Oxygen and carbon dioxide are transported to the tissues in the blood. Oxygen is not very soluble in plasma. It enters red blood cells and binds to hemoglobin, which allows high concentrations of oxygen in the blood, in spite of its low solubility.
Carbon dioxide is more soluble in plasma, but most of it is converted to bicarbonate in the red blood cells. The bicarbonate is released into the plasma, so most carbon dioixide is transported as bicarbonate in the plasma. Both gases exchange only from the plasma.
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