Respiratory system is responsible for the process where gas exchange takes place to continuously provide oxygen and remove the carbon dioxide formed in respiration. During the breathing/gas exchange process, the gaseous exchange step occurs due to a pressure gradient that follows the principle of diffusion. Since the inhaled air and exhaled air use the same route for moving in and out of the body of organisms, we can say the gas movement is bidirectional and not unidirectional. Every respiratory system does not have a series of tubes for air intake and outtake (humans have a single passage for taking in air and giving out (trachea), whereas some insects have multiple passages. Gases move through a moist membrane in all animals. For example, at alveoli, gas diffuses across the membrane of cells which have nearly 70% water. Same goes for trachea, gills and subcutaneous breathing.