---
title: "Haemoglobin, the Dissociation Curve and the Bohr Shift"
description: "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 f"
canonical: https://lightmysky.com/learn/science/haemoglobin-the-dissociation-curve-and-the-bohr-shift-mt_MmYJArGxrE
source: https://lightmysky.com/learn/science/haemoglobin-the-dissociation-curve-and-the-bohr-shift-mt_MmYJArGxrE.md
retrieved: 2026-09-12
---

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# 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.

Subject: Science · Area: The Human Body · Ages 17 to 18
Page: https://lightmysky.com/learn/science/haemoglobin-the-dissociation-curve-and-the-bohr-shift-mt_MmYJArGxrE

## Ready when they can

- 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.

## Lesson: How haemoglobin loads and unloads oxygen

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.

**Recap.** Cooperative binding makes the curve S-shaped, and carbon dioxide shifts it right so working tissue unloads first.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Blood Components and Blood Vessels](https://lightmysky.com/learn/science/blood-components-and-blood-vessels-mt_aWZ2sGXGMn)
- [Ventilation in Insects, Fish and Mammals](https://lightmysky.com/learn/science/ventilation-in-insects-fish-and-mammals-mt_Thd06O06Vr)

## Opens up

- [Ventilation and Perfusion Matching, and the Control of Breathing](https://lightmysky.com/learn/science/ventilation-and-perfusion-matching-and-the-control-of-breathing-mt_O89KdcN_Vv)
- [Allosteric Enzymes, Cooperativity and the Sigmoid Curve](https://lightmysky.com/learn/science/allosteric-enzymes-cooperativity-and-the-sigmoid-curve-mt_u8f5QLHq-g)