By Nina Thach
As shown in Figure 1, in A. tridactylum, the lungs are elongated. Both the right and left lungs extend caudally from the heart. The larger right lung spans the entire ventral cavity while the tip of the smaller left lung is located just cranial to the colon. The size disparity between lungs is similar to the cat, whose right lung is larger than the left. However, in the cat, the lungs are segmented into specific lobes, whereas the Amphiuma lungs are each unicameral, which is an undivided sac (Hill 2012). In comparison to the shark, lungs are absent because gas exchange occurs in their gills between the water and the blood rather than between the air and the blood as in the amphiuma and cat.

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Figure 1. (a) length of each lung. (b) ventral view of lungs in the body cavity. (c) lungs reflected from the body. Photo credit: M. Lang & N. Thach.

Amphiuma breathe in a systematic approach called the "four stroke buccal pump" as shown in Figure 2 (Kardong 2015). They breathe every 45 to 75 minutes when they swim to the surface for air. To aid in the limited amount of breathing the Amphiuma does, they shed carbon dioxide through their skin (see cutaneous respiration) (Toews 1969).
Figure 2. The process of the four stroke buccal pump in A. tridactylum respiration. The steps are as follows: 1. The floor of the mouth expands with nostrils closed, drawing air up from the lungs, 2. The volume of the mouth is reduced while nostrils are open, pushing air out of the nostrils, 3. The mouth expands again while the nostrils are open, drawing air from the environment into the nostrils, 4. The volume of the mouth decreases, pushing the air into the lungs. Illustration by N. Thach, after Kardong (2015).

Hill, R., Wyse, G., and Anderson, M. (2012). Animal physiology. third ed. Sunderland: Sinauer Associates Inc. Print. 596-596 p.

Kardong, K. V. (2015). Vertebrates: Comparative anatomy, function, evolution. seventh ed. New York: McGraw Hill Education. Print. 422-423 p.

Toews, D.P., Shelton, G., and Randall, D.J. (1970). Gas tensions in the lungs and major blood vessels of the urodele amphibian, Amphiuma tridactylum. Vancouver: University of British Columbia. 61 p.