The respiratory membrane is the diffusion barrier across which respiratory gases are swapped between the blood and the air in the alveoli is muscular layer of the capillary wall.
External respiration is the Swapping of gases with the outer environment, and occurs in the alveoli of the lungs.
Inner respiration is the exchange of gases with the interior environment, and takes place in the tissues.
The basic exchange of gases occurs due to simple diffusion.
Thus, option "C" is correct, muscular layer of the capillary wall.
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