The respiratory system facilitates gas exchange between the body and environment. It involves the intake of oxygen and exhalation of carbon dioxide. Various animal groups have evolved different respiratory surfaces and structures to efficiently carry out this gas exchange, such as lungs, gills, and tracheal systems. In humans, air enters through the nose and mouth and moves through the trachea and bronchi to the lungs, where oxygen diffuses into blood in the alveoli and carbon dioxide diffuses out.