The lung is the primary respiratory organ in all air breathing animals, which group includes most of the tetrapods, i.e. all vertebrates other than fishes, and a few fishes such as the Australian lung fish. The main job of the lung is to transfer oxygen from atmospheric air into the animal’s bloodstream and extract carbon dioxide that has accumulated in the blood.
Mammalian lungs, of which the human lung is typical, exhibits a spongy texture and is honeycombed with epithelia, i.e. the specialized cell tissue that covers the outer surface of the body as well as the mucous membrane connected with it as well as the closed cavities of the body. Because of the profusion of these cells inside the lung, the lung actually has a considerably greater total surface area than would be indicated by its outer surface area. The lung contains specialized cells, alveoli, which cluster up to form millions of tiny thin walled sacs, alveolar sacs, that facilitate the exchange of gases.
Oxygenated air, taken through the mouth and nose, travel through the trachea which, in humans, is divided into two bronchi which enter into the roots of the lungs. The bronchi subdivide into smaller and smaller connections, bronchioles, which finally connect to the alveolar sacs. These sacs are very tightly wrapped up in blood vessels and it is here that the exchange of gasses, carbon dioxide for oxygen, is made.
Human lungs occupy two cavities on both sides of the heart and, although they are very similar, they are not identical; for on the edges closest to the heart, the right lung runs almost vertical to the heart while the left lung sports an indentation, called the cardiac notch, in order to accommodate the shape of the heart. This notch makes the left lung somewhat smaller than the right lung. Each lung is divided into five lobes, three on the right and two on the left and these are further subdivided into lobules, the smallest subdivision of the lung that can be seen by the naked eye.
The lung comes with very considerable overcapacity for its primary respiratory function so that when one is at rest, for instance, most of lung capacity is unused, more capacity been called into play as exertion is increased. It is because of this built in overcapacity that a lung can be removed by surgery and the single remaining lung is capable of carrying the entire respiratory workload.
Apart from respiration however, the lung also plays some very important additional roles.
First, it acts as a filter removing small blood clots from the veins which could otherwise cause complications as well as micro-bubbles that form in the venous blood stream as is common, for instance, when one is decompressing after diving.
The lungs also secrete immunoglobulin A which provides protection against respiratory diseases and it also gives protection to the heart which it almost totally encloses as the soft tissues of the lungs act as shock absorbers.
The lung also produces mucus which contains antibacterial compounds which help keep the lungs and the entire bronchial tract sterile and infection free. Further, the mucus serves to capture dust and bacteria which might otherwise cause infection.
The lungs of other tetrapods play a similar role to the mammalian lung although in slightly different ways. Avian lungs, for instance, do not have alveoli and are comparatively smaller than mammalian lungs for similarly sized animals. The lungs are made up of millions of tiny passages which permit the inhalation of more oxygen than mammalian lungs so that birds can fly at altitudes where mammals would have trouble breathing and survive just well. Airflow is maintained throughout the body in flight by the existence of some eight or nine air sacs spread throughout the body and two in the bones and air is forced through the sacs by rib and flight muscle actions.
Reptilian ventilation is affected by the expansion and contraction of the ribs using the axial muscles, as well as the mouth muscles. Crocodilians are also equipped to use a muscle attached to the pubic bone which pulls the liver which in turns pulls the lungs causing expansion and contraction. Some reptiles which are unable to move their ribs, e.g. turtles, use their forelimbs and pectoral girdles to cause the lungs to contract and expand. Snakes, limbless lizards and amphisbaenas often have just the one lung as an organ of ventilation with the other lung greatly reduced or even absent.
Amphibian lungs are simple balloon like affairs and gas exchange takes place on the outer lung surface. This arrangement which is quite inefficient in comparison to other tetrapods, is, nonetheless, perfectly suitable for amphibians which have, comparatively, a very slow metabolic process and which, moreover, are able to supplement their oxygen needs by diffusion, i.e. the passage of oxygen into the blood stream through their moist outer skins. The only known lungless tetrapods, e.g. most salamanders, are to be found amongst amphibians, respiration being carried out entirely through the skin and the moist tissues that line their mouths.