In human physiology, which one of the following statements about haemoglobin present in red blood cells (RBCs) is correct regarding its ability to carry respiratory gases?

Introduction / Context:
Haemoglobin is the iron containing pigment found inside red blood cells (RBCs) and is one of the most important proteins in human physiology. It is primarily known for its role in transporting oxygen from the lungs to the tissues, but it also participates in the transport of carbon dioxide back from the tissues to the lungs. This question tests your understanding of the actual gas carrying capacity of haemoglobin and clears a common misconception that it carries only oxygen.

Given Data / Assumptions:

• We are discussing haemoglobin present inside human red blood cells.
• The focus is on which respiratory gases haemoglobin can carry.
• Options mention oxygen, carbon dioxide, both gases, or unrelated roles such as blood clotting.
• Normal physiological conditions in a healthy adult are assumed.

Concept / Approach:
Haemoglobin reversibly binds oxygen in the lungs, forming oxyhaemoglobin, and releases it in the tissues where the partial pressure of oxygen is lower. In addition to oxygen, about 20 to 25 percent of carbon dioxide produced in tissues is transported bound to haemoglobin, mainly as carbaminohaemoglobin. The remaining carbon dioxide is carried dissolved in plasma or as bicarbonate ions. Haemoglobin has no role in blood clotting; clotting is mediated by platelets and clotting factors. Therefore, the correct choice must state that haemoglobin can carry both oxygen and a portion of carbon dioxide.

Step-by-Step Solution:
Step 1: Examine option A, which says haemoglobin can carry only oxygen but not carbon dioxide. This is incomplete and therefore incorrect because haemoglobin also binds some carbon dioxide in the tissues.Step 2: Examine option B, which states that haemoglobin can carry both oxygen and carbon dioxide. This matches standard physiology: haemoglobin transports most of the oxygen and a significant fraction of carbon dioxide.Step 3: Examine option C, which claims haemoglobin can carry only carbon dioxide. This contradicts the well known role of haemoglobin in oxygen transport.Step 4: Examine option D, which suggests haemoglobin is used only for blood clotting. This is clearly wrong because clotting involves platelets and plasma proteins such as fibrinogen, not haemoglobin.Step 5: Conclude that option B is the only statement that correctly describes the gas carrying function of haemoglobin in RBCs.

Verification / Alternative check:
A quick check is to remember typical blood gas transport percentages. Roughly 98 percent of oxygen is carried bound to haemoglobin, with a small fraction dissolved in plasma. For carbon dioxide, about 70 percent is transported as bicarbonate, about 20 to 25 percent bound to haemoglobin, and the remainder dissolved in plasma. Since a significant portion of carbon dioxide is indeed attached to haemoglobin, any option that excludes carbon dioxide binding is incomplete.

Why Other Options Are Wrong:
Option A is wrong because it ignores the carbaminohaemoglobin form through which haemoglobin carries part of the carbon dioxide. Option C is wrong because haemoglobin is the major oxygen carrier; without it, adequate oxygen transport would be impossible. Option D is wrong because haemoglobin is not involved in the clotting cascade; clotting is performed by platelets and clotting factors, while haemoglobin’s main job is gas transport.

Common Pitfalls:
Many students remember haemoglobin primarily as an oxygen carrier and forget its role in carbon dioxide transport. Another common mistake is to confuse the functions of different blood components, such as thinking red blood cells are involved in clotting because they appear in clots. Distinguishing clearly between RBCs, platelets, and plasma proteins helps prevent these errors. Always recall that haemoglobin is a transport protein for respiratory gases, not a clotting factor.

Final Answer:
Haemoglobin present in red blood cells can carry both oxygen and a portion of carbon dioxide.