Q.Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?

If the percentage saturation of haemoglobin with oxygen is plotted against the partial pressure of oxygen, the graph obtained is called the oxygen dissociation curve.

• It is not a straight line but an S-shaped, or sigmoid, curve.
• It is a useful tool for studying how factors such as carbon dioxide pressure and hydrogen ion concentration affect the loading and unloading of oxygen. …

Oxygen is carried through the blood chiefly bound to haemoglobin as oxyhaemoglobin, and whether it binds or is released depends mostly on how much oxygen is available in a given place. To capture this relationship, one plots the percentage saturation of haemoglobin with oxygen along one axis against the partial pressure of oxygen along the other. The line that results is the oxygen dissociation curve.

A striking feature of this curve is that it is not straight. It takes an S-shape, or sigmoid form, which makes it a valuable tool for examining how other conditions — such as the partial pressure of carbon dioxide and the hydrogen ion concentration — influence the loading of oxygen at the lungs and its unloading at the tissues.

Why should the curve be sigmoid rather than a simple diagonal? A reasonable explanation lies in the way haemoglobin itself is built:

• A single haemoglobin molecule can hold at most four oxygen molecules.
• Because the oxygen is taken up in successive steps rather than all in one moment, the response of saturation to rising oxygen pressure is uneven — slow to begin, rapid through the middle range, and then levelling off as the four binding places fill up. …

Method: Explaining the Shape of the Oxygen Dissociation Curve

Use this approach for any question asking you to define the O2 dissociation curve or explain why it is sigmoid rather than a straight line.

Steps

Step 1: State the two axes being plotted

% saturation of haemoglobin with O2 (y-axis) against partial pressure of O2 (x-axis) — this pairing of axes is the definition of the curve itself.

Step 2: Recall the structural fact behind the shape

One haemoglobin molecule carries up to four O2-binding sites, filled in successive steps rather than all at once.

Step 3: Connect the structural fact to the curve's regions …