Respiration involves the diffusion of gases through membranes to facilitate metabolic processes in organisms. Simple organisms rely on direct diffusion through their integument, while more complex animals have respiratory systems. Aquatic animals use gills and many land animals use lungs. Amphibians are unique in that they can respire through their skin, mouth, and lungs. Their respiratory system includes glottis, larynx, bronchi and lungs, with gas exchange aided by a moist, permeable membrane.