Flexi Says: Carbon dioxide passes through cell membranes by simple diffusion. When carbon dioxide builds up in a cell above a certain level, carbon dioxide molecules will pass out of the cell and into nearby capillaries. Capillaries are only one cell thick. Once inside the capillaries, carbon dioxide molecules diffuse across the membranes of red blood cells and bind to hemoglobin. The red blood cells then carry the carbon dioxide to the lungs where it diffuses into the air.
