Cardiovascular system: Oxyhaemoglobin disassociation curve.

The idea

The oxyhaemoglobin dissociation curve is a graph plotting the percentage saturation of haemoglobin with oxygen against the partial pressure of oxygen (pO₂). At high pO₂ in the lungs, haemoglobin loads oxygen to form oxyhaemoglobin; at lower pO₂ in respiring tissues, it unloads oxygen. The curve is S-shaped (sigmoid) because binding the first oxygen molecule changes the shape of haemoglobin and makes binding the next molecules easier, a cooperative effect. The Bohr shift describes the curve moving to the right when carbon dioxide concentration rises, temperature increases or pH falls during exercise, meaning haemoglobin releases more oxygen at the same pO₂. Myoglobin's curve lies to the left, showing its higher affinity.

What earns the marks

• The curve plots percentage saturation of haemoglobin with oxygen on the y-axis against partial pressure of oxygen (pO₂) on the x-axis.
• The curve is sigmoid (S-shaped) because of cooperative binding: the first oxygen molecule bound alters haemoglobin's shape and increases its affinity for further oxygen molecules.
• At high pO₂ in the lungs, haemoglobin has a high affinity for oxygen and becomes almost fully saturated, forming oxyhaemoglobin.
• At low pO₂ in respiring tissues, haemoglobin's affinity for oxygen decreases and oxygen dissociates to be used in aerobic respiration.
• The Bohr shift moves the curve to the right when carbon dioxide concentration increases, pH decreases or temperature rises during exercise, so more oxygen is released at the same pO₂.
• Myoglobin's dissociation curve lies to the left of haemoglobin's, reflecting its higher affinity for oxygen.

Common mistakes and how to put them right

• Reversing the axes: percentage saturation of haemoglobin with oxygen is on the y-axis and partial pressure of oxygen is on the x-axis.

• Describing the curve as a straight line.

  Put it right: It is sigmoid because cooperative binding makes the middle section steeper.

• Confusing a rightward shift with reduced oxygen release.

  Put it right: A rightward (Bohr) shift means haemoglobin releases more oxygen to tissues at the same pO₂.

• Thinking the Bohr shift occurs because haemoglobin has a lower affinity at high pO₂.

  Put it right: The shift is caused by increased carbon dioxide, increased temperature and decreased pH in active muscle.

Exam tips

• Use the terms 'loads' at the lungs and 'unloads' or 'dissociates' at the tissues to describe the two ends of the curve.
• Link the Bohr shift to exercise by naming the specific changes: increased carbon dioxide, increased temperature and decreased pH.
• Compare the myoglobin curve with the haemoglobin curve by stating that myoglobin sits to the left because of its higher oxygen affinity.