Most oxygen is transported in the blood:

Explanation

Bicarbonate ions are involved in the transport of carbon dioxide in the blood. They do not play a significant role in the transport of oxygen. Albumin compounds are proteins that are found in the blood plasma. They can bind to a small amount of oxygen, but this is not a significant way of transporting oxygen in the blood. A small amount of oxygen is dissolved in the plasma. However, this is only a small fraction of the total amount of oxygen that is transported in the blood. Leukocytes are white blood cells. They do not play a role in the transport of oxygen in the blood. Most oxygen is transported in the blood bound to hemoglobin. Only a small amount of oxygen is dissolved in the plasma. Hemoglobin is a protein found in red blood cells. It has a heme group that can bind to oxygen molecules. When oxygen binds to hemoglobin, it forms oxyhemoglobin. Oxyhemoglobin is transported to the tissues, where the oxygen is released. The amount of oxygen that is bound to hemoglobin depends on the partial pressure of oxygen in the blood. The partial pressure of oxygen is the pressure exerted by the oxygen molecules in the blood. The higher the partial pressure of oxygen, the more oxygen that will bind to hemoglobin. The partial pressure of oxygen is highest in the lungs, where the oxygen is taken in. The partial pressure of oxygen is lowest in the tissues, where the oxygen is released. This difference in partial pressure drives the diffusion of oxygen from the lungs to the tissues.