Revise Transport of oxygen and carbon dioxide for Cambridge 9700 (8.2)

Haemoglobin (Hb) is the primary molecule responsible for oxygen transport in your blood. Found within red blood cells, it's a globular protein with a remarkable structure. It's a quaternary protein, meaning it's composed of four polypeptide chains: two alpha (α) chains and two beta (β) chains. Each polypeptide chain is associated with a non-protein haem prosthetic group, which contains a central iron (Fe²⁺) ion. It's this Fe²⁺ ion that reversibly binds to an oxygen molecule. Since there are four haem groups per haemoglobin molecule, each haemoglobin can carry up to four oxygen molecules, forming oxyhaemoglobin (HbO₂). Haemoglobin's ability to bind oxygen strongly in high partial pressures of oxygen (pO₂) in the lungs and release it readily in low pO₂ in the tissues is crucial for life.

The relationship between the partial pressure of oxygen (pO₂) and the percentage saturation of haemoglobin with oxygen is represented by the oxygen dissociation curve. This curve is typically S-shaped (sigmoidal), reflecting haemoglobin's property of cooperative binding. When the first oxygen molecule binds to haemoglobin, it causes a conformational change that increases the affinity of the remaining binding sites for oxygen. This makes it easier for subsequent oxygen molecules to bind, leading to a steep rise in saturation.

Hb + 4O₂ ⇌ Hb(O₂)₄

In the lungs, where pO₂ is high (around 13.3 kPa), haemoglobin becomes almost 100% saturated with oxygen. In the tissues, where cells are respiring and pO₂ is much lower (e.g., 5.3 kPa at rest), haemoglobin readily releases its oxygen. The steep part of the curve indicates that a small drop in pO₂ in the tissues leads to a significant release of oxygen, perfectly supplying active cells.

Active tissues produce more carbon dioxide and heat, which influence haemoglobin's oxygen affinity. The Bohr effect describes how an increase in partial pressure of carbon dioxide (pCO₂), a decrease in pH (more acidic), and an increase in temperature all reduce haemoglobin's affinity for oxygen. These conditions cause the oxygen dissociation curve to shift to the right. This 'right shift' means that at any given pO₂, haemoglobin will release more oxygen. This is a vital adaptation: in metabolically active tissues, oxygen is released precisely where and when it's most needed for respiration.

Carbon dioxide (CO₂) produced during tissue respiration must be transported to the lungs for excretion. It's carried in the blood in three main ways:

1. Dissolved in plasma (~10%): A small amount of CO₂ dissolves directly in the blood plasma.
2. Bound to haemoglobin (~20%): CO₂ can bind reversibly to the amino groups on the globin chains of haemoglobin, forming carbaminohaemoglobin. This binding occurs more readily with deoxygenated haemoglobin, which is prevalent in the tissues.
3. As hydrogencarbonate ions (~70%): This is the most significant method. Inside red blood cells, CO₂ is converted to carbonic acid, which then dissociates. The resulting hydrogencarbonate ions are transported in the plasma.

The chloride shift is a vital mechanism that ensures electrical neutrality across the red blood cell membrane during carbon dioxide transport. As hydrogencarbonate ions (HCO₃⁻), which are negatively charged, move out of the red blood cell into the plasma, an equal number of negatively charged chloride ions (Cl⁻) move into the red blood cell. This prevents a charge imbalance that would otherwise hinder the continuous movement of HCO₃⁻ out of the cell, allowing efficient transport of CO₂ as hydrogencarbonate ions to the lungs. In the lungs, the process reverses: HCO₃⁻ moves back into the red blood cell, and Cl⁻ moves out.