What is breathing?
Most organisms on Earth are aerobic. Such organisms breathe oxygen, i.e. absorb oxygen gas (`O_2` ) and output carbon dioxide gas (`CO_2` ).
Most terrestrial vertebrates and some invertebrates are breathe with lungs. For example, in mammals, reptiles, amphibians and birds, inhaled air passes through the upper airways, trachea, bronchi and reaches the alveoli. Through the alveolar walls, the `O_2` diffuses into the blood and `CO_2` diffuses from the blood.
Shellfish and spiders lungs have a different structure. Insects and centipedes breathe with their trachea. The trachea is a branching tube inside the body, which opens on the surface of the body spiracles. Most aquatic animals breathe with gills. Gills have a folded structure to increase the surface area. Through the gills' surface, `O_2` and `CO_2` are diffused.