Respiration of Tetrapods 



With few exceptions the respiratory organs of tetrapods are 

 paired hollow sacs or lungs. They are absent in some Amphibia, 

 notably salamanders living in torrential streams, and reduced in 

 forms which have external gills as adults. The lungs develop as 

 ventral outgrowths of the pharynx and although their precise 

 phylogenetic relationships are not clear, it is generally accepted 

 that they are homologous with the swimbladder and lungs of 

 fishes. Their blood supply is derived from the sixth aortic arch 

 of the embryo and oxygenated blood is returned directly to the 

 heart in a pulmonary vein. All lungs have a thin epithelial layer 

 abundantly supplied with blood but their internal complexity 

 varies a great deal. In many amphibia it is little more than a 

 hollow sac, as in lungfishes, there being few infoldings of the 

 internal wall to produce an increase in surface area for gaseous 

 exchange (fig. 12). In the common frog, however, the surface is 

 extensively folded and externally it has a foamy appearance. 

 The two lungs communicate separately with a single laryngo- 

 tracheal chamber which is supported by small cartilages but 

 there are no well-defined trachea or bronchial tubes. This cham- 

 ber opens into the hind end of the pharyngeal cavity through a 

 slit-like glottis which is operated by muscles. The respiratory 

 surface of Rana corresponds to no more than 20 sq. cm. for 

 each cc. of air contained within the lung. In comparison with 

 the latter figure, a man's lung has 300 sq. cm. of alveolar surface 

 for each cc. of air. The walls of the lungs contain numerous 

 elastic fibres and plain muscle fibres are also present which 

 enable the lung to make spontaneous contractions when 

 isolated. 



