How does CO2 gets diffused into the red blood cells?
The red cell membrane is highly permeable to gases because the molecules of gases are small, uncharged, and soluble in lipids, such as those of a bilayer.The effect of 4,4′-diisothiocyanato-stilbene-2,2′-disulfonate on CO2 permeability of the red blood cell membrane Remember red blood cells are  cells like any other cells only that they have hemoglobin and no nucleus. Because they have an ordinary cell membrane, it follows that the laws of diffusion which apply in gases and other cells will also apply with gases and RBCs. Oxygen is the major gas which gets into RBCs, so is carbon monoxide and others