How is oxygen primarily transported in the blood?

Oxygen is primarily transported in the blood by hemoglobin in red blood cells. Hemoglobin is a specialized protein that binds to oxygen in the lungs and carries it through the bloodstream to various tissues and organs in the body. Each hemoglobin molecule can carry up to four oxygen molecules, allowing for efficient transport and delivery.

The affinity of hemoglobin for oxygen is crucial as it enables the release of oxygen to cells where it is needed for metabolic processes while allowing for the binding of oxygen in the lungs where concentrations are higher. This mechanism ensures that oxygen delivery is finely regulated according to the body's demands, particularly during activities that require increased oxygen, such as exercise.

The other choices provided do not account for the primary transportation method of oxygen. Plasma does carry a small amount of dissolved oxygen, but this is not sufficient for the body’s needs. White blood cells play a role in the immune response, not in oxygen transport. Lastly, carbon dioxide is transported in the blood primarily in the form of bicarbonate ions rather than as dissolved carbon dioxide, emphasizing the significance of hemoglobin in oxygen transport rather than other gases.