What protein carries oxygen in the blood?

• A. Blood
• B. Hemoglobin
• C. Lymph capillaries
• D. Veins

Answer: (B)

Hemoglobin is the oxygen-carrying protein in red blood cells. In the lungs, iron in the heme groups binds oxygen, so each molecule of hemoglobin can transport up to four oxygen molecules. This binding is cooperative, meaning once one oxygen molecule attaches, the others are more likely to bind, making loading in the lungs efficient and unloading in tissues effective. In tissues, conditions like lower pH, higher carbon dioxide, and higher temperature reduce hemoglobin’s affinity for oxygen, promoting release where it’s needed. While a small amount of oxygen dissolves directly in plasma, most is carried bound to hemoglobin. Blood is the fluid that transports substances, but the protein that actually binds and carries the oxygen is hemoglobin; lymph capillaries are part of the lymphatic system, and veins are vessels that return blood to the heart.