3.4.5 The Bohr Shift

Key Points of the Bohr Shift:

1. Effect of Carbon Dioxide:

• Respiring cells produce carbon dioxide ($CO₂$) as a by-product of respiration.
• An increase in $pCO₂$ lowers haemoglobin's affinity for oxygen.
• This causes oxygen to be released more readily in tissues.

2. Mechanism:

• Carbon dioxide reacts with water to form carbonic acid ($H₂CO₃$):

$$CO₂ + H₂O \rightarrow H₂CO₃$$

• Carbonic acid dissociates into H⁺ ions and bicarbonate ions ($HCO₃⁻$):

$$H₂CO₃ \rightarrow H⁺ + HCO₃⁻$$

• The increase in H⁺ ions lowers the pH, creating a slightly acidic environment.
• This alters haemoglobin's shape, reducing its ability to bind oxygen.

3. Result:

• Oxygen is released from haemoglobin in high $pCO₂$ or low pH conditions, which occur in actively respiring tissues.

Effect on the Oxygen Dissociation Curve:

1. Shift to the Right:

• The oxygen dissociation curve shifts to the right in the presence of high $pCO₂.$
• At the same partial pressure of oxygen ($pO₂$), haemoglobin is less saturated.
• This means oxygen is released more easily in tissues with high metabolic activity.

2. Biological Advantage:

• Ensures oxygen is supplied where it is most needed, such as in tissues producing high levels of carbon dioxide.

Key Adaptations:

1. Exercise:

• During exercise, increased respiration raises $pO₂$, enhancing oxygen delivery to active muscles.

2. Foetal Haemoglobin:

• Less affected by the Bohr Shift than adult haemoglobin, ensuring oxygen transfer from mother to foetus.

Summary of the Bohr Shift:

Condition	Effect on Haemoglobin	Result
High $pCO₂$ or Low pH	Reduced oxygen affinity	Oxygen released to respiring tissues.
Low $pCO₂$ or High pH	Increased oxygen affinity	Oxygen binds tightly (e.g., in lungs).

Key Terms:

• Bohr Shift: The reduction in haemoglobin's oxygen affinity due to high $pCO₂$ or low $pH$.
• Partial Pressure ($pCO₂$): The concentration of carbon dioxide in the environment.
• Affinity: The tendency of haemoglobin to bind oxygen.
• Right Shift: Movement of the oxygen dissociation curve due to the Bohr Shift.