Which cell carries oxygen in the blood?

• A. Leukocyte
• B. Erythrocyte
• C. Platelet
• D. Monocyte

Answer: (B)

Oxygen transport in the blood is handled mainly by red blood cells because they are packed with hemoglobin, a protein with iron-containing heme groups that reversibly binds O2. In the lungs, oxygen binds to hemoglobin to form oxyhemoglobin; when blood flows to tissues, oxygen is released where it’s needed for cellular metabolism. This creates a high-capacity, efficient system for moving oxygen throughout the body. The other cells listed—white blood cells (immune cells) and platelets (clotting factors)—do not carry oxygen, and only a tiny amount of oxygen is dissolved directly in plasma.