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Respiration -- the exchange of gases between an organism and its environment -- in adult frogs can occur in three different ways, either independently or in tandem. Adult frogs breathe through their lungs and exchange gases through their skin and the lining of their mouths. At the larval stage of their development, frogs lack functional lungs but are able to take in oxygen through a set of gills.
All frogs go through a process called metamorphosis, in which they mature from their larval, or tadpole, stage into adult frogs. One of the hallmarks of this process for most frog species is the transition from having gills to having functional lungs. A newly hatched tadpole's gills are external. These gills take in oxygen when water passes over them. As the tadpole matures, the gills are absorbed by the body and become an internal part of the tadpole's anatomy.
Once mature, frogs lose their gills and are able to bring oxygen into their bodies through functioning, though comparatively underdeveloped, lungs. Frogs rely on their lungs to breathe when they are active and need more oxygen than skin respiration alone can provide. Unlike mammals that draw air continuously into their lungs, frogs only breathe through lungs when necessary. Because they lack a diaphragm to help regulate the pressure in their lungs, frogs must use their throats, nostrils and mouths together to bring in and expel gases.
Though they have functional lungs, much of a frog’s respiration occurs through the skin. A frog’s moist skin is thin and marbled with blood vessels and capillaries close to the surface. The moisture on the skin dissolves oxygen from the air and water surrounding the frog and transmits it into the blood. Though moist skin is essential for this process, frogs are not limited to skin respiration only when under water. Glands in the frog’s skin produce mucus that keeps the skin moist and allows for respiration even on dry land.
Frogs have an additional surface for respiration other than their skin -- the moist lining of the mouth. When the frog's mouth is not submerged completely in water, this respiratory lining is in constant use, bringing oxygen into the bloodstream from the surrounding air and diffusing excess carbon dioxide back into the environment.
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