Oxygen is carried in the blood bound to hemoglobin in the red blood cells.  Carbon dioxide dissolves in the plasma, where most of it forms carbonic acid, HÒCOÓ, which dissociates to form bicarbonate, HCOÓÍ, and hydrogen, HÌ, ions (carbonic anhydrase, an enzyme found within erythrocytes, facilitates the production of carbonic acid).  Carbon dioxide also moves into red blood cells where it combines with hemoglobin.  In this lung capillary, oxygen is diffusing into the blood and carbon dioxide is diffusing into the alveolus.