Figure 7 (below). Diagram of the respiratory membrane (a), abstracted from figure 6 (above - as if the blood vessel in the box were a capillary. The actual capillaries are too small to be seen clearly.) The respiratory membrane includes the alveolar epithelium (b), capillary endothelium (c), and both of their basement membranes (e & f) that fuse together (d - between arrowheads). It is all that separates the atmosphere (atm) from the blood (asterisk).