, it lets go.
Loading is not like filling a bucket, because each oxygen changes the protein. When the first oxygen attaches, it twists the shape of haemoglobin just enough to make the next spot grab oxygen more easily, and so on. That cooperative binding is why a graph of saturation against pressure is S-shaped instead of a straight line.
Hardworking muscle rewrites the deal with its waste. Extra carbon dioxide makes haemoglobin release oxygen more easily, shifting the whole curve to the right, a change called the Bohr shift. At any given pressure the shifted haemoglobin holds less, so busy tissue making lots of carbon dioxide gets first dibs on oxygen. Acid does the same loosening: if failing kidneys let acid build up, blood pH drops and haemoglobin affinity falls.
Read the curve in two zones. The flat top means haemoglobin is nearly full, so extra pressure adds little. The steep middle means a small pressure drop unloads a lot, which is exactly where tissues collect their oxygen. A right shift at the same pressure always means lower saturation and more release.
Cooperative binding makes the curve S-shaped, and carbon dioxide shifts it right so working tissue unloads first.
The rewritten readings were written for this dataset.
More to read
How haemoglobin delivers oxygen
Red blood cells contain haemoglobin, a protein that transports oxygen to tissues. A single molecule can bind four oxygen molecules.
Attaching the first oxygen changes haemoglobin's shape and helps further oxygen molecules bind. This cooperation produces an S-shaped oxygen dissociation curve.
High oxygen partial pressure in the lungs favours loading. Lower pressure in tissues favours release, supplying cells with oxygen.
Working muscles produce carbon dioxide. Its increase, together with the resulting fall in pH, reduces haemoglobin's oxygen affinity. The dissociation curve shifts right, so haemoglobin releases more oxygen at a given oxygen partial pressure. This helps match oxygen delivery to the needs of active muscles.
Reading oxygen loading from the curve
Most blood oxygen travels bound to haemoglobin. Its four protein chains each contain an iron-bearing group that binds one oxygen molecule.
Binding changes the protein's shape, increasing the affinity of the remaining sites. This cooperative behaviour gives a graph of percentage saturation against oxygen partial pressure its S shape.
Lung oxygen pressure lies near the curve's upper plateau, where haemoglobin is highly saturated. Active tissues have lower oxygen pressure and produce more carbon dioxide, favouring oxygen release. Increased carbon dioxide and decreased pH lower oxygen affinity and shift the curve right. This change is the Bohr shift.
Explore more
2 · Watch
به انگلیسیTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.