Oxygen is primarily transported in human beings by hemoglobin in red blood cells. Hemoglobin binds oxygen in the lungs' alveoli, forming oxyhemoglobin, and carries it through the bloodstream to tissues where oxygen is released for cellular use. Only a small amount of oxygen dissolves directly in the blood plasma. Carbon dioxide, produced as a metabolic waste in cells, is mostly transported dissolved in blood plasma, but a significant amount is also carried inside red blood cells converted into bicarbonate ions. In the lungs, carbon dioxide is released from bicarbonate, diffuses into the alveoli, and is exhaled out. Thus, oxygen is transported mainly bound to hemoglobin, while carbon dioxide is transported mainly dissolved in plasma or as bicarbonate within red blood cells.