Blood entering the lung capillaries is full of carbon dioxide that it has collected from the tissues. Most of the carbon dioxide is dissolved in the plasma either in the form of sodium bicarbonate or carbonic acid. A little is transported by the red blood cells. As the blood enters the lungs the carbon dioxide gas diffuses through the capillary and alveoli walls into the water film and then into the alveoli. Finally it is removed from the lungs during breathing out (see diagram 9.2). (See chapter 8 for more information about how oxygen and carbon dioxide are carried in the blood).