section_path: Other animals/Amphibians
section_name: Amphibians
paragraph_text: Amphibians
  
  The lungs of most frogs and other amphibians are simple and balloon-like, with gas exchange limited to the outer surface of the lung. This is not very efficient, but amphibians have low metabolic demands and can also quickly dispose of carbon dioxide by diffusion across their skin in water, and supplement their oxygen supply by the same method. Amphibians employ a positive pressure system to get air to their lungs, forcing air down into the lungs by buccal pumping. This is distinct from most higher vertebrates, who use a breathing system driven by negative pressure where the lungs are inflated by expanding the rib cage. In buccal pumping, the floor of the mouth is lowered, filling the mouth cavity with air. The throat muscles then presses the throat against the underside of the skull, forcing the air into the lungs.
  
  Due to the possibility of respiration across the skin combined with small size, all known lungless tetrapods are amphibians. The majority of salamander species are lungless salamanders, which respirate through their skin and tissues lining their mouth. This necessarily restricts their size: all are small and rather thread-like in appearance, maximising skin surface relative to body volume. Other known lungless tetrapods are the Bornean flat-headed frog and Atretochoana eiselti, a caecilian.
  
  The lungs of amphibians typically have a few narrow internal walls (septa) of soft tissue around the outer walls, increasing the respiratory surface area and giving the lung a honeycomb appearance. In some salamanders even these are lacking, and the lung has a smooth wall. In caecilians, as in snakes, only the right lung attains any size or development.