Which component carries oxygen in the bloodstream?

• A. Red Blood Cells
• B. Plasma
• C. White Blood Cells
• D. Platelets

Answer: (A)

Oxygen transport in the blood relies on red blood cells. These cells are packed with hemoglobin, a protein that contains iron-containing heme groups capable of binding oxygen. In the lungs, high oxygen levels saturate hemoglobin to form oxyhemoglobin, allowing four oxygen molecules to attach to each hemoglobin molecule. When blood reaches tissues where oxygen is needed, conditions such as lower oxygen pressure, higher carbon dioxide, and a lower pH promote the release of oxygen from hemoglobin to be used by cells. A small portion of oxygen does dissolve directly in plasma, but this amount is tiny compared to what hemoglobin carries, so plasma cannot meet metabolic needs on its own. The other components—plasma, white blood cells, and platelets—do not transport oxygen; plasma mainly carries fluids, nutrients, and gases dissolved in the blood, while white blood cells and platelets have roles in immunity and clotting, respectively.