The Oxyhaemoglobin Dissociation Curve

Cooperative Binding: Haemoglobin is a quaternary protein with four subunits, each containing a haem group. The binding of the first oxygen molecule induces a conformational change in the protein structure, which significantly increases the affinity of the remaining subunits for oxygen.

The Steep Phase: This cooperative effect results in a steep middle section of the curve. In this range, a small increase in $PO_2$ leads to a large increase in saturation, allowing for rapid loading in the lungs and rapid unloading in the tissues.

The Plateau Phase: As haemoglobin approaches $100\%$ saturation, the curve flattens out. This occurs because most binding sites are already occupied, making it statistically less likely for the final oxygen molecules to find an empty site.

Mechanism: The Bohr effect describes the shift of the dissociation curve to the right in the presence of elevated carbon dioxide ($CO_2$) levels. Increased $CO_2$ leads to a higher concentration of hydrogen ions ($H^+$), which lowers the pH and reduces haemoglobin's affinity for oxygen.

Physiological Advantage: This shift occurs primarily in actively respiring tissues where $CO_2$ production is high. By lowering the affinity, the Bohr effect ensures that oxygen is released more readily exactly where it is needed most for aerobic respiration.

Graphical Representation: On a graph, the Bohr shift moves the curve downward and to the right. This means that at any given $PO_2$, the percentage saturation of haemoglobin is lower than it would be under normal conditions.

Right Shift (Lower Affinity): Factors causing a right shift include increased temperature, increased $CO_2$ concentration, and decreased pH. These conditions are typical of exercising muscle, facilitating the 'unloading' of oxygen to the cells.

Left Shift (Higher Affinity): Factors causing a left shift include decreased temperature, decreased $CO_2$, and increased pH. A left shift means haemoglobin binds oxygen more tightly, which is beneficial for 'loading' oxygen in the lungs or in environments with low oxygen availability.

Fetal Haemoglobin: Fetal haemoglobin ($HbF$) has a higher affinity for oxygen than adult haemoglobin ($HbA$), resulting in a left-shifted curve. This allows the fetus to successfully extract oxygen from the mother's blood across the placenta.

Interpret the X-axis: Always check the units for $PO_2$ (kPa vs. mmHg). In the lungs, $PO_2$ is typically high (around $13$ kPa), while in resting tissues it is lower (around $5$ kPa), and even lower in exercising tissues.

The $P_{50}$ Value: The $P_{50}$ is the partial pressure of oxygen at which haemoglobin is $50\%$ saturated. A higher $P_{50}$ indicates a right shift (lower affinity), while a lower $P_{50}$ indicates a left shift (higher affinity).

Common Pitfall: Students often mistakenly think a right shift is 'bad' because saturation is lower. In reality, a right shift is a vital adaptation that allows tissues to receive more oxygen during periods of high metabolic demand.