8.2 Transport of Oxygen and Carbon Dioxide

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe the role of red blood cells in transporting oxygen and carbon dioxide, including the roles of haemoglobin, carbonic anhydrase, haemoglobinic acid, and carbaminohaemoglobin.
  2. Describe the chloride shift and explain why it is important.
  3. Describe the role of plasma in transporting carbon dioxide.
  4. Describe and explain the oxygen dissociation curve of adult haemoglobin.
  5. Explain the importance of the oxygen dissociation curve at the partial pressures of oxygen found in the lungs and in respiring tissues.
  6. Describe the Bohr shift and explain why it is important.

1. The Role of Red Blood Cells in Transporting Oxygen and Carbon Dioxide

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Structure of Red Blood Cells (RBCs)

Red blood cells (also called erythrocytes) are specially adapted to carry gases around the body. When they mature, they lose their nucleus and mitochondria. This frees up space for a large amount of the protein haemoglobin, which is the main gas-carrying molecule.

Each mature RBC also contains the enzyme carbonic anhydrase, which is essential for carbon dioxide transport (explained below).

Transport of Oxygen: The Role of Haemoglobin

Haemoglobin (Hb) is a large protein made of four subunits — two alpha (α) chains and two beta (β) chains. Each subunit contains a haem group, which is a special region with an iron ion (Fe²⁺) at its centre. This iron ion is where one oxygen molecule (O₂) binds.

Because there are four haem groups in total, one haemoglobin molecule can carry four oxygen molecules at once.

When oxygen binds to haemoglobin, it forms oxyhaemoglobin:

Hb + 4O₂ ⇌ HbO₈ (oxyhaemoglobin)

This reaction is reversible — haemoglobin picks up oxygen in the lungs and releases it in the tissues. The binding of oxygen is also cooperative: when the first oxygen molecule binds, the haemoglobin changes shape slightly (called an allosteric change), making it easier for the next three oxygen molecules to bind. This is known as co-operative binding.

About 95–97% of oxygen in the blood is carried as oxyhaemoglobin. The remaining 3–5% is simply dissolved in the plasma.

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