What blood component carries oxygen in the bloodstream?

• A. Hemoglobin
• B. Plasma
• C. Platelets
• D. White blood cells

Answer: (A)

Oxygen in the blood is carried mainly by hemoglobin inside red blood cells. Hemoglobin has four heme groups, each with iron that binds one molecule of oxygen. In the lungs, oxygen loads onto hemoglobin to form oxyhemoglobin; in tissues, oxygen is released as the affinity of hemoglobin for oxygen decreases in response to factors like higher carbon dioxide, lower pH, and higher temperature. This cooperative binding allows hemoglobin to pick up oxygen efficiently where it’s abundant and drop it off where it’s needed.

Plasma can dissolve a small amount of oxygen, but this dissolved portion is minimal compared to what hemoglobin carries. Platelets and white blood cells do not transport oxygen; they have other roles in clotting and immunity, respectively.