Haemoglobin enables maximum oxygen uptake via oxyhaemoglobin formation

Steps:

• Identify the process: Oxygen diffuses into blood in lung capillaries and binds haemoglobin to form oxyhaemoglobin, saturating red blood cells.
• Evaluate capacity: Haemoglobin's structure allows binding up to four O2 molecules per unit, but maximum uptake relies on efficient transport mechanism.
• Consider binding dynamics: Cooperative binding ensures rapid saturation, but the key is how oxyhaemoglobin enhances overall oxygen carrying.
• Select best explanation: Option D directly links oxyhaemoglobin to increased red cell transport capacity, maximizing volume uptake.

Why D is correct:

• Oxyhaemoglobin formation binds oxygen reversibly to haemoglobin, increasing red blood cell oxygen-carrying capacity by up to 70 times compared to dissolved oxygen alone (per Fick's law of diffusion).

Why the others are wrong:

• A: Describes basic binding capacity but not the mechanism for maximum uptake in lungs.
• B: Incorrect; carboxyhaemoglobin involves carbon monoxide, not CO2, which doesn't dissociate to free haemoglobin for O2.
• C: False; initial O2 binding actually increases haemoglobin's affinity for subsequent O2 via positive cooperativity.

Final answer: D