Biology – Transport of oxygen and carbon dioxide | e-Consult

The oxygen dissociation curve is sigmoidal, demonstrating a non-linear relationship between pO₂ and the saturation of hemoglobin. This shape is crucial for efficient oxygen uptake in the lungs and delivery to respiring tissues.

In the lungs (high pO₂): At high pO₂ in the lungs (typically around 100 mmHg), hemoglobin is almost fully saturated (approximately 98%). The curve is relatively flat in this region, indicating that even small changes in pO₂ have a minimal impact on hemoglobin saturation. This ensures that hemoglobin can efficiently load oxygen from the inhaled air.

In respiring tissues (low pO₂): In respiring tissues, pO₂ is significantly lower (typically around 40-50 mmHg). Here, the curve shows a steep decline in hemoglobin saturation. This means that even a small decrease in pO₂ leads to a substantial release of oxygen from hemoglobin. This is vital because it allows oxygen to be readily unloaded and delivered to cells that need it. The steepness of the curve ensures that oxygen is efficiently released in the areas of high metabolic demand.

Factors contributing to the shape: Several factors contribute to the sigmoidal shape:

• Cooperative binding: The binding of oxygen to hemoglobin is cooperative. The binding of one oxygen molecule increases the affinity of the remaining binding sites for oxygen. This makes it easier for oxygen to bind as pO₂ decreases.
• Deoxyhemoglobin conformation: The conformational change in deoxyhemoglobin facilitates oxygen binding.
• pH (Bohr effect): A decrease in pH (increased acidity) reduces hemoglobin's affinity for oxygen, promoting oxygen release in metabolically active tissues. This is because H⁺ ions bind to hemoglobin and alter its structure.
• Temperature: Increased temperature reduces hemoglobin's affinity for oxygen, also promoting oxygen release. This is linked to increased metabolic activity and heat generation.
• 2,3-Bisphosphoglycerate (2,3-BPG): Increased levels of 2,3-BPG in red blood cells reduce hemoglobin's affinity for oxygen, facilitating oxygen release. 2,3-BPG binds to deoxyhemoglobin, causing a conformational change that reduces oxygen affinity.

The sigmoidal shape of the oxygen dissociation curve is therefore a crucial adaptation that ensures efficient oxygen uptake in the lungs and efficient delivery to respiring tissues, meeting the metabolic demands of the body.