Which structure is primarily responsible for oxygen transport in the blood?

Hemoglobin is the protein found within red blood cells that is primarily responsible for the transport of oxygen in the bloodstream. This molecule has a high affinity for oxygen, allowing it to bind effectively in the lungs where oxygen concentrations are high. Once bound, hemoglobin carries the oxygen through the circulatory system and releases it in tissues where oxygen levels are lower, facilitating cellular respiration and energy production.

Additionally, hemoglobin is crucial in determining the blood's overall ability to carry oxygen, significantly influencing the body's metabolism and functionality. The integration of hemoglobin's structure, comprising four subunits, allows it not only to transport oxygen but also to aid in the transport of carbon dioxide back to the lungs for exhalation.

The other options, while important components of the blood, do not serve this specific function. For instance, platelets are primarily involved in blood clotting, leukocytes are responsible for immune responses, and plasma acts as the liquid medium that transports cells and various substances but does not specifically transport oxygen.