In human blood, haemoglobin is a vital pigment found predominantly in which specific component of the blood?

Introduction / Context:
This question focuses on the composition of blood and asks where haemoglobin is located. Haemoglobin is the iron containing respiratory pigment responsible for transporting oxygen from the lungs to tissues. Identifying the correct cellular component that carries haemoglobin is essential knowledge in physiology and general science.

Given Data / Assumptions:
- The options list white blood cells, red blood cells, plasma, and an all inclusive choice. - The question asks where haemoglobin is an important component. - Basic understanding of blood cell types and plasma is assumed.

Concept / Approach:
Blood consists of plasma and formed elements. The main formed elements are red blood cells, white blood cells, and platelets. Haemoglobin is a conjugated protein present in high concentration inside red blood cells, or erythrocytes. It binds oxygen in the lungs and releases it in tissues. White blood cells are involved in defence and do not contain significant haemoglobin. Plasma is the liquid component and does not carry haemoglobin in solution under normal conditions. Therefore, haemoglobin is specifically associated with red blood cells.

Step-by-Step Solution:
Step 1: Recall that red blood cells are responsible for oxygen and carbon dioxide transport in mammals. Step 2: Remember that haemoglobin molecules are packed inside red blood cells and give blood its red colour. Step 3: Recognise that white blood cells perform immune functions, such as phagocytosis and antibody production, and do not contain haemoglobin. Step 4: Note that plasma is mainly water with proteins, nutrients, hormones, and waste products dissolved in it. Step 5: Understand that if haemoglobin were freely dissolved in plasma, it would affect blood viscosity and kidney function, so it is safely contained within red blood cells. Step 6: Conclude that haemoglobin is a key component of red blood cells.

Verification / Alternative check:
Textbook diagrams show red blood cells as biconcave discs filled with haemoglobin. Laboratory tests such as haemoglobin estimation or red cell indices are all based on properties of red blood cells. Anaemia is described as a deficiency of haemoglobin concentration in red blood cells. White blood cell counts and plasma proteins are measured separately, confirming that haemoglobin is not a vital component of these. This consistent description verifies that red blood cells are the correct answer.

Why Other Options Are Wrong:
White blood cells: These immune cells contain nuclei and are involved in defence but do not carry significant haemoglobin. Plasma: The fluid part of blood carries many substances in solution, but haemoglobin is not normally present here as a free pigment. All options are correct: This is incorrect because haemoglobin is specifically located in red blood cells, not equally distributed among all components.

Common Pitfalls:
A common mistake is to over generalise and think that all parts of blood must contain haemoglobin since the whole blood is red. In reality, the colour comes from red blood cells suspended in plasma. Another pitfall is to confuse plasma proteins such as albumin and globulins with haemoglobin. Keeping clear distinctions between each blood component and recalling that erythrocytes are oxygen carriers helps prevent such errors.

Final Answer:
Haemoglobin is an important component of Red blood cells.