What primarily transports oxygen in the bloodstream?

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

Answer: (A)

Oxygen transport in the bloodstream is primarily accomplished by red blood cells (erythrocytes). These specialized cells contain hemoglobin, a protein that binds oxygen in the lungs and carries it through the circulatory system to tissues and organs. When red blood cells travel through the pulmonary capillaries in the lungs, they pick up oxygen as it diffuses from the alveoli into the blood. Hemoglobin can carry up to four oxygen molecules per molecule of hemoglobin, significantly increasing the blood's capacity to transport oxygen compared to plasma alone.

While plasma does carry a small amount of dissolved oxygen, its capacity is insufficient to meet the body's needs during physical activity or increased metabolic demand. White blood cells are primarily involved in immune responses and do not play a role in oxygen transport. Platelets are involved in blood clotting and also do not contribute to the transport of oxygen. The specialization and efficiency of red blood cells in binding and releasing oxygen are why they are the primary means of oxygen transport in the bloodstream.