Haemoglobin is a protein with a quaternary structure found in red blood cells.

Haemoglobin binds to oxygen and carries it in the blood from the lungs to tissues where it's needed. It then releases the oxygen for cells to use.

The cooperative nature of oxygen binding means that the binding of the first oxygen molecule to haemoglobin makes it easier for subsequent oxygen molecules to bind.

1. Some carbon dioxide dissolves in plasma
2. Some carbon dioxide binds to haemoglobin in red blood cells to form carbaminohaemoglobin
3. Some carbon dioxide reacts with water, catalysed by carbonic anhydrase, to form carbonic acid, which dissociates to form hydrogen carbonate ions and hydrogen ions

Plasma transports these ions and red blood cells to the lungs, where they are converted back to carbon dioxide and exhaled

The chloride shift is the movement of negatively charged chloride ions into red blood cells as negatively charged hydrogen carbonate ions move out.

The chloride shift helps to maintain the electrical balance of the red blood cells.

In the lungs, the partial pressure of oxygen is high, so haemoglobin binds to oxygen.

In respiring tissues, the partial pressure of oxygen is low, so haemoglobin releases oxygen.

Haemoglobinic acid is formed when haemoglobin binds to hydrogen ions.

This is one way that haemoglobin helps to maintain the pH of the blood when pCO₂ is high.

The Bohr effect, or Bohr shift, refers to the rightward shift in the oxygen dissociation curve in response to increased carbon dioxide concentration.

Higher carbon dioxide levels make haemoglobin release oxygen more readily, enhancing oxygen unloading where it's most needed.

The oxygen dissociation curve is a graph showing how the saturation of haemoglobin with oxygen changes when the partial pressure of oxygen changes.

It is sometimes called the oxyhaemoglobin dissociation curve.

Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, allowing it to draw oxygen from the mother's blood supply.