The actual exchange of oxygen for carbon dioxide takes place between the capillaries and the alveoli of the lungs by a process known as diffusion. This is due to a difference in the partial pressure of the gases in these structures. Solution The actual exchange of oxygen for carbon dioxide takes place between the capillaries and the alveoli of the lungs by a process known as diffusion. This is due to a difference in the partial pressure of the gases in these structures..