Which protein transports oxygen from the lungs to other parts of the body?

• A. Albumin
• B. Hemoglobin
• C. Lipoprotein
• D. Retinol-binding protein

Answer: (B)

Hemoglobin is the protein that carries oxygen from the lungs to the tissues. It’s a tetramer in red blood cells, with each subunit containing a heme group that holds an iron atom capable of binding one O2 molecule. That means a single hemoglobin molecule can transport up to four oxygen molecules.

In the lungs, the high oxygen partial pressure allows oxygen to bind to the iron, forming oxyhemoglobin. In tissues, the lower oxygen pressure, along with higher carbon dioxide, lower pH, and the presence of 2,3-bisphosphoglycerate, reduces hemoglobin’s affinity for oxygen and promotes release where it’s needed. This cooperative binding creates the characteristic sigmoidal oxygen-hemoglobin dissociation curve, enabling efficient pickup in the lungs and delivery to tissues.

Other proteins listed have different roles: albumin helps maintain blood volume and transports various small molecules but not oxygen; lipoproteins carry lipids; retinol-binding protein transports vitamin A. Myoglobin stores oxygen in muscle but does not transport it in the bloodstream.