Which protein in erythrocytes carries oxygen?

• A. Myoglobin
• B. Fibrinogen
• C. Albumin
• D. Hemoglobin

Answer: (D)

Hemoglobin is the protein in red blood cells that carries oxygen. It’s a tetramer with four heme groups, each containing iron that can reversibly bind one molecule of O2. This means a single hemoglobin molecule can carry up to four oxygen molecules from the lungs to tissues that need it, releasing them where oxygen pressure is low or conditions favor unloading (like higher CO2, lower pH, or higher temperature).

The other options don’t fit because they aren’t the oxygen-carrying protein inside red blood cells. Myoglobin stores oxygen in muscle tissue but is located in muscle cells, not in erythrocytes. Fibrinogen is a plasma protein involved in clotting, and albumin is the main plasma protein that helps maintain blood volume and osmotic pressure, not oxygen transport inside red blood cells.