RESPIRATORY SYSTEM
In amphibians, gas exchange always occurs across a moist surface. Although cutaneous respiration is important in both larval and adult forms, as a general rule larval amphibians utilize gill structures for respiration, while adults use lungs, although there are many exceptions to this.
There are three modes of respiration described in adult caecilians and anurans: pulmonic, buccopharyngeal and cutaneous. A fourth mode exists in adult urodeles, that being branchial respiration from retained gill structures seen in neotenic species such as sirens, mudpuppies, axolotls and Texas blind salamanders (Goin et al. 1978; Mitchell et al. 1988; Wright 1996, 2001c).In most amphibians, gill structure shows some variability depending on the species and their environment. The gills of larval anurans are usually smaller and simpler than those of salamander larvae. While the branchial arches of tadpoles are covered by an operculum, in many species of salamander, especially neotenic species, the gills are external. The gills of most caecilians are resorbed before birth or hatching, while the gills of anurans resorb during metamorphosis. Most terrestrial salamander species lose their gills and develop lungs like anurans; however, many of the aquatic neotenic species will retain their gills and still develop normal lungs. A few families of salamander, notably the Plethodontidae and Hynobiidae, lack lungs or have lungs that are reduced in size (Goin et al. 1978; Mitchell et al. 1988; Wright 1996, 2001c).
The lungs of amphibians are simple saclike structures that lack true alveoli. As a result, most lungs are subdivided internally by delicate reticulate infoldings of the pulmonic tissue that significantly increase the surface area for gas exchange. Complete cartilaginous rings support the tracheal tissues. The trachea is variable in length depending on the species, but in general is considered short, and bifurcates quickly into main bronchi.
Amphibians
CLINICAL NOTE
Care must be taken if intubating the amphibian patient, or passing a tube to perform tracheal washes or intra-tracheal treatments, to prevent damaging the pulmonic epithelium. Also, owing to the delicate nature of the lung, one must make sure not to overinflate the lungs during anesthesia as they easily rupture (Green 2001; Mitchell et al. 1988; Wright 1996, 2001c).
Amphibians lack a diaphragm so they rely on coordinated movements of their axial and appendicular muscles for gas exchange in the lungs. Buccopharyngeal gas exchange occurs through the pumping action of the larynx during inspiration and expiration. During periods of reduced oxygen availability (such as hibernation) amphibians may switch to cutaneous respiration. As cutaneous respiration is not as efficient as pulmonic respiration, many amphibians have developed specialized integumentary structures, such as lateral folds, costal grooves or cutaneous “hairs”, as seen in the African hairy frog (Trichobatrachus spp.) (Mitchell et al. 1988; Wright 1996, 2001c).