The component that serves as major buffer in blood is

Analyze the question: It asks which blood component acts as the major buffer, meaning the part that helps keep blood pH steady (about 7.35-7.45).

Explain the concept: A buffer resists changes in pH. In blood, the main buffering system is the bicarbonate buffer (carbonic acid/bicarbonate ions, H₂CO₃/HCO₃⁻), together with plasma proteins such as albumin and phosphate ions. All of these are dissolved in the liquid part of blood, which is the plasma.

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

• If blood becomes too acidic, HCO₃⁻ mops up the excess H⁺.
• If blood becomes too alkaline, H₂CO₃ releases H⁺.

Evaluate the options:

• A. Plasma: It contains bicarbonate, plasma proteins and phosphates, which make up the main buffer system of blood. ✔
• B. Erythrocytes: Red blood cells carry oxygen. Haemoglobin does buffer H⁺ inside the cell, but the question asks for the major buffer, and this is not the main one.
• C. Leucocytes: White blood cells fight infection and play no major role in buffering.
• D. Lymph: This is a tissue fluid that drains into the lymphatic system. It is not the blood's main buffer.

Answer: The correct option is A. plasma.

Exam tip: Remember "Plasma = the liquid with the chemicals". Anything about dissolved substances, such as buffers, proteins, hormones, and nutrients, usually points to plasma. Anything about carrying oxygen points to red blood cells, and anything about defence points to white blood cells.