Transport of oxygen
Carrying oxygen
- Oxygen is carried by haemoglobin in red blood cells.
- How tightly haemoglobin holds oxygen depends on how much oxygen is around.
- The oxygen dissociation curve captures this in one S-shaped graph.
Loading and unloading
- The curve plots % saturation (how full of oxygen) against the partial pressure of oxygen.
- Where oxygen is high (the lungs), haemoglobin loads up and becomes almost fully saturated.
- Where oxygen is low (respiring tissues), haemoglobin unloads its oxygen for the cells.
Practice
Where the partial pressure of oxygen is high (the lungs), haemoglobin:
At high oxygen partial pressure (lungs), haemoglobin loads up and becomes nearly fully saturated.
Practice
In respiring tissues, where oxygen is low, haemoglobin:
At low oxygen partial pressure (tissues), haemoglobin releases oxygen to the cells.
Practice
The oxygen dissociation curve is S-shaped (sigmoid).
It is S-shaped: binding one oxygen makes the next bind more easily, giving the characteristic sigmoid curve.
The Bohr shift
- Active tissues release more carbon dioxide, which lowers the pH.
- This makes haemoglobin release oxygen more easily — the curve shifts to the right.
- So oxygen is given up exactly where it is needed most. This is the Bohr shift.
Practice
The Bohr shift describes how:
More CO₂ lowers pH, shifting the curve right so haemoglobin releases more oxygen where tissues need it.
You've got it
Key idea
- haemoglobin carries oxygen; the dissociation curve is S-shaped
- high oxygen (lungs) → loads / saturated; low oxygen (tissues) → unloads
- Bohr shift: more CO₂ → lower pH → curve moves right → more oxygen released
- the Bohr shift delivers oxygen where active tissues need it most