Oxygen-carrying pigment in red blood cells.

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

Answer: (B)

Hemoglobin is the oxygen-carrying pigment in red blood cells. It’s a protein made of four subunits, each with a heme group that contains iron capable of binding one oxygen molecule. In the lungs, oxygen binds to these iron-containing heme groups; as blood circulating to tissues reaches areas with lower oxygen and higher carbon dioxide, hemoglobin releases the oxygen where it’s needed. It also helps shuttle some carbon dioxide back to the lungs to be exhaled. The other options don’t fit this role: albumin is a plasma protein that helps maintain blood volume and carries various substances in the plasma; fibrinogen is a clotting factor that becomes fibrin to form a blood clot; myoglobin stores and releases oxygen in muscle tissue and is not contained in red blood cells.