Cardiovascular system: Bohr shift.

The idea

The Bohr shift describes how a rise in carbon dioxide concentration, and the accompanying fall in pH, reduces haemoglobin's affinity for oxygen so that oxygen dissociates more readily from oxyhaemoglobin. During vigorous exercise, respiring muscle produces extra carbon dioxide, which forms carbonic acid and lowers blood pH; this shifts the oxyhaemoglobin dissociation curve to the right at any given partial pressure of oxygen. The same percentage saturation therefore releases more oxygen to the active tissues. A useful method is to compare two curves on one graph: at a fixed oxygen partial pressure, read across to each curve and note that the right-hand curve gives the lower saturation.

What earns the marks

• Carbon dioxide concentration increases during exercise because respiring muscles produce more carbon dioxide.
• Increased carbon dioxide lowers blood pH, making the blood more acidic.
• Lower pH reduces haemoglobin's affinity for oxygen.
• The oxyhaemoglobin dissociation curve shifts to the right.
• At a given partial pressure of oxygen, haemoglobin saturation is lower, so more oxygen dissociates.
• More oxygen is therefore delivered to the active muscles.

Common mistakes and how to put them right

• Thinking the Bohr shift means haemoglobin carries more oxygen.

  Put it right: It actually means haemoglobin releases oxygen more readily at the tissues.

• Confusing a rightward shift with a leftward shift.

  Put it right: A rightward shift shows reduced affinity and easier unloading.

• Believing carbon dioxide directly displaces oxygen.

  Put it right: The effect works mainly through the fall in pH caused by carbon dioxide.

• Assuming the shift changes the maximum saturation of haemoglobin.

  Put it right: It changes the position of the curve, not the maximum achievable saturation.

Exam tips

• Link the shift explicitly to exercise so the answer explains why more oxygen reaches active muscle.
• Use the phrase 'reduced affinity of haemoglobin for oxygen' rather than saying haemoglobin 'holds less oxygen'.
• When a graph is given, quote the partial pressure of oxygen and compare the two saturation values at that point.
• State the causal chain in order: more carbon dioxide, lower pH, reduced affinity, rightward shift, greater dissociation.