They are found most often in larval or paedomorphic amphibians. Lungs are found among fishes found in warm or stagnant water, as well as in primitive fishes, and allow the fish to gulp air and undergo diffusion in an environment with relatively low dissolved oxygen levels. Swim bladders are similar to lungs, but are found in fishes that live in more oxygen-rich environments. Thus, the air-filled spaces serve less of a purpose in respiration and function more as a hydrostatic organ. The swim bladder can also be used in sound production by fishes.
Gas is secreted into the swim bladder from blood by action of the gas glands.
Gas glands are associated with the rete mirabile, which is a network of small arteries or capillaries. Lungs and their ducts Tetrapod lungs are paired organs surrounded by pleura and contained in the pleural cavity. They have a higher surface to area volume than the gills, and are joined to the ventral side of the gut tube by the trachea. In tetrapods, any increase in overall body size leads to an increased amount of compartmentalization of the lungs. During respiration, air enters through the mouth, or into the external nares to the choanae, and then passes into the pharynx.