Which of the following blood components has the greatest affinity for oxygen and carbon (IV) oxide?

Analyze the question: It asks which blood component binds oxygen and carbon (IV) oxide most strongly. The answer is the cell that carries these gases.

Explain the concept:

• Erythrocytes (red blood cells) contain haemoglobin, a protein with iron (Fe²⁺) that combines readily with oxygen to form oxyhaemoglobin. Haemoglobin also carries some CO₂ as carbaminohaemoglobin.
• Erythrocytes also contain the enzyme carbonic anhydrase, which helps convert CO₂ into bicarbonate for transport.

Check each option:

• A. Lymphocyte: a white blood cell that produces antibodies for immunity. It does not carry gases.
• B. Erythrocyte: contains haemoglobin, which has a high affinity for O₂ and also carries CO₂. This is correct.
• C. Blood plasma: transports most CO₂ as dissolved bicarbonate, and only a very small amount of O₂ is dissolved in it. Its affinity is much lower than that of haemoglobin.
• D. Thrombocyte (platelet): helps blood clot. It has no role in gas transport.

Answer: The erythrocyte (red blood cell) is correct, which is option B.

Exam tip: Remember "Erythrocyte = Exchange of gases" (and Red = Respiratory gases). Likewise, the white cells (leucocytes/lymphocytes) are for defence, and thrombocytes are for clotting.