The lung is the essential respiration organ in air-breathing animals, including most tetrapods, a few fish and a few snails. The most primitive animals with a lung are the lungfish (vertebrate) and the pulmonate snails (invertebrate). In mammals and the more complex life forms, the two lungs are located in the chest on either side of the heart. Their principal function is to transport oxygen from the atmosphere into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere. This exchange of gases is accomplished in the mosaic of specialized cells that form millions of tiny, exceptionally thin-walled air sacs called alveoli.