By Nina Thach with edits by Heidi Schutz

The Amphiuma heart (Fig. 1) consists of two chambers: the atrium and ventricle. Deoxygenated blood from the systematic organs and tissues arrives in the sinus venosus, and flows into the atrium. The pulmonary arteries from the Amphiuma lungs connect to the atrium, causing a mixture of deoxygenated and oxygenated blood. The blood then drains into the ventricle and is pumped through the bulbus arteriosus, where the blood disperses throughout the body. In the bulbus arteriosus, the spiral valve aids in separating the deoxygenated and oxygenated blood. The deoxygenated blood, which carries a higher content of CO2, travels to the lungs and some travels to the integumentary capillary beds, where cutaneous gas exchange occurs and later joins the oxygenated blood from the lungs that will flow through the body.

In general, amphibian larvae begin with a two chambered heart. However, during metamorphosis, the heart develops three chambers, a condition not seen in Amphiuma which as described above, only have two chambers.It is hypothesized that Amphiuma tridactlyum underwent paedomorphosis and retained the juvenile condition of two chambers instead of three (Duellman 1986). One possible evolutionary explanation for this is that
Amphiuma is predominantly aquatic compared to its family members that are both terrestrial and aquatic. With an increase in gravitational force and increased metabolism, ancestral amphibians developed a third chamber to help their heart pump blood throughout the body more efficiently. Because of its primarily aquatic lifestyle, the effects of gravity are offset by water buoyancy, and therefore Amphiuma does not need to invest in a third chamber.

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Figure 1. Ventral view of the Amphiuma heart. Photo credit: M. Lang & N. Thach.

Comparative Anatomy:

When comparing Amphiuma to the shark, both organisms have a two chambered heart that is quite similar, likely due to the fact that they are both primarily aquatic. On the other hand, the cat has four chambers. As discussed above, terrestrial and aquatic vertebrates have different gravitational forces acting on their blood flow. In addition, due to the fact that cats are endothermic and maintain a constant elevated temperature, the cat must support a more elevated metabolism to Amphiuma, the four chambers provide complete separation of oxygenated and deoxygenated blood, which maximizes oxygen delivery to the systematic organs and tissues.

Among the amphibian class, Amphiuma have the largest red blood cells, known as erythrocytes, with a large nucleus and an elliptical shaped cell membrane (Duellman 1986). They do not uptake oxygen as frequently as other amphibians and even less so than a mammal. Their enlarged erythrocytes can store more oxygen in the body for a longer period of time and as shown in Fig. 2 below, Amphiuma, shark, amphibians, and other aquatic animals retain a nucleus in their erythrocytes. However, the cat and other mammals do not have a nucleus present in their blood cells which may be an adaptation to maximize the oxygen carrying capacity of these smaller cells .

Figure 2. 40x magnification. Comparison of erythrocyte size in the Amphiuma, Anura (frog), and mammal (human). Photo credit: H. Schutz.


Duellman, W., and Trueb, L. (1986). Biology of amphibians. New York: McGraw Hill. 670 p.