Which protein carries most of the oxygen in the blood?

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

Answer: (A)

Oxygen is carried in the blood mainly by hemoglobin, a protein inside red blood cells that has four heme groups, each capable of binding one O2 molecule. In the lungs, oxygen binds to the iron in these heme groups, and as blood travels to tissues where oxygen is needed, hemoglobin releases it. This binding and release are cooperative, meaning the binding of one O2 increases the likelihood of others binding and also facilitates unloading where oxygen is scarce. Myoglobin stores oxygen in muscle tissue and releases it under very low oxygen conditions, but it does not transport oxygen in the bloodstream. Albumin and fibrinogen have other roles in the blood (transport of various substances and clotting, respectively) and do not carry significant amounts of O2. So the protein responsible for most of the blood’s oxygen transport is hemoglobin.