Hypaxial Muscles Aid in Respiration

Biological Strategy

Greater siren

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Distribute Gases

Gases of particular importance to living systems are oxygen, carbon dioxide, and nitrogen. Oxygen and carbon dioxide are involved in respiration, so distributing these gases efficiently and effectively is important for a living system’s survival. However, gases are difficult to contain because they disperse easily. To accommodate this, living systems have strategies for confining gases and using gases’ properties to their advantage. For example, prairie dogs and mound-building termites build systems of tunnels and mounds that take advantage of wind to ventilate their underground homes.

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Expel Gases

The most familiar forms of discharging gases are through respiration when many living systems release carbon dioxide, and when plants release oxygen as the end product of plant photosynthesis. Because gases cannot be effectively moved through pushing, a different kind of force is needed to expel them. Creating that force requires energy, even at the cellular level, so living systems must have efficient strategies worth the energy investment or use an external force. This typically entails strategies that build up pressure or use other forces to propel gases. For example, a human exhales about 15% of its spent air per breath. In contrast, when a whale surfaces, it exhales 90% of its spent air in just one spouting.

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Amphibians

Class Amphibia (“to live a double life”): Frogs, salamanders, toads

Amphibians live a “double life” by being able to survive both on land and in water. Unlike reptiles with dry, scaly skin, amphibians have smooth skin that needs to be kept moist. Many live in the water through development and start life with gills, and then transition to breathing with lungs. They can get even more oxygen through cutaneous respiration, or breathing through their skin. This class represents the first animal group to be able to efficiently walk on land—aided by the well-defined limbs that gave them a “leg up” on their finned competitors.

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The hypaxial muscles of some salamanders cause exhalation by actively contracting to increase abdominal pressure and force air out of the lungs.

Historically, amniotes were the only known animals to use the muscles of their thorax and abdomen to expand the body and pull air into the lungs. The next animals in the evolutionary chain, such as air-breathing fishes and amphibians, use a method known as a buccal pump that compresses air stored in an extended jaw (Gans, 1970b; Liem, 1985 via Brainerd and Monroy 1998). Siren lacertina, however, is a salamander (a non-amniote) that uses a system similar to the aspiration pump used by many amniotes to breathe. The new method of breathing discovered to be used by the salamanders is known as an exhalation pump and exhalation occurs in two phases. The first phase is passive; air is forced out of the lungs by hydrostatic pressure and the elasticity of the lungs. The second phase is an active one involving the traverse abdominal muscle. By engaging the muscles on the lower side of the body (the hypaxial muscles) the salamander creates active pressure within the body cavity that forces air out of the lungs (Brainerd and Monroy 1998: 673).

Last Updated September 14, 2016

References

Journal article

Lung ventilation in salamanders and the evolution of vertebrate air-breathing mechanisms

Biological Journal of the Linnean Society | 16/09/2002 | E Brainerd

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Reference

Journal article

Mechanics of lung ventilation in a large aquatic salamander, Siren lacertina

The Company of Biologists Limited | Brainerd, EL; Monroy, JA

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