The lung is the essential respiratory organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and most other vertebrates, two lungs are located near the backbone on either side of the heart. Their function is to extract oxygen from the atmosphere and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, a process of gas exchange in the respiratory system.The air that enters, or ventilates, the lungs enters the body through the mouth or nose, and travels through the pharynx, larynx, and trachea (windpipe). The trachea divides into two bronchi one for the right and one for the left lung, which then progressively subdivide into a system of smaller secondary and tertiary bronchi and smaller bronchioles. This division ends in alveoli, which are thin-walled sacs where gas exchange of carbon dioxide and oxygen, takes place.Respiration is driven by different muscular systems in different species. Mammals, reptiles and birds use their musculoskeletal systems to support and foster breathing. In humans, the primary muscle that drives breathing is the diaphragm. In early tetrapods, air was driven into the lungs by the pharyngeal muscles via buccal pumping, a mechanism still seen in amphibians. Medical terms related to the lung often begin with pulmo-, such as in the (adjectival form: pulmonary) or from the Latin pulmonarius (""of the lungs""), or with pneumo- (from Greek πνεύμων ""lung"").