In human blood, oxygen is transported from the lungs to body tissues mainly by which iron-containing protein found inside red blood cells?

Introduction / Context:
Efficient transport of oxygen is essential for aerobic respiration in human cells. While oxygen dissolves slightly in plasma, the vast majority is carried by a specialised protein in red blood cells. This protein binds oxygen in the lungs and releases it in tissues where it is needed. The question tests your knowledge of the name and role of this key oxygen-carrying protein in blood.

Given Data / Assumptions:

• We are considering proteins related to structural support, muscle, clotting, and oxygen transport.
• Options include keratin, myoglobin, collagen, haemoglobin, and fibrinogen.
• We assume basic understanding of red blood cells and blood components.
• We also assume that the main pathway of oxygen transport involves red blood cells, not free plasma alone.

Concept / Approach:
Haemoglobin is the iron-containing protein found in red blood cells (erythrocytes) that binds oxygen molecules. Each haemoglobin molecule has four haem groups, each containing an iron ion that can bind one molecule of oxygen. This allows haemoglobin to carry up to four oxygen molecules per protein. In the lungs, haemoglobin binds oxygen where its partial pressure is high, forming oxyhaemoglobin. In tissues, where the oxygen level is lower and carbon dioxide level is higher, haemoglobin releases oxygen. Other proteins listed serve different functions: myoglobin stores oxygen in muscle cells, keratin forms hair and nails, collagen provides structural support in connective tissues, and fibrinogen is involved in blood clotting.

Step-by-Step Solution:
Step 1: Identify that the question is specifically about oxygen transport in blood, not oxygen storage in muscles or structural support. Step 2: Recall that red blood cells (erythrocytes) contain a pigment protein responsible for carrying oxygen. Step 3: Recognise that this pigment is haemoglobin, an iron-containing protein that gives blood its red colour. Step 4: Compare this with myoglobin, which is similar but found mainly in muscle tissue, not as the primary oxygen carrier in blood. Step 5: Note that keratin, collagen, and fibrinogen do not function as oxygen carriers. Step 6: Conclude that haemoglobin is the correct answer.

Verification / Alternative check:
Physiology textbooks state that more than 98 percent of oxygen transported in the blood is bound to haemoglobin in red blood cells, while a small fraction is dissolved in plasma. The oxygen-haemoglobin dissociation curve describes how haemoglobin loads and unloads oxygen depending on partial pressure. Clinical measurements of blood oxygen content often involve haemoglobin saturation. These multiple references confirm that haemoglobin is the main molecule responsible for oxygen transport in human blood.

Why Other Options Are Wrong:
Option A: Keratin is a structural protein found in hair, nails, and the outer layer of skin; it does not carry oxygen. Option B: Myoglobin is an oxygen-binding protein in muscle cells that stores oxygen for muscle activity but is not the main carrier in blood. Option C: Collagen is a fibrous protein in connective tissue, providing strength and support; it does not transport gases. Option E: Fibrinogen is a blood plasma protein involved in clot formation; it is not used for oxygen transport.

Common Pitfalls:
Learners sometimes confuse haemoglobin and myoglobin because both can bind oxygen and both contain haem groups. The key difference is location and main function: haemoglobin is in red blood cells and transports oxygen through the circulation, while myoglobin is in muscle and serves as a local oxygen reserve. Another pitfall is to assume that any major blood protein, such as fibrinogen, might be involved in gas transport, but in reality gas transport is handled primarily by haemoglobin (for oxygen and part of carbon dioxide transport) and by plasma for some dissolved gases.

Final Answer:
Oxygen in human blood is transported mainly by haemoglobin, an iron-containing protein located inside red blood cells.